16.13: Summary of Homeostatic Mechanisms
1. Temperature control
The biochemical and physiological processes in the cell are sensitive to temperature. The optimum body temperature is about 37 C [99 F] for mammals, and about 40 C [104 F] for birds. Biochemical processes in the cells, particularly in muscles and the liver, produce heat. The heat is distributed through the body by the blood and is lost mainly through the skin surface. The production of this heat and its loss through the skin is controlled by the hypothalamus in the brain which acts rather like a thermostat on an electric heater. .
(a) When the body temperature rises above the optimum, a decrease in temperature is achieved by:
- Sweating and panting to increase heat loss by evaporation.
- Expansion of the blood vessels near the skin surface so heat is lost to the air.
- Reducing muscle exertion to the minimum.
(b) When the body temperature falls below the optimum, an increase in temperature can be achieved by:
- Moving to a heat source e.g. in the sun, out of the wind.
- Increasing muscular activity
- Shivering
- Making the hair stand on end by contraction of the hair erector muscles or fluffing of the feathers so there is an insulating layer of air around the body
- Constricting the blood vessels near the skin surface so heat loss to the air is decreased
2. Water balance
The concentration of the body fluids remains relatively constant irrespective of the diet or the quantity of water taken into the body by the animal. Water is lost from the body by many routes (see module 1.6) but the kidney is the main organ that influences the quantity that is lost. Again it is the hypothalamus that monitors the concentration of the blood and initiates the release of hormones from the posterior pituitary gland. These act on the kidney tubules to influence the amount of water (and sodium ions) absorbed from the fluid flowing along them.
(a) When the body fluids become too concentrated and the osmotic pressure too high, water retention in the kidney tubules can be achieved by:
- An increased production of antidiuretic hormone (ADH) from the posterior pituitary gland, which causes more water to be reabsorbed from the kidney tubules.
- A decreased blood pressure in the glomerulus of the kidney results in less fluid filtering through into the kidney tubules so less urine is produced.
(b) When the body fluids become too dilute and the osmotic pressure too low, water loss in the urine can be achieved by:
- A decrease in the secretion of ADH, so less water is reabsorbed from the kidney tubules and more diluted urine is produced.
- An increase in the blood pressure in the glomerulus so more fluid filters into the kidney tubule and more urine is produced.
- An increase in sweating or panting that also increases the amount of water lost.
Another hormone, aldosterone, secreted by the cortex of the adrenal gland, also affects water balance indirectly. It does this by increasing the absorption of sodium ions (Na-) from the kidney tubules. This increases water retention since it increases the osmotic pressure of the fluids around the tubules and water therefore flows out of them by osmosis.
3. Maintenance of blood volume after moderate blood loss
Loss of blood or body fluids leads to decreased blood volume and hence decreased blood pressure. The result is that the blood system fails to deliver enough oxygen and nutrients to the cells, which stop functioning properly and may die. Cells of the brain are particularly vulnerable. This condition is known as shock.
If blood loss is not extreme, various mechanisms come into play to compensate and ensure permanent tissue damage does not occur. These mechanisms include:
- Increased thirst and drinking increases blood volume.
- Blood vessels in the skin and kidneys constrict to reduce the total volume of the blood system and hence retain blood pressure.
- Heart rate increases. This also increases blood pressure.
- Antidiuretic hormone (ADH) is released by the posterior pituitary gland. This increases water re-absorption in the collecting ducts of the kidney tubules so concentrated urine is produced and water loss is reduced. This helps maintain blood volume.
- Loss of fluid causes an increase in osmotic pressure of the blood. Proteins, mainly albumin, released into the blood by the liver further increase the osmotic pressure causing fluid from the tissues to be drawn into the blood by osmosis. This increases blood volume.
- Aldosterone, secreted by the adrenal cortex, increases the absorption of sodium ions (Na+) and water from the kidney tubules. This increases urine concentration and helps retain blood volume.
If blood or fluid loss is extreme and the blood volume falls by more than 15-25%, the above mechanisms are unable to compensate and the condition of the animal progressively deteriorates. The animal will die unless a vet administers fluid or blood.
4. Acid/ base balance
Biochemical reactions within the body are very sensitive to even small changes in acidity or alkalinity (i.e. pH) and any departure from the narrow limits disrupts the functioning of the cells. It is therefore important that the blood contains balanced quantities of acids and bases.
The normal pH of blood is in the range 7.35 to 7.45 and there are a number of mechanisms that operate to maintain the pH in this range. Breathing is one of these mechanisms.
Much of the carbon dioxide produced by respiration in cells is carried in the blood as carbonic acid. As the amount of carbon dioxide in the blood increases the blood becomes more acidic and the pH decreases. This is called acidosis and when severe can cause coma and death. On the other hand, alkalosis (blood that is too alkaline) causes over stimulation of the nervous system and when severe can lead to convulsions and death.
(a) When vigorous activity generating large quantities of carbon dioxide causes the blood to becomes too acidic it can be counteracted in two ways:
- By the rapid removal of carbon dioxide from the blood by deep, panting breaths
By the secretion of hydrogen ions (H+) into the urine by the kidney tubules.
(b) When over breathing or hyperventilation results in low levels of carbon dioxide in the blood and the blood is too alkaline, various mechanisms come into play to bring the pH back to within the normal range. These include:
- A slower rate of breathing
- A reduction in the amount of hydrogen ions (H+) secreted into the urine.
SUMMARY
Homeostasis is the maintenance of constant conditions within a cell or animal’s body despite changes in the external environment.
The body temperature of mammals and birds is maintained at an optimum level by a variety of heat regulation mechanisms. These include:
- Seeking out warm areas,
- Adjusting activity levels,
blood vessels on the body surface,
- Contraction of the erector muscles so hairs and feathers stand up to form an insulating layer,
- Shivering,
- Sweating and panting in dogs.
Animals maintain water balance by:
- adjusting level of antidiuretic hormone(ADH)
- adjusting level of aldosterone,
- adjusting blood flow to the kidneys
- adjusting the amount of water lost through sweating or panting.
Animals maintain blood volume after moderate blood loss by:
- Drinking,
- Constriction of blood vessels in the skin and kidneys,
- increasing heart rate,
- secretion of antidiuretic hormone
- secretion of aldosterone
- drawing fluid from the tissues into the blood by increasing the osmotic pressure of the blood.
Animals maintain the acid/base balance or pH of the blood by:
- Adjusting the rate of breathing and hence the amount of CO2 removed from the blood.
- Adjusting the secretion of hydrogen ionsinto the urine.