Homeostasis is the process which maintains a constant internal body environment. The process relies on the mechanisms which keep this internal environment within very narrow limits, despite fluctuating external conditions

* Breathing - muscle contracting is controlled by nerve impulses from the medulla in the brain, to the diaphragm and intercostals muscles. Receptors, in this case stretch receptors in lung inhalation send inhibitory signals to the respiratory centre to cause exhalation (the effect). This is a negative feedback system where damage to the lungs by overstretching is prevented.

* Heart rate - Nerves from the medulla in the brain can change heart rate by using negative feedback. If chemoreceptors in aortic and carotid bodies detect that the blood needs to be pumped around quicker, e.g. because there is too much carbon dioxide in blood which causes pH changes, a nerve impulse would be sent down the sympathetic nerve to speed up the heart rate. As well as this, pressure receptors can cause vasoconstriction or vasodilation of arterioles. This example of homeostasis allows a person to undergo exercise without experiencing problems due to lack of oxygen etc by being able to change their heart rate and stroke volume.

* Osmoregulation - water potential of blood must be kept constant to prevent osmotic damage to cells by the movement of water. In this instance, levels are controlled by osmoreceptor cells in the hypothalamus in the brain, which detect changes in water potential of blood. For example if water potential falls as water is lost through sweating, receptor cells would detect this and make the organism feel thirsty. The receptor cells would also cause the pituitary to release more ADH to allow increased water reabsorption in the kidneys. Both of these changes all of which resulting in more water in the blood so water potential would return to its normal levels.

* Thermoregulation - In ectotherms temperature is controlled by external surroundings, so they rely almost completely on behavioural mechanisms. This could be sitting in the shade or going into water in the heat of the day in order to maintain a constant internal temperature. However endotherms such as humans rely on physiological processes. If the receptors in the hypothalamus detect blood temperature is too high/low, impulses are sent to appropriate effects to adjust the body temperature. If you were too hot, vasodilation of arterioles would occur and the sweat glands would produce sweat. If you were too cold, vasoconstriction of arterioles would occur, your body would shiver and adrenal or thyroid glands would also work to encourage respiration in cells to produce heat.

* Blood sugar levels - When the pancreas detects a rise or fall in blood glucose concentration, the islets of langerhans produce one of two antagonistic hormones. This is either insulin to decrease concentration in blood or glucagons to increase the concentration in blood. At the same time the liver and muscle cells respond by either:

1. Removing more glucose from the blood and activating enzymes which convert glucose to glycogen to decrease blood sugar level to normal concentrations.

2. Activating enzymes which convert glycogen to glucose and returning more glucose to the blood to increase blood sugar level to normal concentrations.

It is clear from these examples that the cells in any living organism would not function properly without homeostasis to maintain correct conditions internals. Without is we see conditions such as hyperthermia when body temperature falls too low, osmotic damage when water balance is uncontrolled, or severe diseases such as diabetes when homeostasis goes wrong. A great deal of the hormonal system and the autonomic nervous system are dedicated to homeostasis. We could not survive without it unless we had some kind of alternate method to maintain our internal conditions.

Joanne Phillips