Homeostatic control mechanisms
A homeostatic control system maintains body temperature when room temperature decreases. The "Begin" sign indicates where to start. The arrows next to each term within the boxes denote increases or decreases. The arrows connecting any two boxes in the figure denote cause and effect; that is, an arrow can be read as "causes" or "leads to." (For example, decreased room temperature "leads to" increased heat loss from the body.) In general, you should add the words "tends to" in thinking about these cause and- effect relationships. For example, decreased room temperature tends to cause an increase in heat loss from the body, and curling up tends to cause a decrease in heat loss from the body. Qualifying the relationship in this way is necessary because variables like heat production and heat loss are under the influence of many factors, some of which oppose each other.
The activities of cells, tissues, and organs must be regulated and integrated with each other so that any change in the extracellular fluid initiates a reaction to correct the change. The compensating mechanisms that mediate such responses are performed by homeostatic control systems.
Consider an example of the regulation of body temperature. Our subject is, a resting, lightly clad man in a room having a temperature of 20°C and moderate humidity. His internal body temperature is 37°C, and he is losing heat to the external environment because it is at a lower temperature.
However, the chemical reactions occurring within the cells of his body are producing heat at a rate equal to the rate of heat loss.
Under these conditions, the body undergoes no net gain or loss of heat, and the body temperature remains constant. The system is in a steady state, defined as a system in which a particular variable-temperature, in this case-is not changing, but energy-in this case, heat-must be added continuously to maintain a constant condition.
Steady state differs from equilibrium, in which a particular variable is not changing but no input of energy is required to maintain the constancy. The steady-state temperature in our example is known as the set point, sometimes termed the operating point, of the thermoregulatory system.
This example illustrates a crucial generalization about homeostasis. Stability of an internal environmental variable is achieved by the balancing of inputs and outputs. In the previous example, the variable (body temperature) remains constant because metabolic heat production (input) equals heat loss from the body (output).