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 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 a synapse.

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 next cell.

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.

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 different circumstances.

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.

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 of Ranvier.

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.

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.

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.

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 organs.

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 neurons.

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 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 cells.

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 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 brain.

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 vestibular nerve.

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 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 bloodstream.

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, and cortisol.

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 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:

The two hormones made in the hypothalamus and released by the storage facility in the posterior pituitary are:

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 of Langerhans.

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.

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.

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.

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 over again.

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 on genetics.

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 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.

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.

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.

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 can collapse.

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.

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 by cells.

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.

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 anus.

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 stores food.

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 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 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.

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.

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.

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 digestion.

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.

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 tubule.

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.

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.

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:

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.

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:

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:

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 cells.

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 significant heat.

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 parthenogenesis.

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 has two major functions:

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.

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 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 phase.

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.

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 the pregnancy.

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.

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 birth.

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 reverse.

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 their behavior.