Structure and Function of Animals
In order to survive, animals must be able to coordinate
the functions of their many specialized cells, take in and digest
food, pull oxygen from the air, circulate nutrients and oxygen to
their cells, eliminate wastes, move, maintain body temperature,
and reproduce. Animals have also developed various behaviors that
help them to survive.
Humans and other highly evolved animals have developed
two main systems for coordinating and synchronizing the functions
of their millions of individual cells. The nervous system works
rapidly by transmitting electrochemical impulses. The endocrine
system is a slower system of control; it works by releasing chemical
signals into the circulation. In addition to coordinating essential
bodily functions, these two control systems allow the animal to
react to both its external and internal environments.
The Nervous System
The nervous system functions by the almost instantaneous
transmission of electrochemical signals. The means of transmission
are highly specialized cells known as neurons, which are
the functional unit of the nervous system.
The neuron is an elongated cell that usually consists
of three main parts: the dendrites, the cell
body, and the axon. The typical neuron contains
many dendrites, which have the appearance of thin branches extending
from the cell body. The cell body of the neuron contains the nucleus
and organelles of the cell. The axon, which can sometimes be thousands
of times longer than the rest of the neuron, is a single, long projection
extending from the cell body. The axon usually ends in several small
branches known as the axon terminals. Neurons are often connected
in chains and networks, yet they never actually come in contact
with one another. The axon terminals of one neuron is separated
from the dendrites of an adjacent neuron by a small gap known as
The electrical impulse moving through a neuron begins
in the dendrites. From there, it passes through the cell body and
then travels along the axon. The impulse always follows the same
path from dendrite to cell body to axon. When the electrical impulse
reaches the synapse at the end of the axon, it causes the release
of specialized chemicals known as neurotransmitters.
These neurotransmitters carry the signal across the synapse to the
dendrites of the next neuron, starting the process again in the
The Resting Potential
To understand the nature of the electrical impulse that
travels along the neuron, it is necessary to look at the changes
that occur in a neuron between when it is at rest and when it is carrying
an impulse. When there is no impulse traveling through a neuron,
the cell is at its resting potential and the inside of the cell
contains a negative charge in relation to the outside.
Maintaining a negative charge inside the cell is an active
process that requires energy. The cell membrane of the neuron contains
a protein called Na+/K+ ATPase
that uses the energy provided by one molecule of ATP to pump three
positively charged sodium atoms (Na+) out
of the cell, while simultaneously taking into the cell two positively
charged potassium ions (K+).
The sodium-potassium pump builds up a high concentration of sodium
ions outside the cell and an excess of potassium ions inside the
cell. These ions naturally want to diffuse across the membrane to
regularize the distribution. However, one of the special properties
of phospholipid cell membranes is that they bar passage to ions
unless there is a special protein channel that allows a particular
ion in or out. No such channel exists for the sodium that is built
up outside the cell, though there are potassium leak channels that
allow some of the potassium ions to flow out of the cell. The difference
in ion concentrations creates a net potential difference across
the cell membrane of approximately –70 mV (millivolts),
which is the value of the resting potential.
The Action Potential
While most cells have some sort of resting potential from
the movement of ions across their membranes, neurons are among only
a few types of cells that can also form an action potential. The
action potential is the electrochemical impulse that can travel
along the neuron. In addition to the Na+/K+ ATPase
and potassium leak channel proteins, the neuron membrane contains voltage-gated proteins.
These proteins respond to changes in the membrane potential
by opening to allow certain ions to cross that would not normally
be able to do so. The neuron contains both voltage-gated sodium
channels and voltage-gated potassium channels, which open under
The action potential begins when chemical signals from
another neuron manage to depolarize, or make less negative, the
potential of the cell membrane in one localized area of the neuron
cell membrane, usually in the dendrites. If the neuron is stimulated
enough so that the cell membrane potential in that area manages
to reach as high as –50 mV (from the resting potential
of –70 mV), the voltage-gated sodium channels in that
region of the membrane open up. The voltage at which the voltage-gated
channels open is called the threshold potential, so
the threshold potential in this case is –50 mV. Since
there is a large concentration of positive sodium ions just outside
the cell membrane that have been pumped out by Na+/K+ ATPase,
when the voltage-gated channels open, these sodium ions follow the
concentration gradient and rush into the cell. With the flood of
positive ions, the cell continues to depolarize. Eventually the
membrane potential gets as high as +35 mV, at which
point the voltage-gated sodium channels close again and voltage-gated
potassium channels reach their threshold and open up. The positive
potassium ions concentrated in the cell now rush out of the neuron,
repolarizing the cell membrane to its negative resting potential.
The membrane potential continues to drop, even beyond –70 mV,
until the voltage-gated potassium channels close once again at around –90 mV.
With the voltage-gated proteins closed, the Na+/K+ ATPase
and the potassium leak channels work to restore the membrane potential
to its original polarized state of –70 mV. The whole
process takes approximately one millisecond to occur.
The action potential does not occur in one localized area
of the neuron and then stop: it travels down the length of the neuron.
When one portion of the neuron’s cell membrane undergoes an action
potential, the entering sodium atoms not only diffuse into and out
of the neuron, they also diffuse along the neuron’s length. These
sodium ions depolarize the surrounding areas of the neuron’s cell
membrane to the threshold potential, at which point the voltage-gated
sodium channels in those regions open, creating an action potential.
This cycle continues to occur along the entire length of the neuron
in a chain reaction.
During the time it takes the neuron to repolarize back
from +35 mV to –70 mV, the voltage-gated
sodium channels will not reopen. This lag prevents the action potential
from moving backward to regions of the cell membrane that have already
experienced an action potential.
Speeding Up the Action Potential
Axons of many neurons are surrounded by a structure
known as the myelin sheath, a structure that helps
to speed up the movement of action potentials along the axon. The sheath
is built of Schwann cells, which wrap themselves around the axon
of the neuron, leaving small gaps in between known as the nodes
The sodium and potassium ions that cause the action potential
are only able to cross the cell membrane at the nodes of Ranvier,
so the action potential does not have to occur along the entire
length of the axon. Instead, when the action potential is triggered
at one node, the sodium ions that enter the neuron will trigger
an action potential at the next node. This causes the action potential
to jump from node to node, greatly increasing its speed. This jumping
of the action potential is called saltatory conduction. Some diseases
such as multiple sclerosis can damage the myelin sheaths, greatly
impeding conduction of impulses along the neurons.
Strength of the Signal
There is no such thing as a stronger or weaker action
potential. If a neuron reaches the threshold to trigger an action
potential, then the entire sequence of events, from depolarization
to repolarization, will occur, and the same threshold potentials
will be reached. But it’s obvious that every signal can’t trigger
an identical response, or else neurons would never be able to convey
any useful information. For example, if the feel of lukewarm water and
the burn of a hot iron triggered the same response, our sense of
touch would be rather useless.
The body communicates a stronger message not by creating
a larger action potential, but by firing action potentials more
rapidly. The burn of an iron may cause the heat receptors in our
skin to fire action potentials at a rate of up to one hundred action
potentials per second, while lukewarm water might trigger action
potentials at less than half that rate.
Transmitting an Impulse Between Neurons
Neurons cannot directly pass an action potential from
one to the next because of the synapses between them. Instead, neurons
communicate across the synaptic clefts by the means of chemical
signals known as neurotransmitters. When an action potential reaches
the synapse, it causes the release of vesicles of these neurotransmitters,
which diffuse across the gap and bind to receptors in the dendrites
of the adjacent neuron. The neurotransmitters can be excitatory,
causing an action potential in the next neuron, or inhibitory, preventing one.
Excitatory neurotransmitters cause the target neuron to allow positive
ions to enter it, which may or may not be enough to cause the membrane
to reach the threshold potential of –50 mV that is
needed to open the -voltage-gated sodium channels and initiate an
action potential. Inhibitory neurotransmitters cause the target
neuron to allow entrance to negative ions, carrying the neuron further
from threshold and preventing it from firing an action potential.
To form the nervous system, neurons are organized in a
dense network. Each neuron shares a synapse with many other neurons,
exposing each neuron to excitatory and inhibitory neurotransmitters
simultaneously. The effects of all of the neurotransmitters working on
a neuron at a given time are added up to determine whether or not
an action potential will be fired. After a neurotransmitter has
its effect on the target neuron, it usually either diffuses away
from the synapse, is deactivated by enzymes in the synapse, or is
absorbed by surrounding cells.
Nervous System Organization
As animals became more complex, their nervous systems
evolved from the simple, unorganized networks of nerves that are
found in cnidarians, such as jellyfish, and became more complicated
and coordinated by a central control. Annelids and mollusks have
simple, organized clusters of neurons known as ganglia.
Many ganglia fuse in the head region of these organisms to form
a primitive brain. Arthropods exhibit a more complex nervous system
that includes many sensory organs such as antennae and compound
eyes. Vertebrates mark the culmination of nervous system evolution.
The vertebrate system is highly centralized, with a large brain
that can process complex information and numerous specialized sensory
The Vertebrate Nervous System
The vertebrate nervous system contains billions of individual
neurons but can be divided into two main parts: the central nervous
system (CNS) and the peripheral nervous system (PNS). The central
nervous system, as its name implies, acts as central command. It receives
sensory input from all regions of the body, integrates this information,
and creates a response. The central nervous system controls the
most basic functions essential for survival, such as breathing and
digestion, and it is responsible for complex behavior and, in humans,
consciousness. The peripheral nervous system refers to the pathways
through which the central nervous system communicates with the rest
of the organism.
In highly evolved systems, such as the human nervous system,
there are actually three types of neural building blocks: sensory,
motor, and interneurons.
After an organism’s sense organs receive a stimulus from
the environment, sensory neurons send that information back to the
central nervous system. Also called afferent neurons.
In response to some stimulus or as a voluntary action,
motor neurons carry information away from the central nervous system
to an organ or muscle. Also called efferent neurons.
Provide the connection between sensory neurons and motor
The Central Nervous System
The central nervous system consists of the brain and
the spinal cord. The spinal cord is a long cylinder
of nervous tissue that extends along the vertebral column from the
head to the lower back.
Composed of many distinct structures working together
to coordinate the body, the brain is a highly complex (and poorly
understood) organ. Luckily, you don’t have to “understand” the brain
for the SAT II Biology. You just need to know its basic structures and
their functions. The brain is made up almost entirely of interneurons.
- The cerebrum is the largest
portion of the brain and the seat of consciousness. The cerebrum
controls all voluntary movement, sensory perception, speech, memory,
and creative thought.
- The cerebellum does not initiate voluntary
movement, but it helps fine-tune it. The cerebellum makes sure that
movements are coordinated and balanced.
- The brainstem, specifically a portion of
it known as the medulla oblongata, is responsible for
the control of involuntary functions such as breathing, cardiovascular
regulation, and swallowing. The medulla oblongata is absolutely essential
for life and processes a great deal of information. The medulla
also helps maintain alertness.
- The hypothalamus is responsible for the maintenance
of homeostasis. It regulates temperature, controls hunger and thirst,
and manages water balance. It also helps generate emotion.
The spinal cord contains all three types of neurons. Axons
of motor neurons extend from the spinal column into the
peripheral nervous system, while the fibers of sensory neurons merge
into the column from the PNS. Interneurons link the motor and sensory
neurons, and they make up the majority of the neurons in the spinal
column. In addition to the neurons, cells called glial cells are
present to provide physical and metabolic support for neurons. The
spinal cord serves as a link between the body and the brain, and
it can also regulate simple reflexes.
The brain and spinal cord are bathed in a fluid called
the cerebrospinal fluid, which helps to cushion these delicate organs
against damage. The cerebrospinal fluid is maintained by the glial
The Peripheral Nervous System
The peripheral nervous system consists of a sensory
system that carries information from the senses into the
central nervous system from the body and a motor system that
branches out from the CNS to targeted organs or muscles. The motor
division can be divided into the somatic system and
the autonomic system.
The somatic nervous system is responsible for voluntary,
or conscious, movement. The neurons only target the skeletal muscles
responsible for body movement. All of the neurons in the somatic
system release acetylcholine, an excitatory neurotransmitter
that causes skeletal muscles to contract. None of the neurons in
the somatic nervous system has an inhibitory effect.
The autonomic system controls tissues other than skeletal
muscles, including smooth and cardiac muscle, glands, and organs.
The system controls processes that an animal does not have voluntary
control over, such as the heartbeat, the movements of the digestive tract,
and the contraction of the bladder. Autonomic neurons can either
excite or inhibit their target muscles or organs. The autonomic
nervous system can itself be subdivided into the sympathetic division
and parasympathetic division. These two systems act antagonistically
and often have opposite effects.
- The sympathetic division prepares
the body for emergency situations. It increases the heart rate,
dilates the pupils, increases the breathing rate, and diverts blood
from the digestive system so that it can be used to oxygenate skeletal
muscles that may be needed for action. The sympathetic division
also stimulates the medulla of the adrenal glands to release epinephrine
and norepinephrine into the bloodstream, hormones that help to reinforce
the direct effects of the neurons. Together, the actions of the
sympathetic nervous system are often called the “fight or flight”
response. The neurotransmitter most often released by sympathetic
neurons is norepinephrine.
- The parasympathetic division is most active
when the body is at rest. It slows the heart rate, increases digestion,
and slows breathing. The effects of the parasympathetic division
are sometimes called the “rest and digest” response. The neurotransmitter
most often associated with the parasympathetic division of the autonomic
nervous system is acetylcholine.
The sensory organs provide information about the environment
through the peripheral nervous system to the central nervous system.
Complex organs like the eyes and ears, as well as more simple sensory
receptors, such as those found in the skin and joints, provide raw
information about the environment by firing action potentials under
special circumstances. The modified neurons of the eye fire when
exposed to light, while those of the ear respond to vibration. This
sensory information is processed and perceived by the brain.
The eyes can determine the intensity of the
light as well as its color, or frequency. The retina of the eye
contains specialized photoreceptors called rods and cones, which
can sense the different properties of the light that hits them.
Rods are very sensitive and respond to low levels of illumination,
a property that is important for night vision. Cones respond to
brighter light and are responsible for color vision. Pigments in
the photoreceptor cells change their molecular shapes when stimulated
by light, leading to the firing of an action potential from neurons
in the eye. The impulse passes along the optic nerve to the occipital
lobe of the brain, where the visual information is processed. Light
is focused onto the retina by the lens of the eye, which can change
shape in order to maintain a focused image. The pupil is the hole
in the eye that regulates how much light can pass through to the
lens; the diameter of the pupil is adjusted by the muscular iris.
The cornea is the clear, outer layer of the eye and helps to bend
light through the pupil toward the lens.
In the ears, sound energy causes the eardrum,
or tympanic membrane, to vibrate at the same frequency as the sound.
The vibration is conducted through three small bones, the auditory
ossicles, which amplify the vibration and direct it to the cochlea.
Hair cells in the cochlea convert the vibrations of the cochlea
into action potentials. The frequency and amplitude of the vibration
affect which hair cells are stimulated and how often they fire. The
action potentials are transmitted down the auditory nerve to the
Everyone knows the ear is involved in hearing, but few
know that the ear also helps maintain balance. Three semicircular
canals in each ear contain specialized hair cells that detect
the movement of a fluid that fills the canals. When the position
of the head changes, the fluid inside the canals moves. The changing
pressure on the hair cells affects the rate at which they fire action
potentials. This information is transmitted to the brain along the
Taste and smell:
Taste and smell detect the presence of chemical substances,
either dissolved in the saliva in the case of taste or dissolved
in the mucus of the nose in the case of smell. Chemoreceptors that
respond to taste are concentrated in structures known as taste buds present
on the surface of the tongue. The taste buds respond to the four
main taste sensations—sour, salty, bitter, and sweet—creating action
potentials that travel to the brain along the facial and glossopharyngeal
nerves. Smell originates when molecules of a substance pass along
the olfactory epithelium, a region near the top of
the nasal cavity. The molecules dissolve in the mucus that
coats the olfactory epithelium and bind to surface receptors. It is
believed that there are approximately one thousand different receptor
types in the nose, each responding to a different chemical signal.
When these receptors are activated, they transmit their signals
to the brain through the olfactory nerve.
In addition to the special senses discussed above, there
are many sensory nerve endings throughout the body: in the skin,
on the body wall, in the muscles, tendons, and joints, in the bones,
and in certain organs. These senses are often called the somatic senses,
and they include senses of touch, pressure, the senses of posture
and movement, temperature, and pain. The specifics of these senses
are not tested on the SAT II Biology, but it is important to know
that the senses arise when receptor cells are stimulated to produce
action potentials, which are interpreted in the brain.
The Endocrine System
The endocrine system works in concert with the nervous
system to control and coordinate the functions of the other organ
systems. The endocrine system, however, functions on a slower time
scale than the nervous system does. The organs that make up the
endocrine system are called the endocrine glands, and they communicate
with the body by releasing chemical messengers known as hormones into
The hormones released by the endocrine glands usually
target specific organs in an entirely different part of the body.
The cells of the target organ for a specific hormone will have receptors
to which only that hormone can bind. Organs without those particular receptors
will remain unaffected. A hormone can affect targeted cells for
a matter of minutes, such as the regulation of blood sugar, or over
several days, months, or even years, as happens in puberty.
Two major classes of hormones exist: peptide hormones
and steroid hormones. Peptide hormones are composed of amino acids
and can range in size from a short chain of only three or four amino
acids to small polypeptides. Examples of peptide hormones include insulin
and antidiuretic hormone (ADH). Because amino acids cannot freely
cross cell membranes, peptide hormones must be secreted through
special vesicles and also must convey their information by binding
to receptors that exist on the outside of a targeted cell. By binding
with the receptor, the hormone generates a chain reaction of signals
into the cell that eventually causes changes in specific enzymes
within the cell itself. Peptide hormones generally work rather quickly,
on the order of minutes and hours rather than days and months.
Steroid hormones are ring-shaped lipids made from cholesterol.
Because they are hydrophobic, steroid hormones can easily pass into
the bloodstream from the endocrine cells that produce them; they
can also pass directly into their target cells. The receptors for steroid
hormones are located on the interior of the target cells. When hormone
and receptor bind, they enter the nucleus of the cell and can activate
or deactivate genes coding for specific proteins. Since
steroid hormones exert their influence by changing the rates of
protein synthesis, steroid hormones act more slowly than peptide
hormones do. Examples of steroid hormones are testosterone, estrogen,
The endocrine system contains a great variety of glands,
all of which produce different hormones and regulate different processes
or areas of the body. The SAT II Biology does occasionally ask questions
about the major endocrine glands.
The Pituitary gland:
The pituitary gland is a tiny gland located
at the base of the brain in the center of the head. It is made up
of two separate lobes, the anterior pituitary and the posterior
pituitary, each of which is responsible for the secretion of a different
set of hormones.
The pituitary is a very important part of the endocrine
system because the hormones it produces control the secretions of
many of the other endocrine organs. The pituitary itself is controlled
by the hypothalamus. For each of the six hormones produced by the
anterior pituitary gland, the hypothalamus produces a specific hormone-like
substance known as a releasing factor that stimulates the anterior
pituitary to release that particular hormone. The two hormones released
by the posterior pituitary are produced directly in the hypothalamus and
merely stored in the pituitary gland until they are secreted into
the blood. The six hormones released by the anterior pituitary are:
- Growth hormone (GH) stimulates
growth in many body tissues. The hormone is particularly important
for growing children. In adults, it affects the rate at which older
cells are replaced by new ones.
- Follicle-stimulating hormone (FSH) stimulates
the maturation of ova in the ovaries and sperm in the testes and
can cause the gonads to release sex hormones.
- Luteinizing hormone (LH) triggers ovulation
and the development of a structure known as the corpus luteum in
females. In males it stimulates the release of testosterone by the
- Prolactin is released after pregnancy and
stimulates milk production in the female mammary glands.
- Thyroid-stimulating hormone (TSH) stimulates
the thyroid, another endocrine gland, to release its hormone, thyroxine.
- Adrenocorticotrophic hormone (ACTH) stimulates
the adrenal cortex to release its hormones, the corticoids.
The two hormones made in the hypothalamus and released
by the storage facility in the posterior pituitary are:
- Antidiuretic hormone (ADH) regulates
the kidneys to reduce water loss in the urine.
- Oxytocin stimulates uterine contraction during
The Thyroid gland:
Located in the back of the neck, the thyroid gland produces
the hormone thyroxine, which increases the metabolism of most of
the cells in the body. Iodine is needed to produce thyroxine, so
iodine deficiencies can greatly affect the functioning of the -thyroid
gland. If the thyroid gland produces too little thyroxine, a condition
known as hypothyroidism develops. A person who suffers from hypothyroidism
has a lower metabolic rate, which can cause obesity and sluggishness.
The opposite condition, known as hyperthyroidism, occurs when the
thyroid produces too much thyroxine. It can lead to excessive perspiration,
high body temperature, loss of weight, and a faster heart rate.
Four small but important glands known as the parathyroid
glands are embedded on the posterior surface of the thyroid gland.
The parathyroid glands produce a hormone appropriately named parathyroid
hormone, or parathormone, which regulates the level of calcium in
the bloodstream. When parathyroid hormone is released, it stimulates
the bones to secrete extra calcium into the bloodstream, raising
the levels of calcium ions in the blood plasma and decreasing them
in the bone tissue. Calcium is important for many reasons, including
the functioning of muscles and neurons and the blood-clotting process.
The pancreas is a large organ located behind the stomach.
It serves two major functions. First, it is a digestive organ, releasing
digestive enzymes into the small intestine by means of the pancreatic
duct. But it also functions as an endocrine organ, releasing the hormones insulin and glucagon directly
into the bloodstream from specialized cells known as the islets
Insulin stimulates cells to absorb glucose from the bloodstream
when glucose levels are high, such as after a meal. The hormone
also stimulates the liver to remove glucose from the blood and store
it as glycogen, decreasing blood sugar levels. Glucagon has the
opposite effect. Released when blood glucose levels are low, glucagon
stimulates the liver to break down glycogen into glucose and to
release it into the bloodstream, raising blood sugar levels.
The adrenal glands are located on the kidneys.
They consist of two distinct parts: the adrenal cortex,
the external portion of the gland, and the adrenal medulla,
the interior portion.
The sympathetic nervous system stimulates the adrenal
medulla to release its hormones, norepinephrine and epinephrine,
into the bloodstream. Like the sympathetic nervous system, these
two hormones ready the body for stress: they increase heart rate
and breathing rate, they divert blood from the digestive system
to the skeletal muscles, and they dilate the pupils. (The
similarities between the effects of the sympathetic nervous system
and adrenal medulla hormones are easy to understand, considering
norepinephrine is the neurotransmitter that is released
by the sympathetic neurons.) The only difference between the effects
of the adrenal medulla and the sympathetic nervous system is that
hormones released by the adrenal medulla remain in the bloodstream
for a long time, usually several minutes and sometimes more, while
the effects of the sympathetic nervous system are short-lived.
The adrenal cortex releases three types of steroid hormones.
The glucocorticoids affect glucose levels in the blood. Mineralocorticoids
affect the rate at which the kidneys absorb certain minerals from
the blood. Sex steroids have some effect on sexual characteristics
and processes but are generally overshadowed by the hormones produced
by the gonads.
The gonads—the testes in the male and the ovaries in the
female—are the sex organs that produce gametes. In addition, the
gonads produce steroid sex hormones. In males the primary sex hormone
is testosterone, which is necessary for sperm production. In
addition to facilitating the production of sperm, testosterone is
responsible for developing and maintaining the secondary sex characteristics
of males, starting at puberty. These characteristics include a deeper
voice, facial and body hair, and broad shoulders. In females, the
ovaries produce estrogen and progesterone.
Estrogen helps to develop and maintain the female secondary sex
characteristics, such as the development of mammary glands, a narrower
waist and wider hips, axillary and pubic hair, and a higher-pitched voice.
Estrogen also stimulates growth of the uterine lining for pregnancy,
while progesterone prepares the uterus for embryo implantation and
helps to maintain pregnancy.
The Circulatory System
In the simplest multicellular animals such as the cnidarians,
almost all cells are in contact with the external environment, so
there is little need to transport materials internally. Any cell
can get its nutrients from the surrounding water and can expel its
waste directly back from where it came. As animal body plans evolved
to further complexity, however, a need developed for a circulatory
system that could transport materials such as nutrients, oxygen,
and waste products throughout the body. Annelids have a simple closed
circuit of blood vessels with five small hearts, which are really
just pulsating vessels themselves. Insects and other arthropods
have an open circulatory system that bathes their internal organs.
The open circulatory system consists of one dorsal vessel that pulsates,
keeping the blood moving throughout the body of the insect.
Vertebrate Circulatory Systems
Vertebrates have evolved an intricate closed circulatory
system that consist of a heart and three principal types of blood
vessels: arteries, capillaries, and veins.
Arteries carry blood away from the heart and have thick,
elastic, muscular walls that can dilate or contract to control blood
pressure within the vessels. Because blood in the arteries has been
relatively recently pumped out of the heart, arterial blood pressure
tends to be high. The blood in arteries is usually rich in oxygen,
since it is being pumped out to the body to provide oxygen and other
nutrients to the cells. The only exceptions are the pulmonary arteries,
which carry blood to the lungs to pick up its supply of oxygen.
Since blood in the pulmonary arteries hasn’t yet reached the lungs,
it is oxygen poor.
Arteries are too large to service every little cell in
the body. As arteries get farther from the heart, they begin to
branch into smaller and smaller vessels, which eventually branch into
thousands of capillaries. The walls of the smallest capillaries
are only one cell thick, allowing nutrients, waste products, oxygen,
and carbon dioxide to diffuse between the blood and the surrounding
tissues. After providing nutrients and oxygen and picking up waste,
capillaries begin to merge into larger and larger vessels, eventually
converging into veins.
Veins carry blood toward the heart. The blood in veins
is not pushed by pumping of the heart, so the blood pressure and
forward momentum of the blood in veins is lower than in arteries.
Blood in veins is largely pushed along by the contractions of the
skeletal muscles as the organism moves around. To ensure that the
blood in veins flows toward the heart, veins contain unidirectional
valves. Venous blood has already provided nutrients to cells, so
it is usually deoxygenated, giving it a characteristic blue color.
The lone exception, once again, is the pulmonary veins. Since this
blood is flowing back to the heart from the lungs, it is fully oxygenated
and bright red.
Patterns of Circulation in Vertebrates
As vertebrates have evolved, they have developed increasingly
efficient circulatory systems. The circulatory system in fish is
one closed loop: blood is pumped from the heart to the gill capillaries,
where oxygen is picked up from the surrounding water. The blood
then continues on to the body tissues, and the vessels eventually
become capillaries again to allow for nutrient and gas exchange
in the tissues. Then the deoxygenated blood is returned to the heart
and pumped to the gills once more. This system is inefficient because the
blood loses a lot of momentum in the gill capillaries. After leaving
the gill capillaries, it travels slowly and with a lower pressure,
affecting the delivery of oxygen to the body tissues.
Amphibians, reptiles, birds, and mammals have overcome
this problem by evolving two circuits within the circulatory system:
the pulmonary circuit and the systemic circuit. After
the blood is pumped from the heart to the lungs to be oxygenated,
it is returned to the heart before it is pumped out to the rest
of the body.
Amphibian and reptile hearts are inefficient because they
make no distinction between oxygenated and deoxygenated blood. Their
hearts have only two chambers: one chamber for receiving blood from
the lungs and the body, and another for pumping that blood back out.
These two-chambered hearts allow oxygen-rich blood returning from
the lungs to mix with oxygen-poor blood returning from the systemic
circuit. The blood pumped to the body never contains as much oxygen
as it could.
The avian (bird) and mammalian heart is four-chambered.
It consists of two halves, one for oxygenated blood and the other
for deoxygenated blood. Each half has one atrium and one ventricle,
separated by one-way atrioventricular valves. The atrium is the
chamber where blood returns to the heart, while the ventricle is
the chamber where blood is pumped out of the heart. Oxygen-poor
blood returning from the body enters the right atrium and then moves
into the right ventricle, which pumps the blood through the pulmonary
artery to the lungs, where it picks up oxygen and releases carbon
dioxide. This newly oxygenated blood returns to the left atrium
of the heart through the pulmonary veins. Blood in the left atrium
moves into the left ventricle, from where it is pumped out through
the aorta, the largest artery, into other arteries,
arterioles, and capillaries. The blood provides oxygen to the cells,
picks up carbon dioxide, and gathers back into veins. Eventually
the deoxygenated blood flows through the superior vena cava and
inferior vena cava back into the right atrium, starting the process
The vertebrate heart is composed of special muscle tissue
called cardiac muscle. These muscles are stimulated to contract
in a regular and controlled rhythm by an electric pulse generated
in a region of the heart called the sinoatrial node, or pacemaker.
The pacemaker cells fire impulses spontaneously, without any stimulation
from the nervous system. This impulse spreads among the heart cells,
stimulating the atria to contract, forcing blood into the ventricles.
At the junction of the atria and the ventricles, the impulse reaches
another node, called the atrioventricular node. The atrioventricular
node sends an impulse that causes the ventricular walls to contract,
forcing blood out of the heart and into the aorta and pulmonary
arteries. Although the heartbeat can be maintained without external
stimulation by the nervous system, the autonomic nervous system
can regulate the heart rate by speeding it up or slowing it down.
The entire purpose of the circulatory system
is to move oxygen and nutrient-rich blood to where it needs to go.
Blood is a liquid tissue that is composed of a fluid called plasma and
three types of specialized cells: red blood cells, white
blood cells, and platelets.
The plasma of the blood is composed mainly of
water, allowing it to contain many dissolved substances, such as
the glucose that provides cells with energy; carbon dioxide in the
form of carbonic acid; hormones that carry important chemical signals
to their target organs; salts such as calcium, potassium, and sodium;
lipids; and nitrogenous waste. The plasma also contains proteins
that assist in blood clotting, in the immune response, and in preventing
the loss of too much blood fluid from the capillaries.
Red blood cells are biconcave disks with no nucleus and
no major organelles (if you took a ball of putty and squashed it
between two fingers, it would look like a red blood cell). Red blood
cells are the most abundant cell type in the blood. Their primary
function is to transport oxygen through the blood. Red blood cells
are filled with hemoglobin, an iron-containing protein
that can bind to oxygen molecules. When the concentration of oxygen
is high, as it is in the lungs, one molecule of hemoglobin can bind
up to four molecules of oxygen. When the concentration of oxygen
is very low, as it is in the capillaries of oxygen-poor tissue,
the hemoglobin gives up its oxygen, releasing it into the tissues
where it is needed.
White blood cells are important in fighting off infectious
disease. There are two general classes of white blood cells: phagocytes
and lymphocytes. These cells will be explained more fully during
the discussion of the immune system later in this chapter.
The third type of blood cell is the platelet.
Platelets are not really cells at all; they are packets of cytoplasm
that release the enzyme thromboplastin when they come into contact
with a foreign substance within the blood or the rough edges of
an open wound. Thromboplastin sets off a chain reaction that converts
fibrinogen, a soluble protein found in the blood plasma, into fibrin,
a tough, insoluble fibrous protein that traps red blood cells and
thereby forms blood clots that stop blood loss from an open wound.
Red blood cells manufacture proteins called antigens that
coat the cell surface. These proteins help the immune system to
determine if a cell is a foreign invader or part of the body’s normal
tissues. In the case of human red blood cells, there are two major
types of antigens that can be formed: antigen A and antigen B. According
to genotype, an individual might have one or both of these antigens
expressed, or she may have neither. If a person’s red blood cells
contain only antigen A, she is said to have type A blood. If only
antigen B is present, the blood is type B. Type AB blood contains
both antigens, and type O blood contains neither antigen A nor B.
In order to combat foreign cells, the blood plasma contains
antibodies for all antigens that are not expressed
on its own red blood cells. These antibodies would cause any foreign blood
cells to clump together, forming a dangerous clot. A person with
type A blood has anti-B antibodies in the plasma, a person with
type B blood has anti-A antibodies in the plasma, a person with
type AB blood has no antibodies in the plasma, and a person with type
O blood has both anti-A and anti-B antibodies. A person with type
AB blood can therefore receive a blood transfusion of any type because
his or her blood contains no antibodies that would clump up the
foreign cells. For this reason, AB blood is often called the universal
recipient. In contrast, a person with type O blood can receive only type
O blood in a transfusion because she has both anti-A and anti-B
antibodies in the plasma, which would immediately clump any blood
that contained antigens A or B. But since type O blood has no antigens,
it could be given to a person of any type blood in a transfusion
without clumping. Type O blood is called the universal donor.
Blood type is a codominant trait; we explained the inheritance
patterns of blood type in the codominance section of the chapter
The Lymphatic System
Because the capillaries are so small, the pressure inside
them is often high enough to force some of the plasma out of the
blood and the capillary and into the surrounding tissue. If the fluid
remained in the tissue, it would cause swelling. The lymphatic system
is responsible for returning this fluid to the circulatory system.
The fluid, known as lymph, collects in small lymph
capillaries, which contain valves similar to veins. These lymph
capillaries converge into larger lymph vessels, and they eventually
drain into the subclavian vein.
The lymph is a popular route for invading microorganisms
that are trying to enter the bloodstream, so it must be well defended. Lymph
nodes contain white blood cells that can destroy bacteria
or viruses that are present in the lymph. Additional organs such
as the spleen and tonsils are considered part of the lymphatic system
because they aid in filtering the blood to remove foreign invaders.
The Immune System
The immune system is responsible for keeping foreign invaders
out of the body and destroying those entities that do manage to
invade the tissues. Immune system defenses can be either passive
or active. Passive defenses are physical barriers that prevent microorganisms
from entering the body. Skin is the most obvious example. The sticky
mucus lining the respiratory tract and stomach acid, which kills
many microorganisms that might otherwise enter through the digestive
system, are other examples of passive defenses.
The active defenses of the immune system are primarily
made up of white blood cells. There are two classes of white blood
cells: phagocytes and lymphocytes. Phagocytes
resemble amoebas and can crawl through the body’s tissues ingesting
any foreign invaders they come upon. Lymphocytes are more specific
in the invaders they target. There are three general types of lymphocytes.
B cells identify pathogens by producing antibodies that
recognize the protein coats of specific viruses or bacteria. Helper
T cells coordinate the immune response by activating other immune
system cells. Killer T cells kill infected cells.
The Respiratory System
All aerobic organisms need a way to exchange
gases with their surrounding environment. Oxygen must
be brought to the cells in order for aerobic respiration to take
place, and the carbon dioxide created as a by-product of respiration
must be removed. The acquisition of oxygen and simultaneous elimination
of carbon dioxide is called gas exchange. As animals have evolved,
they have developed increasingly efficient methods of gas exchange.
In the simplest multicellular animals, the cnidarians,
gas exchange occurs by simple diffusion. Since almost all of a jellyfish
or hydra’s cells are in contact with its water environment, each
cell has direct access to outside water as both a source of oxygen
and dumping ground for carbon dioxide. Annelids also exchange gases
by diffusion. In an earthworm, for example, the circulatory system
comes very close to the surface skin, allowing oxygen and carbon
dioxide to diffuse across the worm’s skin. To make gas diffusion
possible, the worm’s skin must remain moist at all times. Insects
and other arthropods have a system of tracheae for gas exchange.
Tracheae are hollow, branched tubes that penetrate the arthropod’s
deep tissues. Air flows into the tracheae and oxygen and carbon
dioxide diffuse into and out of the body tissues through the trachea
walls. The insect does not actively draw air into the tracheae;
respiration is a passive process in arthropods.
Vertebrate Respiratory Systems
Vertebrates such as fish, birds, and mammals have evolved
specialized structures for gas exchange. Fish gills are made of
a delicate tissue with many fine filaments that maximize surface
area. The fish pumps water across the gills, and oxygen and carbon
dioxide are exchanged across the filament walls. Fish gills are
made especially efficient because blood flows through the gills
against the current of the water. In this way, the water is always more
oxygen rich than the blood in the gills, and the concentration gradient
always moves from the water to the blood.
Terrestrial vertebrates have evolved internal structures
for gas exchange known as lungs. Lungs are basically inverted gills.
Lungs are internal because a gas exchange surface would quickly
dry up if exposed to air, a problem that fish need not deal with.
The amphibian lung is often shaped like one large sac. In higher
vertebrates, such as mammals, the lungs divide into millions of
tiny sacs known as alveoli, which greatly increases
surface area and oxygen absorptive power. After air is sucked into
the lungs, gas exchange takes place across the surfaces of the alveoli,
which are dense with capillaries. After the blood in the capillaries
has given off its carbon dioxide and taken in oxygen, air is once
again released from the lungs. Birds have evolved an even more efficient
breathing system that uses air sacs to maintain a constant, countercurrent,
unidirectional flow of air across the lung surfaces. Bird lungs
do not contain dead-end sacs like the alveoli of mammalian lungs,
but rather contain millions of tiny tubes known as parabronchi,
through which air is constantly flowing in one direction.
Respiration in Humans
The human respiratory system has two parts: the upper
portion channels air to the lower portion, the lungs, where the
respiration takes place.
Air enters the respiratory system either through the nose
or mouth. The nose contains many tiny hairs and sticky mucus that
traps airborne particles and prevents them from entering the lungs.
Air is also moistened and warmed in the nasal and oral passages.
From the nose and mouth, air flows down the pharynx,
through the larynx, and into the trachea. The
larynx is a structure made of cartilage that contains the vocal
cords. When air passes out of the larynx, the vocal cords can be
tensed and made to vibrate, producing sound, which, when shaped
by the mouth, produces speech. The trachea is a cartilaginous tube that
branches into two bronchi, which in turn branch into
smaller and smaller bronchioles within the lung.
Eventually the air reaches the lungs and the clusters
of alveoli. The blood is low in oxygen and the inhaled air is rich
with it, while the blood contains a higher concentration of carbon
dioxide than air does. These two gases passively diffuse across
the thin surface of the alveoli, following the concentration gradients.
After gas exchange takes place, the oxygen-poor air is expelled
from the lungs. Most of the surfaces of the respiratory system, including
the surfaces of the bronchioles, bronchi, trachea, and pharynx,
are coated with epithelial cells that are capable of producing mucus.
This mucus traps particles of dust, bacteria, and viruses that may
be entering the respiratory system; cilia on these cells help to sweep
this mucus up away from the lungs and eventually out of the body.
The lungs suck in air by using negative pressure. The diaphragm is
a large, flat muscle at the base of the thoracic (chest) cavity.
When it contracts during inhalation, it moves downward, expanding
the volume of the thorax and lungs. Air rushes into the lungs to balance the
drop in pressure caused by this expansion. To exhale, the diaphragm
relaxes to its original position, increasing air pressure and forcing
the air back out of the chest cavity. Breathing is only possible
if the thoracic cavity remains airtight. When an accident causes
any sort of puncture in the chest cavity, one or both of the lungs
Blood pH Regulation
In addition to its obvious function of gas exchange, the
respiratory system also helps maintain the pH of the blood at a
constant level of about 7.4. Because carbon dioxide is transported
through the blood plasma as carbonic acid, the rate of carbon dioxide
exhalation can affect the pH level of the blood. Breathing faster
will increase blood pH by getting rid of more carbon dioxide and
carbonic acid. Breathing slower will have the opposite effect. A
small receptor in the carotid artery measures blood pH and transmits
this information to the medulla oblongata of the brain. The medulla
then adjusts the breathing rate in order to correct for any fluctuations
in blood pH. When we feel out of breath, it is not because our body
is sensing that we need more oxygen; it is actually telling us that
we need to get rid of more carbon dioxide.
The Digestive System
Unlike plants, animals cannot synthesize the majority
of their own organic building blocks, such as fatty acids, sugars,
and most amino acids. Instead, animals must ingest other organisms
and digest them into the essential molecules that they need. Animals
get their energy from sugars, fats, and proteins, and they use them
to construct more complex molecules such as enzymes. The digestive
system has evolved to process the food that animals ingest by breaking
it down, or digesting it, into simple building blocks that can be used
Digestion consists of two main processes: mechanical digestion
and chemical digestion. Mechanical digestion refers to the physical
breaking down of food into smaller particles without changing the
food’s chemical nature. Chewing food is an example of mechanical digestion,
as is the churning of food that takes place in the stomach. Chemical
digestion, which occurs through the action of special digestive
enzymes, breaks the chemical bonds in food and hydrolyzes larger
molecules into simpler components.
Simple Digestive Systems
In the simplest of animals and in animal-like protists,
much of the digestion process takes place within each individual
cell. An amoeba engulfs its food by phagocytosis, and a lysosome
fuses with the food vacuole and chemically digests its contents.
Paramecia have a ciliated oral groove that facilitates the creation
of the food vacuole. Cnidarians digest some of their food extracellularly
by releasing enzymes into their water-filled gastrovascular cavity,
but a large portion of their food is digested intracellularly as
well. Flatworms, such as planarians, take food in through their
mouth and into the gastrovascular cavity. The food is digested intracellularly
by the cells that line the cavity and is absorbed into the tissues. Waste
products are expelled back out of the mouth, which also serves as
an anus in this case.
Most higher animals, such as annelids, arthropods, and
vertebrates, possess a complete digestive tract, with a mouth that
is separate from the anus. Food is moved in one direction through
a tubular system that contains many specialized parts that perform
different functions. In the earthworm, for example, food passes
through the mouth, down a tube called the esophagus, and into a
chamber known as the crop, which acts as a storage chamber.
Next it enters the gizzard, which has thick, muscular walls that
mechanically grind the food. The pulverized food passes into the
intestine, where enzymes chemically break it down into simpler molecules.
These molecules are absorbed into the circulatory system. In the
last portion of the intestine, some water is absorbed from the food,
and the indigestible portions of the food are expelled through the
The Human Digestive System
The human digestive system
is somewhat similar to the earthworm’s in basic design, though it is
more complicated and efficient. The human digestive system is composed
of the alimentary canal, which is the actual tube through
which the food travels, and the glands that aid in digestion
by releasing enzymes and other secretions into the alimentary canal.
The alimentary canal begins with the mouth, where teeth
and the tongue pulverize food through mechanical digestion into
what is called a bolus. The tongue also tastes the food, which helps
to determine if it is fit to be ingested. Six salivary glands release
saliva into the mouth cavity through ducts that open under the tongue
and on the roof of the mouth. Saliva is composed mainly of water,
but it includes mucus and an enzyme called salivary amylase. The
water and mucus in the saliva help to dissolve and lubricate the
food in preparation for swallowing. Salivary amylase starts the
process of chemical digestion of starches by breaking down complex
polysaccharides into the disaccharide maltose. When the food is
sufficiently chewed, it is swallowed. The food moves through the
pharynx, or throat, to the esophagus.
The esophagus is a long tube that connects the mouth and
the stomach. Food in the esophagus is propelled downward by waves
of muscular contraction known as peristalsis. Between
the stomach and the esophagus is a tight ring of muscle known as
the cardiac sphincter. This sphincter, which is normally closed,
acts as a valve to prevent stomach contents from moving upward into
the esophagus. During peristalsis the sphincter opens to allow the
food to pass into the stomach.
The stomach has thick, muscular walls that contract to
churn and mix the food, continuing the process of mechanical digestion.
In addition, the walls of the stomach secrete hydrochloric acid
and the enzyme pepsin. The hydrochloric acid gives the stomach a
pH of less than 2, and this extremely acidic environment serves
to kill many microorganisms that might be ingested along with the
food. Pepsin is produced by the stomach in an inactive form known
as pepsinogen. Pepsinogen is only activated into pepsin in a very
low pH environment, so when it comes into contact with the hydrochloric
acid, it becomes pepsin. Pepsin begins the digestion of protein
by cleaving long chains of amino acids into shorter chains. In addition
to its roles in mechanical and chemical digestion, the stomach temporarily
The walls of the stomach are protected from the hydrochloric
acid by a thick layer of mucus. If this mucosal lining wears away,
an ulcer can develop.
The Small Intestine
The small intestine is the major site of food breakdown,
chemical digestion, and cellular absorption of food. Chemical digestion
is carried out by secretions from the liver and pancreas.
When the stomach empties, the partially digested food,
now called chyme, passes through the pyloric sphincter into the duodenum,
the upper portion of the small intestine. At this point the chyme
encounters bile. Bile is a complex solution of salts, pigments,
and cholesterol that is produced in the liver and stored and concentrated
in a small sac called the gallbladder before entering
the duodenum. Bile does not actually change the chemical nature
of the chyme; instead it emulsifies—breaks down—fats. Because fats
and oils are not soluble in water, the fat content in chyme tends
to separate and collect into large globules. Bile breaks these large
fat globules into tiny droplets. The surface area of many droplets
of fat is much greater than the surface area of a few large globules,
and so by increasing the surface area of these fat droplets, bile
exposes more fat to the enzymes that will eventually digest it.
The pancreas is a large gland that sits behind the stomach.
As mentioned in the section on the endocrine system, the pancreas
plays an important role in regulating blood sugar levels by producing
the hormones insulin and glucagon. But it plays just as vital a
role in the digestive system. The pancreas produces a basic secretion
that helps to neutralize the stomach acid. It also produces many
digestive enzymes. Lipase digests fats into glycerol and fatty acids,
while trypsin and chymotrypsin continue the breakdown of amino acid chains
into shorter ones. Both trypsin and chymotrypsin are produced in
inactive forms in the pancreas and are not activated until they
reach the small intestine; if this were not the case, the pancreas
would digest itself! The pancreas also secretes pancreatic amylase, which,
like salivary amylase, breaks down polysaccharides into disaccharides,
but on a much larger scale.
The walls of the small intestine secrete the
remaining few enzymes necessary for digestion. Maltase, lactase,
and sucrase break down the disaccharides maltose, lactose, and sucrose
into monosaccharides. Aminopeptidases cleave off individual amino
acids from the short chains that are left after the action of trypsin
and chymotrypsin from the pancreas. At this point, digestion is
completed. As the digested food travels through the long, convoluted
small intestine, it is absorbed through its walls into the bloodstream.
The walls of the small intestine contain millions of tiny, fingerlike
projections known as villi that increase the surface
area of the intestinal wall, maximizing absorption of nutrients.
The villi contain capillaries into which the digested amino acids
and monosaccharides pass. Fats are processed in the cells of the
intestinal lining and enter the lymphatic system before reaching
the bloodstream. The blood leaving the intestines flows directly
to the liver, where it enters the capillaries of the hepatic portal
system for processing.
The Large Intestine and Rectum
The undigested food that is not absorbed in the small
intestine is waste. It eventually passes into the large intestine,
or colon, where its water content is reabsorbed into the body. A mutually
symbiotic bacteria named E. coli lives in the large
intestine, feeding on waste and producing vitamin K, which is absorbed
by the intestine into the body. The final segment of the large intestine
is the rectum, a sac that stores feces temporarily before they are
eliminated through the anus, another sphincter muscle.
Minerals and Vitamins
In addition to the nutrients that form the building blocks
of proteins, fats, and carbohydrates, the body also absorbs important
minerals and vitamins during digestion. Minerals are inorganic molecules
that are required by the body. Important minerals are iron, a necessary
component of hemoglobin; iodine, which is essential for making thyroid
hormone; and calcium, which is required by the bones and for many
cellular processes. Sodium, chlorine, and potassium are important
components of body fluids, and phosphorus is an important ingredient
of nucleic acids.
Vitamins are more complex molecules that usually
serve as coenzymes, assisting in physiological processes. Vitamin
A is necessary to make retinal, an important chemical for vision. Vitamin
B complex contains many molecules essential for cellular respiration
and DNA replication. Vitamin C is important for making collagen,
a tough material that is found in the body’s connective tissue.
Vitamin D allows the body to absorb calcium, essential
for the teeth and bones. Vitamin E helps prevent the rupture of
red blood cells, and it also helps maintain healthy liver and nerve
function. Vitamin K is important in the blood-clotting
process. Vitamins A, D, E, and K are the fat-soluble vitamins, while
the vitamins of the B complex and vitamin C are water-soluble vitamins.
The Excretory System
While carrying out the physiological processes
that are necessary for life, animal cells produce waste that must
be eliminated. Carbon dioxide and water, two of the main waste products,
are removed from the body by the respiratory system. The third type
of waste produced by metabolic processes is the nitrogenous wastes
urea and uric acid, which are created when amino and nucleic acids
are broken down. Animals have developed a variety of systems to
excrete nitrogenous waste. In many animals these excretory systems
also play important roles in regulating water and salt balance.
Excretion in Invertebrates
As in respiration, cnidarians rely on simple
diffusion to solve the problem of nitrogenous waste. Since most
cells of a cnidarian are in contact with the external environment,
nitrogenous wastes can diffuse across the cell membranes and into
the surrounding water. Annelids have a more complex system for excretion.
Two small tubes called nephridia exist in each of the
annelid’s body segments. These tubes are surrounded by capillaries.
Nitrogenous waste in the form of urea is passed from the blood into
the nephridia. The waste collected in the nephridia eventually exits
the worm through pores in the skin. Arthropods have their own specialized
means of excreting waste: a system of structures known as the Malpighian
tubules that are bathed in the fluid of the arthropod’s open
circulatory system. Nitrogenous waste in the form of uric acid collects
in the tubules. From there the waste empties into the digestive
tract, which reabsorbs all of the water that was lost in the excretory
process. Without water, urea converts to solid crystals of uric
acid, which are excreted along with the solid waste produced by
Excretion in Humans
Vertebrates have evolved a different answer to the problem
of water balance and nitrogenous waste excretion: the kidneys.
The two kidneys filter blood, removing urea in the form of urine,
while also regulating the levels of water and salt present in the
blood plasma. From each kidney, urine travels through a large duct
called the ureter and empties into the urinary
bladder. The bladder is a muscular organ that expands to
store urine. When the bladder contracts, urine is pushed through
another duct called the urethra and out of the body.
The basic functional unit of the kidney is the nephron,
a tiny tubule whose special structure makes it ideal for its blood-filtering
task. Each nephron consists of a cluster of capillaries called the
glomerulus, which is surrounded by a hollow bulb known as Bowman’s
capsule. The Bowman’s capsule leads into a long, convoluted tubule
that has four sections: the proximal tubule, the loop of Henle,
the distal tubule, and the collecting duct. The collecting ducts
empty into the central cavity of the kidney, the renal pelvis, which
connects to the ureter, which carries urine to the bladder. A kidney
is made up of millions of nephrons.
How a Nephron Filters Blood
Blood enters the kidney through renal arteries, which
quickly split into smaller vessels and then branch further into
the very narrow clusters of capillaries that make up the glomerulus
of the nephron. Because the glomerulus’s capillaries are so narrow,
blood pressure is high. The high pressure squeezes the liquid portion
of blood through a sieve structure and into the Bowman’s capsule,
leaving the blood cells, platelets, and large protein molecules behind.
This process is called filtration, and the liquid blood that is
pushed through the sieve structure is called filtrate. The filtrate
contains large amounts of water, glucose, salts, and amino acids
in addition to the urea that is to be excreted.
From Bowman’s capsule the filtrate enters the proximal
tubule of the nephron. In the proximal tubule, important molecules
for life, such as sodium, water, amino acids, and glucose, are pumped
out of the proximal tubule to be reabsorbed by the blood, as they
are too valuable to be excreted. The return of these molecules to
the blood is called reabsorption. After reabsorption, the filtrate
is called urine.
By the time urine enters the next portion of the nephron,
the loop of Henle, it has already lost approximately 75 percent
of its initial water content and volume. The loop of Henle descends
from the outer region of the kidney, the cortex, into the medulla.
The walls of the descending loop are permeable to water but not
to salt. In addition, the medulla of the kidney contains a high
salt concentration, creating a concentration gradient: water is drawn
out of the descending loop and into the medulla, leaving the salts
behind. By the time the urine reaches the ascending part of the
loop of Henle, only 6 percent of the original water content remains.
The ascending loop of Henle is impermeable to water, but it is permeable
to salt. Because the urine lost so much water content in
the descending loop, the salt content at this point is very high.
Salt now diffuses from the ascending loop into the medulla of the
kidney (helping to maintain the high salt content of medulla). When
it is finished traveling through the ascending loop of Henle, only
about 4 percent of the original salt content of the filtrate remains.
With much of the water gone as well, the urine consists mainly of
urea and other waste products at this point.
The urine then enters the distal tubule, which operates
very similarly to the proximal tubule—salt is pumped out of the
urine, and water follows osmotically. By the end of the distal tubule,
only 3 percent of the original water content remains in the urine, and
the salt content is negligible. In the distal tubules, a third process,
in addition to filtration and reabsorption, takes place: secretion.
While the salts and water are leaving the tubules, some substances, such
as hydrogen and potassium ions, are actively transported from the
blood into the urine of the tubule so that they can be excreted
from the body.
From the distal tubule, the urine enters the collecting
duct. Like the loop of Henle, the collecting duct extends deep into
the medulla portion of the kidney. Because the medulla has a high
salt content, as much as three-fourths of the remaining water can be
reabsorbed as the urine travels through the collecting duct. The
actual amount of water that is reabsorbed is dependent on the permeability
of the walls of the duct, which is regulated by the antidiuretic
hormone (ADH) secreted by the posterior pituitary gland. ADH acts
on the walls of the collecting ducts to make them more permeable
to water, but if ADH levels are low, less water will be reabsorbed.
If a person is dehydrated and needs to conserve water, his or her
levels of ADH will rise. In contrast, a person with sufficient levels
of water in the blood will have low ADH levels, resulting in less
reabsorbed water and more dilute urine. In addition to being permeable
to water, the lower portions of the collecting duct are permeable
to urea, allowing some of it to enter the medulla of the kidney.
This release of urea allows the medulla to maintain its high ion
concentration, an important factor in the functioning of the nephron.
Another hormone that has an effect on the nephron in addition
to ADH is aldosterone, which is produced in the adrenal cortex.
Aldosterone increases the sodium and water reabsorption in the distal
The Kidneys and Blood Pressure
In addition to controlling the amount of water that is
reabsorbed from the filtrate, which has an effect on blood volume
and blood pressure, the kidneys release an enzyme, renin, into the
blood. Renin sets off a series of reactions in the blood that results
in the production of another enzyme, angiotensin II. Angiotensin
II constricts blood vessels, causing a rise in blood pressure. It
also causes the adrenal cortex to release more aldosterone, which
raises blood volume and blood pressure.
Support and Locomotion
One of the biggest differences between animals and plants
is also one of the simplest: animals move, plants don’t. While the
simplest animals are propelled by cilia on their cell surfaces,
most animals are too large for such tiny structures to have a significant
effect on their locomotion. Many cnidarians have primitive contractile
fibers that allow them to propel themselves through water. A number
of invertebrates, such as earthworms, possess what is known as a hydrostatic
skeleton, in which muscles are arranged longitudinally down
the length of the body and in circular rings around the body. When
either of these types of muscle contract, an incompressible fluid
maintains the body at a constant volume but allows the worm to change
shape. Contraction of the circular muscles lengthens the body, while
longitudinal muscle contraction shortens the body. Earthworms are
segmented and can control the muscles within each segment independently.
By contracting the muscles in waves along its body, the earthworm
can propel itself through the soil. Tiny hairs called setae on
the worm’s surface provide traction against the soil.
Arthropod muscles connect to a rigid exoskeleton that
encloses the body and is made of chitin. When arthropod muscles
contract, they pull on inward extensions of the exoskeleton, causing
it to move. Range of motion is provided by joints connecting different
sections of the exoskeleton. While the exoskeleton works well for
animals as small as insects, it would be too heavy and impractical
for larger animals.
The Vertebrate Skeletal System
In direct contrast to arthropods, which live inside an
exoskeleton, vertebrates have evolved a hard internal skeleton,
or endoskeleton. The skeleton is made of two tissues: bone and cartilage.
Bones are rigid structures composed of living cells rooted
in a matrix of calcium, phosphate salts, and collagen fibers. Blood
vessels and nerves pass through a central canal in the bone; blood
makes its way to embedded cells through tiny pores. Bones form the
majority of the endoskeleton in higher vertebrates, including humans,
and provide structural support to all the other tissues in the body.
In addition, bones:
- Protect the soft, delicate organs and structures
within the body. The skull and rib cage are examples of hard bone
protecting the vital organs in the head and chest.
- Store minerals such as calcium. When the calcium supply
in the blood is high, it is stored in bones. When the supply is
low, bones give off calcium.
- Have marrow, found in cavities at the centers of bones,
that produces blood cells.
Bones meet each other at joints that are held together
by ligaments and are often bathed in a lubricating
and cushioning fluid called synovial fluid. Joints allow bones to
meet and bind together without actually grinding together. In this
way, joints allow for smooth skeletal movement.
Cartilage is firm but somewhat flexible. It will bend
under strain and spring back to its original shape when the force
is removed. The skeletons of sharks and rays are composed entirely
of cartilage, as are the skeletons of developing embryos. In higher
vertebrates, cartilage is retained in portions of the skeleton that
need to remain flexible, such as in the rib cage, which needs to
expand during inhalation, the tip of the nose and ears, at the end
of bones, and in joints. Cartilage contains no blood vessels or
nerves, and it takes a very long time to heal when damaged.
The Muscular System
Joints allow a skeleton to move. Muscles actually make it
move. Bones interface with muscles by way of tendons.
Movement is achieved when muscles contract, pulling on the bones to
which they are attached, bending the joints. An extensor muscle
straightens the bones in a joint. An example is the triceps muscle
in your upper arm, which straightens out the elbow joint. A flexor
muscle bends a joint. The bicep muscle, which bends your elbow,
is a flexor. (Note that both extensors and flexors perform their
functions by contracting; when an extensor contracts, it straightens
a joint, and when a flexor contracts, it bends a joint.) Muscles
also help the skeleton support and protect the body. Vertebrates
have three classes of muscles:
- Skeletal muscles, also called
striated muscles, are associated with the skeletal system and are
primarily involved in voluntary movement. A vertebrate generally
has conscious control over its skeletal muscles. Each skeletal muscle cell
contains many nuclei.
- Smooth muscle is found in the walls of the
internal organs such as the stomach, intestines, and urinary bladder
and is an involuntary muscle and not under voluntary control.
- Cardiac muscle makes up the heart. Cardiac
muscles are involuntary and can contract without stimulation from
the nervous system.
Muscles can be thought of as the enactors of the nervous
system. Through voluntary impulse or involuntary instinct, nerves
send messages to muscles. Muscles turn these messages into movement
and action by contracting or relaxing.
The interaction between two proteins, actin and myosin,
is responsible for muscular contraction. In skeletal muscles, actin
and myosin are arranged into units known as sarcomeres.
Long filaments of actin extend from each end of the sarcomere
toward the middle, almost meeting, but not actually touching. In
between these actin filaments are short, fat filaments of myosin
that are arranged longitudinally. The myosin does not connect to
the ends of the sarcomere. The ends of the sarcomere to which the
actin filaments are attached are called the Z lines. When the muscle
is stimulated to contract by a neuron, an influx of calcium ions
(Ca++) causes the
myosin to pull on the actin filaments by means of tiny connections known
as cross bridges. Neither the myosin nor the actin filaments change
in length, but when the myosin pulls the actin together, the actin
pulls the Z lines together and the whole sarcomere contracts.
Sarcomeres are arranged end to end into long fibers known
as myofibrils, which bundle to form the primary muscle fibers that
make up the muscle. Contracting sarcomeres cause muscle fibers to
contract, which, in turn, cause the whole muscle to contract.
It’s easy to think of the skin as just a thin covering
for the important internal organs. But skin itself is an organ,
with a multitude of functions:
- Protects against infection, abrasion, and
- Contains nerve endings vital for sensation, such as touch,
pain, heat, and cold
- Excretes water to maintain water and salt balance in the
- Produces vitamin D on exposure to the UV
rays in sunlight
- Regulates body temperature (thermoregulation)
Skin has three layers: the epidermis, the dermis,
and the hypodermis. The epidermis is the topmost layer,
which touches the outside environment. Active cell division occurs
in the lower region of the epidermis. As new cells are created,
old cells are pushed toward the surface, where they form a hardened,
dead layer that is constantly shed. The dermis is living tissue
that contains many blood vessels, sweat glands, and sebaceous glands,
which produce oils that keep the skin from drying out. The dermis
also contains nerve endings that are responsive to touch, pressure,
heat, cold, and pain. Hair follicles originate in the inner portions
of the dermis as well. The hypodermis, or subcutaneous layer of
the skin, is mainly composed of loose connective tissue and fat
Skin and Thermoregulation
The skin helps warm-blooded animals maintain constant
body temperature in varying environments. When the body becomes
too warm, blood vessels in the skin dilate, allowing heat to escape
through the surface of the skin. Special glands called sweat glands
produce a salty secretion called perspiration that evaporates off
the surface of the skin, taking heat with it. When the body becomes
too cold, the opposite processes occur. Sweat glands are shut down,
and blood vessels in the skin constrict, keeping the blood away
from the surface of the body, where heat could be lost. In addition,
the muscles begin to contract rapidly and shiver, which generates
The Reproductive System
As dictated by evolution, an organism’s purpose is to
reproduce and ensure the survival of the species. All of the other
organ systems exist just to keep the animal alive long enough to mate
and pass its genetic makeup down to its progeny.
Animals can reproduce either asexually or sexually. Asexual
reproduction usually occurs among less highly evolved animals. Budding,
which can occur in certain cnidarians like the hydra, is a process
by which the offspring literally grow off the side of the parent, producing
a miniature, genetically identical copy. Regeneration occurs
when animals such as earthworms, planarians, or starfish are broken
apart and each piece then grows into a separate organism. Parthenogenesis occurs
when an animal’s egg cell begins to divide mitotically without being
fertilized by a sperm. The embryo that develops from this unfertilized
egg will be genetically identical to the parent. Populations of
animals that reproduce parthenogenically are usually entirely female.
Animals ranging from rotifers to some amphibians reproduce through
Sexual reproduction is when two haploid gametes, one from
each parent, fuse to form a zygote, which develops into an offspring
genetically different from the parents. This fertilization can take
place externally, as is the case for many aquatic organisms that
release their unfertilized gametes into the water, or internally.
As animals have evolved, they have developed special structures
for the production of gametes, for the fertilization process, and,
in the case of viviparous animals that give birth to live young,
for the support and nourishment of the developing young. Collectively,
these structures are known as the reproductive system. In most species,
including humans, the anatomy of the male and the female reproductive
system is significantly different.
The Male Reproductive System
The male reproductive system has two major functions:
- It produces sperm cells, the
male gametes, through the process of spermatogenesis. (Spermatogenesis
is covered in the chapter on genetics.)
- It produces semen, a fluid that acts as a
vehicle and nourishment for sperm as they make their way through
the female reproductive system on their way to fertilize the egg.
The testes are the male gonads: they produce
the sperm, which is the male gamete. More specifically, sperm cells
are produced in the seminiferous tubules of the testes. (In addition to
producing sperm, the testes also produce the hormone testosterone.)
Since sperm can only develop at a temperature slightly lower than
the normal mammalian body temperature, evolution has provided the
needed lower temperature by placing the testes outside the body
in a sac called the scrotum.
The seminiferous tubules empty into a long tube called
the vas deferens, which joins the urethra just below the bladder
and thereby provides a means of exit from the body through the penis.
The penis is a spongy organ that can become erect during periods
of sexual excitement. During erection, arteries in the penis dilate,
engorging the erectile tissue with blood. This simultaneously compresses
the veins that drain blood from the penis, trapping blood in the
spongy tissue, causing it to become rigid.
The Female Reproductive System
While the male reproductive system is designed to produce
and deposit sperm in the female, the female reproductive system
has the more formidable task of receiving the male gametes, producing
the female gametes, and, in the event of fertilization, maintaining
and supporting a pregnancy.
The female reproductive system consists of the external
genitalia, known as the vulva and vagina, the uterus,
which supports the developing fetus, the Fallopian tubes,
which connect the uterus with the two ovaries, and the ovaries,
which produce the ova, or egg cells, in addition to the female sex
hormones. Also included are the mammary glands, which produce milk
to nourish the young.
The Menstrual Cycle
The functioning of the reproductive system in human females
is dependent on cyclical fluctuation of hormone levels that repeats
regularly every 28 days. This cycle, known as the menstrual
cycle, primarily affects the ovaries and the
uterus. The effects of the menstrual cycle on the ovaries are called
the ovarian cycle, while the effects on the uterus
are called the uterine cycle. The entire menstrual
cycle is regulated by a hormonal feedback loop involving the hypothalamus,
anterior pituitary gland, and the ovaries.
The ovarian cycle is usually divided into two stages:
the follicular stage and the luteal stage. During
the follicular stage, which lasts about 14 days, follicle-stimulating
hormone (FSH) from the anterior pituitary gland stimulates a follicle in
the ovary to mature. A follicle is an ovum and the cells that encapsulate
it. As the ovum matures, the surrounding cells of the follicle begin
to produce estrogen. After about 14 days of increasing
estrogen levels, the estrogen in the blood reaches a concentration
that sets in motion a series of events resulting in the release
of luteinizing hormone (LH) from the anterior pituitary gland. Luteinizing hormone
causes the mature follicle to release the now mature ovum into the
Fallopian tube. This is called ovulation.
The second 14 days of the ovarian cycle are
called the luteal phase. After ovulation, the remnants of the follicle
form into a structure called the corpus luteum. Just
as FSH from the anterior pituitary stimulated the follicle to mature,
LH affects the corpus luteum and causes it to release progesterone
and some estrogen. After about 14 days, if the ovum
is not fertilized, the corpus luteum degenerates, progesterone and
estrogen levels fall, and the cycle starts again with the follicular
The estrogen secreted by the follicle and the progesterone
and estrogen secreted by the corpus luteum affect the lining of
the uterus. The cycle of hormones in the ovarian cycle push the
uterus through a 28-day cycle as well. The uterine
cycle contains three phases: menstruation, the proliferative phase,
and the secretory phase.
In the first few days of the follicular phase, after the
corpus luteum has disintegrated and the follicle is in the earliest
stages of maturation, estrogen and progesterone levels are relatively
low. The low levels of these hormones causes the cells of the uterine
lining to slough off, releasing a bloody discharge commonly referred
to as menstruation, or a woman’s period. The onset of menstruation
marks the first day of the 28-day menstrual cycle and
usually lasts about four to five days. The nine-day-long proliferative
phase begins as the follicle continues to mature, estrogen levels
rise, and a new uterine lining begins to build up along the uterine
walls. Were fertilization to occur, the lining would support the
development of an embryo. This lasts about nine days. After ovulation,
the secretory phase begins as the corpus luteum develops and produces
progesterone. Progesterone causes new blood vessels to grow within
the uterine lining. If fertilization does not occur, the corpus
luteum degenerates, hormone levels fall, and the uterine lining
once again sloughs off during menstruation.
Fertilization and Development of the Embryo
As the egg is swept along the Fallopian tubes on its way
to the uterus, it may encounter sperm deposited by the male. Fertilization
occurs if one of these sperm cells is successful in penetrating
the egg. The sperm nucleus fuses with the nucleus of the egg cell
and a diploid zygote is formed. Within 24 hours,
the zygote begins to divide by mitosis. First it divides into two
cells, then four, then eight, until a solid ball of cells known
as the morula forms. The morula eventually encounters
and implants itself in the lining of the uterus.
At this point, the menstrual cycle halts. From its place
in the uterine lining, the dividing embryo releases a hormone known
as human chorionic gonadotropin (HCG). This hormone prevents the
corpus luteum from disintegrating, prolonging the production of progesterone
and estrogen. Rather than slough off, the uterine lining further
thickens. The corpus luteum continues to produce estrogen and progesterone
until the placenta takes over this function about three months into
After it implants in the uterine lining, the solid ball
of cells that is the morula begins to hollow out into a spherical
ball of cells known as the blastula. The blastula has
a round cell wall called the trophoblast, which encloses a hollow
space known as the blastocoel and a small cluster of cells known
as the inner cell mass.
Before the formation of the blastula, all cells were identical,
or undifferentiated. The cells of the inner cell mass and the trophoblast
are the first sign of cell differentiation. Soon, the cells of the
inner cell mass begin to divide more rapidly than the rest of the
cells in the blastocyst. As development continues, the cells of
the inner cell mass will go on to form the embryo,
and the cells of the trophoblast will form the membranes that surround
the developing human.
As the cells of the inner cell mass rapidly divide, a
small pocket begins to form on one end of the blastocyst. This new
structure is called the gastrula. At first the gastrula
consists of two cell layers: an outer layer known as the ectoderm
and an inner layer known as the endoderm. Soon, a third layer, called
the mesoderm, begins to form between the ectoderm and endoderm.
These three cell layers, or germ layers, go on to form the various
organs and structure of the human as they differentiate. Over the
course of about eight weeks, the germ layer cells build all of the
organs, and the embryo is now classified as a fetus.
In humans, the fetus continues to grow and mature in the womb for
an additional seven months, until birth.
Reproductive Support Structures
Numerous structures within the womb support the embryo
as it develops. The largest and most important is the placenta.
The blood vessels of the placenta come in close proximity with the
blood vessels of the mother so that nutrients and oxygen can diffuse
from the mother’s bloodstream into the bloodstream of the developing
embryo or fetus. There is no direct blood contact through the placenta;
the mother and fetus have completely separate blood circulation.
The placenta is connected to the circulatory system of the embryo
or fetus through the umbilical cord.
Four membranes surround, nourish, and protect the embryo.
The first membrane is the yolk sac, which provides stored nutrients
to the developing embryo. The yolk sac is very small in placental
mammals, which get their nutrients through the placenta, but in
animals that develop in eggs, such as birds and reptiles, the yolk
will be the sole source of nutrients throughout the entire course
of development. The second membrane is the amnion, which is filled
with a clear fluid known as the amniotic fluid. This fluid provides
a cushion for the embryo so that bumps and jolts within the womb
or in the external egg do not cause damage. The third membrane is
the allantois. In placental mammals, the allantois develops into the
umbilical cord, but in vertebrates that develop externally, the
allantois is the site of waste disposal for the developing embryo.
The fourth membrane is the chorion, which in humans forms the placenta.
In vertebrates that develop in eggs, the chorion lines the inside of
the shell and allows for gas exchange with the environment.
Animals’ abilities to move create the possibility for
copious interactions between organisms, giving rise to quite complex
patterns of behavior. Animal behavior can come in the form of instincts
and learned behavior. Instincts are inheritable, genetically coded
behavior patterns that an animal possesses at birth. Learned behaviors
are established and maintained as an animal responds to new situations.
Learned behaviors are not passed down from parent to offspring genetically,
but they can be taught.
Instinctual behavior can take the form of simple
reflexes or fixed-action patterns. Simple reflexes
are automatic responses to specific stimuli. Reflex behaviors do
not originate from the brain in vertebrates. Instead, they are processed
in the spinal cord. For example, if you touch a hot iron, the pain
and heat receptors in your fingers send signals down a sensory neuron
to your spinal cord, where a motor neuron is immediately stimulated
to cause you to pull back your arm. The signal is actually sent
to the brain after it has been acted on by the spinal cord—you do
not perceive pain until the brain processes the information.
Fixed-action patterns are complex behaviors that, like
reflexes, are triggered by a specific stimulus. The stimuli that
cause fixed-action behavior are often more complex than the stimulus
behind simple reflex behavior. Once triggered, fixed-action patterns
often proceed to completion, even if the stimulus is removed. For
example, female geese demonstrate a fixed-action pattern
called egg rolling. If a female goose spots an egg outside of her
nest, the mother goose will use her beak to roll the egg back into
the nest. If the egg is taken away in the middle of this process,
she will continue to move her neck and beak as if she were rolling
an egg, even though the egg is no longer there. Fixed-action patterns
do not need to be learned; they are present in an individual from
Many animals, most notably birds, exhibit a special type
of learned behavior called imprinting. Imprinting occurs
when an animal quickly learns, during a short critical period, to
recognize an individual, object, or location. The most common example
of imprinting is the case of birds that can walk soon after hatching.
Newly hatched infant birds must follow their mother to survive.
Soon after they hatch, these birds go through a critical period
during which they treat the first moving object they see as their
mother. If the first organism a young bird sees is a pig, it will
imprint the pig as its mother. Imprinting is nearly impossible to
Unlike instincts, which are present at birth, an individual
organism learns some behavior over the course of its life. The simplest
form of learning is known as habituation. Habituation
occurs when a nonharmful stimulus that would normally cause an animal
to respond is repeated over and over again until the animal learns
to ignore it. The classic example of habituation is seen in the
common garden snail. When its body is poked, a snail will withdraw
into its shell. However, if it is poked repeatedly without any real
harm done, the snail ignores the stimulus and ceases to retreat
into its shell.
Conditioning, or associative learning,
occurs when an animal learns to associate a specific stimulus with
a set behavior. There are two types of conditioning: classical conditioning and
operant conditioning. Classical conditioning is merely the association
of a new stimulus with a stimulus that is recognized by instinct.
The most famous example of classical conditioning is Pavlov’s dog.
In an experiment, Russian scientist Ivan Pavlov would ring a bell
a few moments before feeding a dog. Every time he fed the dog, he
would first ring the bell. The sight and smell of food causes a
dog to salivate instinctually. But after ringing the bell before
feeding the dog a number of times, Pavlov discovered that the dog
would salivate whether or not food was present. The dogs associated
the sound of the bell with the stimulus of food.
Operant conditioning is sometimes called trial-and-error
learning. It involves the establishment of a new behavior or the
avoidance of an old behavior because of the association of a reward
or punishment. For example, a rat will learn to press a lever in
order to release its food. It learns a new behavior, the pressing
of the lever, because it associates this behavior with a reward.
Similarly, the rat can be trained to avoid a certain colored spot
in its cage if standing in that spot becomes associated with a mild
electrical shock. Normally the rat would have no reason to avoid
such a spot, but because of the association of a punishment with
this behavior, it stays away.
Both classical and operant conditioning can be undone
if the association between stimulus and behavior or behavior and
punishment/reward does not last. For example, if the rat presses
the lever and no food comes out for several tries, it will cease
to press the lever. This unlearning is called extinction.
Many animal behaviors, such as sleep and wakefulness,
foraging times, and metabolic rate, operate according to daily cycles
known as circadian rhythms. These rhythms can be traced to the periods
of light and dark in the day, but the rhythms remain even if for
a short time an animal cannot see the changing of the light. In
other words, animals have a sort of internal clock that regulates