Homeostatic Mechanisms
There are two kinds of feed back in a chain like event in human body. The mechanisms are called as negative feed back and positive feed back respectively. Take a case of chain reaction in which A is converted to B; B is converted to C; C to D and D to E, Quite often every change is controlled by activity of an enzyme. The rate of every enzymatic reaction can vary. The slowest reaction is termed as rate limiting step. If any of the intermediate from A to E is over produce, then higher concentration of the intermediate slows down the reaction leading to its generation. This is termed as negative feed back. Thus, negative feedback limits higher production of an intermediate through a self controll mechanism. Alternatively, if production of an intermediate is reduced below certain limits, then related enzymes may be activated to restore the level of the intermediate. This is called as positive feedback mechanism. Control of arterial pressure involves pressure monitoring system, volume monitoring system, hormonal mechanisms, reflex regulation, autonomic control etc. Bradycardia due to higher blood pressure is an example of negative feed back mechanism. In a cascade of reactions in coagulation, activation of one clotting factor leading to activation of other factors promoting coagulation to limit blood loss is an example of positive feedback mechanism. Operation of the feedback mechanisms involves three components:
• The sensor
• The control center
• The effector
The sensor contains receptors which monitor change of the variable and provide sensory signal to the controlling center as soon as the change is detected. The carotid sinus and the aortic arch contain bats receptors in their valve to detect changes in blood pressure. They send signals to a controlling center located in medullary oblongata. The control centers are usually located in CNS. Centers for blood pressure regulation are located in medulla oblongata and hypothalamus. Effector is the target organ which carries out the demand of the control center to achieve an effective response. Blood vessels and heart are the effector organs related to blood pressure. Through bradycardia or tachycardia at heart and through vasodilation or vasoconstriction, they help in maintaining blood pressure.
The Following control systems are important for regulation of human body:
• Core body temperature
• Blood glucose
• Plasma ionized calcium • Blood partial pressure of oxygen and carbon dioxide
• Blood oxygen content
• Arterial blood pressure
• Extra cellular sodium concentration
• Extra cellular potassium concentration
• Volume of body water
• Extra cellular fluid pH
As an illustration, homeostasis of one parameter i.e. volume of body water is discussed below.
Homeostasis of Volume of Body Water
The volume of water in the body is measured by stretch receptors in the atria of the heart. They are also indirectly sensed by measurement of the osmolality of plasma by the hypothalamus. Measurement of the plasma osmolality gives an indication of the water content of the body. It relies on the fact that water losses from the body through sweat, gut fluids in the form of fecal water and through vomitting/diarrhoea and the exhaled air, are all hypotonic. It means that fluids like saliva, tears are less salty than plasma. Tears have almost the same salt content as that of extra cellular fluids while saliva is hypotonic with respect to plasma. Thus, taste of saliva is not salty while tears are decidedly salty. Nearly all normal and abnormal losses of body water make extra cellular fluids hyper-osmolar. Conversely, excessive water intake dilutes the extra cellular fluids making it hypo-osmolar. Only after loss of water through urine body can become isotonic. Hence excessive water intake leads to frequent urination. When the hypothalamus detects a hyper-osmolar extra cellular environment, it causes secretion of the hormone called antidiuretic hormone (ADH). For ADH, kidney is the effector organ. The effect of ADH on kidney tubules leads to reabsorption of water from distal convoluted tubules and collecting ducts, thus preventing further water loss. Simultaneously, hypothalamus stimulates thirst center in the brain to stimulate urge of drinking water. The caessation of urine flow prevents the hypovolemia and hypertonicity from getting worse. Thus, drinking of water corrects the defect. Hypo-osmolarity leads to very low plasma ADH levels. This results in inhibition of water reabsorption from kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting Ad of excess water in the body.
It is to be noted that urinary water loss, when the body water homeostat is intact, is a compensatory water loss, correcting excess of water in the body. Alternatively thirst reflex is an important second effector mechanism of the body water Homeostatic, correcting any water deficit in the body. Stretching of the right atrium of the heart is a sign of excessive blood volume. It causes stretch receptors to secrete a hormone called as atrial natriuretic peptide (ANP) into blood. It causes kidneys to get rid of sodium along with water loss into urine. The net result is reducing volume of circulating blood. Thus, hypervolemia / hypovolemia is corrected by hormones like ADH / ANP. Water intake through thirst center or urination by kidney leads to homeostasis of volume of body water.
Homeostatic Breakdown
Many diseases are the result of failure of one or more Homeostasis in the body. The body water homeostat can be disrupted by inability to secrete ADH in response to even the normal body water losses via the exhaled air, the feces and sweating. On not receiving ADH signal, the kidneys produce huge unchanging volumes of very dilute urine, causing dehydration and even death. Another example is of type I diabetes mellitus. In this case, blood glucose Homeostatic ceases to function due to destruction of beta cells of pancreas. This means that the glucose sensor is absent and the effector pathway of insulin level in blood remains unchanged. The blood glucose concentration rises to very high levels, while the body's proteins are degraded in to amino acids and are used for generating energy via gluconeogenesis. If untreated, the condition can be fatal.
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