Homeostasis PDF - PCMS Peshawar

HOMEOSTASIS BY DR TASNEEM ULLAH PCMS Peshawar Homeostasis: The Master Balancer of Life’s Equilibrium  HOMEOSTASIS Homeostasis refers to the body’s ability to keep its internal conditions stable, such that it...

HOMEOSTASIS BY DR TASNEEM ULLAH PCMS Peshawar Homeostasis: The Master Balancer of Life’s Equilibrium  HOMEOSTASIS Homeostasis refers to the body’s ability to keep its internal conditions stable, such that its cells can survive. The body maintains homeostasis through a number of self regulating control systems, or homeostatic mechanisms. These mechanisms share three components. 1. Receptors are on the lookout. They provide information about specific conditions (stimuli) in the internal environment. A receptor may be as small as a cell or even a protein that is part of a cell. 2. A control center, or decision-maker, that includes a set point, which is a particular value, such as body temperature at 37°C (Celsius) or 98.6°F (Fahrenheit). 3. Effectors, such as muscles or glands, take action. They cause appropriate responses. 1 A homeostatic mechanism monitors a particular aspect of the internal environment and corrects any changes back to the value indicated by the set point. 2 Homeostatic mechanisms 2 processes Negative feed back and positive feed back Negative feed back Most homeostatic mechanisms operate by a process called negative feedback. In a negative feedback mechanism, effectors are activated that can return conditions toward normal. As this happens, the deviation from the set point progressively lessens, and the effectors gradually shut down.Such a response is called a negative feedback mechanism because the deviation from the set point is corrected (moves in the opposite or negative direction) and because the correction reduces the action of the effectors. This latter aspect is important because it prevents a correction from going too far. Examples of neagative feed back mechanism Control of body temperature Control of blood glucose level Control of blood pressure etc. 3 Positive feedback In some cases, homeostatic mechanisms operate by positive feedback, in which a change is not reversed but intensified, and the effector activity is initially increased rather than turned off. Examples of positive feed back mechanisms Blood clotting after injury Increase force of uterine contractions during child birth. Positive feedback mechanisms usually produce unstable conditions, which might not seem compatible with homeostasis. i-e positive feed back mechanisms are short lived. 4 Organ systems contribute to homeostasis in different ways. For example, resources brought in by the digestive and respiratory 5 Organ systems contribute to homeostasis in different ways. For example, resources brought in by the digestive and respiratory systems are delivered to all body cells by the cardiovascular system. The same blood that brings needed nutrients to cells carries away waste products, which are removed by the respiratory and urinary systems. Homeostatic mechanisms maintain a relatively constant internal environment, yet physiological values may vary slightly in a person from time to time or from one person to another. Therefore, both normal values for an individual and the idea of a normal range for the general population are clinically important. 6 The homeostatic mechanism that regulates body temperature. 7 Hormone secretion is under negative feedback control. 8 Insulin and glucagon function together to stabilize blood glucose concentration. Negative feedback responding to blood glucose concentration controls the levels of both hormones. 9  The amount of prothrombin activator in the blood is directly proportional to the degree of tissue damage. Once a blood clot begins to form, it promotes additional clotting, because thrombin also acts directly on prothrombin activator, causing prothrombin to form still more thrombin. This type of self-initiating action is an example of a positive feedback mechanism, in which the original action stimulates more of the same type of action. Such a mechanism produces unstable conditions and can operate for only a short time in a living system, because life requires the maintenance of a stable internal environment 10 The fetus is usually positioned head downward, so labor contractions force the head against the cervix. This action stretches the cervix, which elicits a reflex that stimulates still stronger labor contractions. Thus, a positive feedback system operates in which uterine contractions produce more intense uterine contractions until effort is maximal. At the same time, dilation of the cervix reflexly stimulates an increased release of oxytocin from the posterior pituitary gland. As labor continues, positive feedback stimulates abdominal wall muscles to contract, helping to propel the fetus through the birth canal (cervix, vagina, and vulva) to the outside. A positive feedback mechanism operates in the birth process. 11