Homeostasis is a term that refers to constancy in a system. To physiologists, homeostasis means "maintaining a constant internal environment." The internal environment usually is thought of as the extra-cellular fluid (ECF) that constantly bathes the cells, providing nutrients and carrying away wastes. If a system is in homeostasis, it is in its normal, or resting, state. If disturbances disrupt the normal state, the system will act to restore the normal state. For example, a person who is standing still has a normal resting respiratory rate. If that person runs as fast as he/she can for 30 seconds, his/her respiratory rate will increase to meet the body's demand for oxygen. During the run, the body uses more oxygen than it does when it is standing still. To maintain homeostasis, the respiratory rate increases to meet the increased demand. Furthermore, when the person returns to a resting state, the respiratory rate eventually will return to a normal rate because of the decreased demand for oxygen.

The cell is the simplest unit of life. Unicellular organisms typically perform all of life's functions. The cells of a multicellular organism, however, depend on other cells for their health, and also are responsible for the health and proper functioning of other cells and the organism as a whole.  To survive, the cell needs a relatively stable internal milieu. Cells within an organism make up body systems. In turn, body systems respond to some external changes in ways that lessen the impact of those changes on the body, and thus, help to maintain the stable internal environment ("internal milieu") needed by cells. 

Claude Bernard was a French physiologist who usually is credited with developing the concept of the "internal milieu" (internal environment). He astutely observed that animals can regulate their internal environments-and maintain a favorable state-even if the external environment changes.

Walter Cannon was a prominent American scientist who coined the term, "homeostasis," when referring to a constant internal environment. He wrote prolifically on the subject and was a pioneer in the study of the autonomic nervous system's role in maintaining homeostasis.

In biology, homeostasis can describe an individual organism's internal regulation as well as the regulation of an entire population of organisms. This presentation focuses on the individual. If an individual organism is unable to maintain homeostasis (i.e., regulate its body temperature, pH, water balance, ion balance, etc.), it will suffer dire consequences. And how do organisms maintain homeostasis? Some organisms, called regulators, are able to buffer the impact of external changes and thus, maintain the internal environment by using various behavioral and physiological mechanisms. Other organisms, called conformers, live in very stable environments and have not evolved such maintenance mechanisms. Since their environments are very stable, the "cost" of such mechanisms outweighs the benefits gained from them.  These organisms' internal environment can change with the external environment because in these cases the external environment changes very little. A microorganism living in extremely salty conditions is a "conformer." Humans, on the other hand, are "regulators."

To maintain homeostasis, a system must have three components:  1) a receptor, 2) a control center, and 3) an effector.  All of these components do specific jobs that allow an organism to regulate its internal environment.  A receptor detects external changes that could influence the internal environment. It then reports these changes to the control center, which, in turn, will activate an effector, whose function is to restore homeostasis. In the diagram above, the thermometer represents the receptor for this homeostatic system. It communicates with the thermostat (the control center) when there is a change in the external environment. The thermostat responds by directing the fan (the effector) to turn off or on. For example, if the thermometer informs the thermostat of a sudden increase in temperature, the thermostat will direct the fan to turn on to cool the area back to a comfortable temperature.

The central nervous system and the endocrine system are the major control systems in the body.  Each is able to respond to signals from receptors and activate effectors accordingly.

The mechanism known as negative-feedback regulation maintains homeostasis. The "receptor, control center, effector" structure of a homeostatic system (described in the previous slide) enables negative-feedback regulation to occur. When a large external change is detected by the receptor, a signal is sent to, and is interpreted by the control center, resulting in a response by the effector to minimize the internal impact of the large external change. This is negative-feedback regulation. The original external change is counteracted internally so that the internal change is small or nonexistent.

Positive-feedback regulation also is seen in biological systems, but it is not utilized for maintaining homeostasis. Rather, positive-feedback is used to augment a change within an organism. For example, during birth, the uterus contracts to expel the infant. Positive-feedback causes these contractions to continue until the birth is complete. So, whereas negative-feedback helps homeostatic systems to remain fairly constant by responding to changes in the external environment, positive-feedback promotes the change.

Glucose is used by many organisms as fuel, but it is vital that glucose levels be tightly regulated. Too little glucose will lead to starvation, while too much is toxic. Glucose homeostasis is accomplished through highly complex mechanisms involving many different molecules, cell types, and organs.

Briefly, when glucose enters the bloodstream (after the digestion of food), it is detected by specialized cells in the pancreas, called β-cells. These cells respond to the rising blood-glucose concentration by releasing the enzyme, insulin. Insulin then signals to other tissues in the body (i.e., muscle cells and adipose tissue) to take in glucose to be used as energy (in muscle cells) or stored for later use (in adipose tissue). The result is a lowering of blood-glucose concentration to non-toxic levels. 

In times of low glucose intake (between meals or in cases of starvation) the α-cells of the pancreas release the enzyme, glucagon. This enzyme directs the liver to break down stored glycogen into glucose and release this glucose into the bloodstream, thereby raising blood-glucose concentration to a desired level. The glucose transporters expressed on the β- and α-cells that bind glucose are the receptors of this homeostatic system. The β- and α-cells, themselves, are the control centers. They process information from the receptors and respond to it in a way that will maintain a constant internal environment. Insulin and glucagon are the effectors. This system is complex; the β- and α-cells working continuously to achieve the optimal, homeostatic blood-glucose concentration.

Here is a diagram of glucose homeostasis. When we eat food, our blood glucose concentration rises, which stimulates insulin secretion from β-cells and eventual glucose absorption by peripheral tissues. In between meals or in times of starvation, we are not taking in glucose and, therefore, experience a drop in blood glucose. During these times, the α-cells release glucagon, which stimulates the liver to make glucose by glycogenolysis and gluconeogenesis, and thereby raise blood glucose to normal levels.

There are numerous ways to disrupt homeostasis, and the results of this disruption can vary in severity. Failure to achieve or restore homeostasis can result in death, which can be considered the ultimate disruption of homeostasis.

Injuries can have severe homeostatic consequences. A punctured lung, for example, will disrupt the flow of oxygen to the body. Cells in the brain cannot be deprived of oxygen for extended periods of time without dire consequences.

Illness also will cause a temporary disruption of homeostasis. Fever, a common symptom of a cold or flu, is a disruption of the body's constant internal temperature.  It usually is a sign that our body is fighting an infection of some type and, therefore, might be considered a good sign. After the illness subsides, the fever breaks and the normal, constant body temperature is re-achieved. However, an unchecked fever can damage neurons and organs, or even result in death..

Some disruptions in homeostasis are genetic. For example, a disease such as diabetes, to which some people have a genetic predisposition, can disrupt homeostasis. In addition, lifestyle factors such as obesity, lack of exercise, and a fatty diet-which also can disrupt homeostasis-have been shown to enhance one's chances of becoming diabetic. Not surprisingly, an individual with poor health habits is more likely to have problems maintaining homeostasis than a person with good health habits.