25.7 Homeostasis

The constant conditions which are maintained in the body might be termed equilibria. That word, however, has come to have a fairly exact meaning as applied to relatively simple physico-chemical states, in closed systems, where known forces are balanced. The coordinated physiological processes which maintain most of the steady states in the organism are so complex, and so peculiar to living beings - involving, as they may, the brain and nerves, the heart, lungs, kidneys and spleen, all working cooperatively - that I have suggested a special designation for these states: homeostasis . The word does not imply something set and immobile, a stagnation. It means a condition - a condition which may vary, but which is relatively constant.

Cannon was an American physiologist who coined and popularized the concept of homeostasis to describe the observations that animals could maintain stable internal body conditions even when the external conditions changed. Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.

Homeostatic process

The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point . While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If blood glucose concentration rises after a meal, adjustments are made to lower the blood glucose level, increasing uptake of glucose from blood into various tissues where it can be converted to storage products like glycogen or triglyceride.

Control of homeostasis

When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals, as described by Cannon in the 1930's.

Negative feedback mechanisms

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop . It can either cause an increase, or a decrease, in the level of the stimulus that triggered the response, In all cases the response is in the opposite direction of the change in the stimulus. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels, as mentioned above. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in [link] . However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released, causing glucose levels to increase. This is still a negative feedback loop, which is defined as a situation where a change in one direction is countered by a response in the opposite direction. Negative feedback loops are the predominant mechanism used in homeostasis.