Homeostasis

Homeostasis (a Greek term meaning same state), is the maintenance of constant conditions in the internal environment of the body despite large swings in the external environment. Functions such as blood pressure, body temperature, respiration rate, and blood glucose levels are maintained within a range of normal values around a set point despite constantly changing external conditions. For instance, when the external temperature drops, the body's homeostatic mechanisms make adjustments that result in the generation of body heat, thereby maintaining the internal temperature at constant levels.

The body's homeostatically cultivated systems are maintained by negative feedback mechanisms, sometimes called negative feedback loops. In negative feedback, any change or deviation from the normal range of function is opposed, or resisted. The change or deviation in the controlled value initiates responses that bring the function of the organ or structure back to within the normal range.

Negative feedback loops have been compared to a thermostatically controlled temperature in a house, where the internal temperature is monitored by a temperature-sensitive gauge in the thermostat. If it is cold outside, eventually the internal temperature of the house drops, as cold air seeps in through the walls. When the temperature drops below the point at which the thermostat is set, the thermostat turns on the furnace. As the temperature within the house rises, the thermostat again senses this change and turns off the furnace when the internal temperature reaches the pre-set point.

Negative feedback loops require a receptor, a control center, and an effector. A receptor is the structure that monitors internal conditions. For instance, the human body has receptors in the blood vessels that monitor the pH of the blood. The blood vessels contain receptors that measure the resistance of blood flow against the vessel walls, thus monitoring blood pressure. Receptors sense changes in function and initiate the body's homeostatic response.

These receptors are connected to a control center that integrates the information fed to it by the receptors. In most homeostatic mechanisms, the control center is the brain. When the brain receives information about a change or deviation in the body's internal conditions, it sends out signals along nerves. These signals prompt the changes in function that correct the deviation and bring the internal conditions back to the normal range.

Figure 1. A negative feedback loop helps regulate blood pressure. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Effectors are muscles, organs, or other structures that receive signals from the brain or control center. When an effector receives a signal from the brain, it changes its function in order to correct the deviation.

An example of a negative feedback loop is the regulation of blood pressure (Figure 1). An increase in blood pressure is detected by receptors in the blood vessels that sense the resistance of blood flow against the vessel walls. The receptors relay a message to the brain, which in turn sends a message to the effectors, the heart and blood vessels. The heart rate decreases and blood vessels increase in diameter, which cause the blood pressure to fall back within the normal range or set point. Conversely, if blood pressure decreases, the receptors relay a message to the brain, which in turn causes the heart rate to increase, and the blood vessels to decrease in diameter. Some set points become "reset" under certain conditions. For instance, during exercise, the blood pressure normally increases. This increase is not abnormal; it is the body's response to the increased demand of oxygen by muscle tissues. When the muscles require more oxygen, the body responds by increasing the blood flow to muscle tissues, thereby increasing blood pressure. This resetting of the normal homeostatic set point is required to meet the increased demand of oxygen by muscles.

Similarly, when the body is deprived of food, the set point of the metabolic rate can become reset to a lower-than-normal value. This lowering of the metabolic rate is the body's attempt to stave off starvation and keep the body functioning at a slower rate. Many people who periodically deprive themselves of food in attempts to lose weight find that after the initial weight loss it becomes increasingly difficult to lose more pounds. This difficulty stems from the lowering of the metabolic set point. Exercise may counteract some of these effects by the increasing metabolic demands.