Homeostasis
To stay alive, all animals need to maintain steady internal conditions, such
as body temperature and the levels of water, salts, nutrients, oxygen, and waste in
the body fluids and cells. All organ systems participate in the maintenance of these
stable internal conditions through a process known as homeostasis,
meaning “steady state.” Although the acceptable ranges for all of these
factors vary from species to species, no animal can survive unless its internal
environment is kept within tolerable limits.
The most common regulatory tool used to maintain homeostasis is the
negative feedback loop. In a negative feedback loop, the end point,
or product, of a specific process regulates the beginning of that same process. If
levels of the product are low in an animal, the process increases; if levels of the
product are high, the process stops. Often the body has two negative feedback loops
for each process: one to raise levels and another to lower them.
EXAMPLE:
The body must maintain a specific level of blood calcium for the proper
functioning of muscles. If blood calcium levels are low, the parathyroid gland
releases a hormone that pulls calcium from the bones and releases it into the
bloodstream. When the concentration of calcium in the blood reaches the proper
level, the parathyroid gland stops releasing the hormone. On the other hand, if
blood calcium levels are high, the thyroid gland releases a hormone that causes
calcium to be drawn out of the bloodstream and deposited into the bones. Once
the blood calcium concentration drops to the proper level, the thyroid gland
stops releasing the hormone.
Although less common than negative feedback loops, positive feedback
loops also work to regulate functions in the body. In a positive feedback
loop, a specific response amplifies until a proper level is reached, at which point
the response is reversed or stopped.
EXAMPLE:
During childbirth, pressure on the mother’s uterus wall and cervix causes
the secretion of the hormone oxytocin. This hormone stimulates contraction of
the uterus walls, which in turn exert pressure on the fetus. Pressure from the
fetus increases as a result, further stimulating the release of oxytoxin. A
positive feedback loop of increasing pressure contin-ues until the fetus is
born, at which point stimulation of both the fetus and uterus wall ceases,
completing the cycle.
