An Introduction to the Analysis of the Homeostasis Concept
Homeostasis is a system of automatic control mechanisms which maintain the internal environment of an organism despite changes in the external environment. The internal environment consists of extracellular fluids that bathe every cell of the body, supplying nutrients and receiving wastes. Regulators (animals which use homeostasis), maintain suitable physical conditions such as body temperature and water potential of cells, and the supply of nutrients e.g. O" and glucose, and removal of wastes e.g. CO" and urea, by the interaction of a number of body systems including the respiratory, digestive, circulatory, excretory and immune systems, controlled by the brain via the endocrine and nervous systems. Homeostasis is based on feedback mechanisms whereby sensory nerves or receptors receive a stimulus, for example a change in temperature, and inform the control centre in the brain. This sensory input is processed and a signal is sent to an effector system, e.g. muscles or glands that cause the response to the stimulus. Most examples are of negative feedback loops in which any deviation from the set point is made smaller or resisted. Positive feedback is occasionally used e.g. release of oxytocin during childbirth - nerve stimulation caused by the baby's head pushing the cervix, stimulates uterine contractions, causing further nerve stimulation and therefore more oxytocin is released until the cut-off point of birth.
Thermoregulation, controlled by the autonomic nervous system, maintains body temperature within an optimal range, enabling cells to function most effectively even as external temperatures fluctuate. This is important as many enzymes in the body work within specific temperature ranges. Thermoreceptors of sensory nerves in the skin, body core and hypothalamus detect a change in temperature and send impulses to the control centre in the hypothalamus. When body temperature is lower than the set point, the heat gain centre is stimulated causing; vasoconstriction (blood flow to the skin is restricted to retain heat), erector-pili muscle contraction - erecting hairs, skeletal muscle contraction -shivering, and reduced sweating, thus generating and conserving heat. The hypothalamus also produces a release factor substance which stimulates its target organ the anterior pituitary gland to secrete thyroid stimulating hormone. This reaches the thyroid gland via the blood and stimulates the release of thyroxine, generating heat by increasing cellular metabolism. Once a higher level of TSH is detected in the blood the hypothalamus inhibits TSH release returning levels to normal in the blood, an example of negative feedback. Impulses to the cerebrum can also cause voluntary behavioural changes, for example, putting on warm clothes. The opposite effect is achieved when the heat loss centre is stimulated. A homeostatic failure is hypothermia, when body temperature falls below tolerable ranges and the respiratory system fails, leading to cardiac arrest.
The control of hunger is also homeostatic helping the organism maintain optimum levels of energy and nutrients. Hormones are released when changes in hunger are detected, and act on the satiety centre in the hypothalamus which sends nervous impulses to the cerebrum making us feel either hungry or satiated. For example, an appetite stimulating hormone is ghrelin, levels of this hormone rise when the stomach is empty inducing hunger and then falls quickly when the stomach is full inducing feelings of satiety. Leptin produced in fat tissues suppresses appetite as its level increases, when body fat decreases, leptin levels fall and appetite increases. A failure of this system is gluttony, the intake of too much food. Cardiac output (heart rate and stroke volume) is adjusted to meet the varying needs of cells nutrient supply and waste removal. Monitored by the medulla, containing the cardio-acceleratory and cardio-inhibitory centres. The CAC and CIC work antagonistically and which centre stimulates the heart is determined by factors such as pH of the blood, (lowered as carbon dioxide levels increase e.g. during exercise). Receptors in the carotid body detect this change and send impulses to the CAC which sends an impulse via the sympathetic nervous system to the sino-atrial node of the heart increasing cardiac output and therefore CO" removal at the lungs. Epinephrine and norepinephrine are also released by the adrenal medulla in times of stress, increasing cardiac output and are counteracted by acetylcholine. The CIC is connected to the SAN via the parasympathetic nervous system and produces the opposite effect. Baroreceptors detect changes in blood pressure and send impulses via the autonomic nervous system to the cardiac centre. During increased blood pressure baroreceptors in the carotid sinus and aortic arch are stretched, stimulating them to send impulses via the parasympathetic nervous system to the CIC decreasing cardiac output, therefore blood pressure falls. The receptors are no longer stimulated by stretch and the CAC becomes activated, a negative feedback mechanism. Homeostasis, by the interaction of many body systems controlled by the brain, maintains a stable environment in organisms' tissues allowing them to function optimally.
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