Concepts of Lifetime Fitness September 1, 1997 Homeostasis is the state of equilibrium in which the internal environment of the human body remains relatively constant. Two excellent examples of homeostasis are how the body maintains a constant temperature and blood pressure during strenuous physical activity or exercise. Although there are many other activities in the body that display homeostasis, I will only discuss these two. Temperature in the human body is usually kept at approximately 37 degrees Celsius. To maintain such a strict temperature, the body has a few functions to combat the outside elements. People cannot make themselves cold as readily as make themselves hot, however I will mention both homeostasis functions.

When the external temperature decreases, a portion of the brain called the hypothalamus detects the drop by means of the blood. To compensate, the brain sends chemical and electrical impulses to the muscles. These impulses tell the muscles to begin to contract and relax at very high intervals. This is commonly known as shivering.

The production of Adenosine Triphosphate or ATP in the mitochondria of the muscles produces heat. If the body temperature does not rise immediately after this, then a second function begins. The brain will signal the blood vessels near the skin to constrict or narrow in diameter. This occurs so the heat deep in the muscles is conserved. Since the vessels are now smaller in diameter, less blood is needed to fill them. Since less blood is needed through the vessels, the heart begins to slow.

If the body remains in this slowed state, hypothermia could result. Hypothermia is the condition in which metabolic processes are inhibited. The medical world has taken advantage of this by inducing hypothermia in patients that are undergoing organ transplants. To fight temperatures higher than normal, as in exercise or on hot days, the body reacts in the opposite way than with cold.

Again, the hypothalamus detects the change of temperature in the blood. The brain signals blood vessels not to constrict, but to dilate. This increases the diameter of the vessels, and results in the need for more blood. Since more blood is needed to fill the vessels, the heart pumps faster and that causes respiration to increase. The increased respiration will make the body exhale some of the internal heat, like placing a fan in a window to cool a room. The blood vessels are dilated so the heat deep in the muscles is easily released.

Another commonly known mechanism to fight heat is sweating. Sweat glands found throughout the body are stimulated by the hypothalamus to excrete sweat and when the sweat evaporates, the skin is cooled. If the body is not cooled by the time all of the internal water supply is used, it could go into hypothermia. This is when the body becomes dehydrated and proteins begin to denature. Hypothermia can result in certain death if the water supply is not immediately replenished.

Some advantages to these mechanisms are the cleansing effect of sweating and weight loss. Sweat, when excreted, removes waste materials such as bacteria and water. Fat material, during exercise, is actually 'eaten' by the body thus reducing overall weight. The second example of homeostasis is blood pressure regulation. When the hydrostatic pressure of blood is above normal, pressure sensors in the blood vessels tell the brain through chemical means. The brain will then stimulate the heart to contract or beat in slower intervals.

This will cause less blood to enter the blood vessels and that will lower the hydrostatic pressure. If the pressure is lower than normal, the exact opposite happens. The sensors in the vessels tell the brain and the brain will then make the heart beat faster so more blood enters the vessels and the pressure is raised. The body uses many mechanisms to regulate temperature and blood pressure. Be it stimuli to the heart from the brain or messages from the blood, the body maintains its internal environment through a process called homeostasis.