Principal Homeostatic processes include the following:
- "Warm-blooded" (endothermic)
animals (mammals and birds) maintain a constant body
temperature, whereas
ectothermic animals (almost all other organisms) exhibit
wide body
temperature variation.[7]
An advantage of temperature regulation is that it allows an
organism to function effectively in a broad range of
environmental conditions. For example,
ectotherms tend to become sluggish at low temperatures,
whereas a co-located endotherm may be fully active. That thermal
stability comes at a price, since an automatic regulation system
requires additional energy.[8]
If the temperature rises, the body loses heat by sweating or
panting, via the
latent heat of
evaporation. If it falls, this is counteracted by increased
metabolic action, by shivering, and—in fur- or feather-coated
creatures—by thickening the coat.
- Regulation of the
pH of the blood at 7.365 (a measure of alkalinity and
acidity).
- All animals also regulate their
blood glucose concentration.
Mammals regulate their blood glucose with
insulin and
glucagon. The human body maintains glucose levels constant
most of the day, even after a 24-hour fast. Even during long
periods of fasting, glucose levels are reduced only very
slightly.[9]
Insulin, secreted by the beta cells of the pancreas, effectively
transports glucose to the body's cells by instructing those
cells to keep more of the glucose for their own use (see
Dynamic equilibrium). If the glucose inside the cells is
high, the cells will convert it to the insoluble
glycogen to prevent the soluble glucose from interfering
with cellular metabolism. Ultimately this lowers blood glucose
levels, and insulin helps to prevent
hyperglycemia. When insulin is deficient or cells become
resistant to it,
diabetes occurs. Glucagon, secreted by the alpha cells of
the pancreas, encourages cells to break down stored glycogen or
convert non-carbohydrate carbon sources to glucose via
gluconeogenesis, thus preventing
hypoglycemia.
- The
kidneys are used to remove excess water and ions from the
blood. These are then expelled as
urine.
The kidneys perform a vital role in homeostatic regulation in
mammals, removing excess water, salt, and
urea
from the blood.
- If the water content of the blood and
lymph
fluid falls, it is restored in the first instance by extracting
water from the cells. The throat and mouth become dry, so that
the symptoms of thirst motivate the animal to drink
- If the oxygen content of the blood falls, or the
carbon-dioxide concentration increases, blood flow is increased
by more vigorous heart action and the speed and depth of
breathing increases.
- Sleep timing depends upon a balance between homeostatic
sleep propensity, the need for sleep as a function of the amount
of time elapsed since the last adequate sleep episode, and
circadian rhythms that determine the ideal timing of a
correctly structured and restorative sleep episode.[10]