During dehydration, blood volume decreases and hence, remaining blood plasma becomes highly concentrated. This physiological changes in the blood volume provide stimulation to the osmoreceptors. These receptors are present in the hypothalamus of the brain, located immediately above the pituitary gland. These osmoreceptors are important in promoting the feeling of thirst and also, they promote the release of antidiuretic hormone (ADH) from pituitary gland. ADH is a hormone secret from posterior pituitary gland. It is also called vasopressin. Generally, in response to an increased osmotic concentration of blood plasma, its secrets. In turn, ADH stimulates kidney to retain more amount of water to keep the body hydrated and to prevent more water loss by excretion. Hence, less amount of water is excreted in urine and due to the feeling of thirst, a dehydrated person drinks more water.
This is a good example of maintaining homeostasis in a biological system. The temperature of the body is regulated by the hypothalamus, a region of the brain. The normal body temperature of a human is around 37°Cor 98.6°F. This value is affected by various factors such as exposure to sunlight, hormones level in the body, metabolic rate and disease condition. Based on these factors, body temperature can go to an excessively high or low value depending upon the condition. This fluctuating body temperature triggers the body's feedback mechanism. This feedback mechanism is carried out through the bloodstream to the brain and that ultimately results in adjustments in breathing rate, the blood sugar level, and the metabolic rate. This is a part of compensatory mechanisms or adjustments. Increase sweating is also a part of this feedback adjustment. Heat loss is also aided by a decrease in activity and by heat exchange mechanism that permits a high amount of blood to circulate near the skin surface. In the case of winter, the body's heat loss is high. This heat loss can be prevented by insulation and also decreased blood circulation to the skin. Homeostatic condition lies between this high and low level of temperature and it is the normal range that sustains life. This is an example of a body's feedback mechanism and it is activated whenever condition approaches any side of extremes.