Homeostasis

As an organism moves from place to place or as the climate changes, vital factors in its external environment change too. However if the organism is to survive, the condition within its body must remain relatively stable. 

In other word body parts function efficiently only when their supplies of water, food and oxygen and the conditions of heat and pressure remain within certain limits. Biologists use the term homeostasis to describe the maintenance of such a stable internal environment.

To better understand this idea, imagine a room equipped with a furnace and an air conditioner. Suppose the room temperature is to remain near 20֯C, so the thermostat is adjusted to a set point of 20֯C. Since a thermostat is sensitive to temperature changes, one it is adjusted it will signal the furnace to start whenever the room temperature drops below the set point. If the temperature rises above the set point, the thermostat will cause the furnace to stop and the air conditioner to start. As a result, a relatively constant temperature will be maintained in the room.

A homeostatic mechanism is the human body achieves similar results in regulating body temperature. The thermostat is a temperature- sensitive region of the brain. In healthy persons the set point of this region is at or near 37֯C.

If a person is exposed to a cold environment and the body temperature begins to drop, the brain’s thermostat can sense this change and trigger heat generating and heat-conserving activities. 

For Example, small groups of muscles may be stimulated to contract involuntarily, an action called shivering. Such muscular contractions produce heat as a by-product, which helps to warm the body. At the same time, blood vessels in the skin may be signaled to constrict so that less warm blood reaches the skin, and heat that might otherwise be lost is held in deeper tissues.

If a person is becoming overheated the brain may trigger a series of changes that leads to increased loss of body heat. It may stimulate the sweat glands of the skin to secrete watery perspiration; as the water evaporates from the surface, some heat is carried away and the skin is cooled. Also blood vessels in the skin may be caused to dilate so blood from deeper tissues reaches the surface in greater volume and loses some of its heat to the outside. 

Homeostasis

Another homeostatic mechanism functions to regulate blood pressure in vessels leading away from the heart. In this instance, pressure-sensitive parts in the walls of these vessels are stimulated if the blood pressure increase above normal. 

When this happens, the sensors signal the brain, and it signals the heart, causing the heart chambers to contract more slowly and with less force. Because of decreased heart action, less blood enters the blood vessels, and the pressure inside the vessels decreases. If the blood pressure is dropping below normal, the brain signals the heart to contract more rapidly and with greater force so that the pressure in the vessels increases. 

In these examples, homeostasis is maintained by a self-regulating control mechanism that can sense changes away from the normal set point and can cause reactions that tend to return conditions to normal. 

Since the changes away from normal stimulate changes away from normal stimulate changes to occur in the opposite direction, the response are called negative, and the homeostatic control mechanism is said to act by a process of negative feedback.