Homeostasis is the process by which the body regulates its internal environment for chemical and biological processes to occur. Some of the more important variables that need to be controlled include temperature, and the levels of blood sugar, oxygen and carbon dioxide. A number of organs are involved in homeostasis, and these include the lungs, pancreas, kidneys and skin.
Respiration is the process by which glucose is used to create energy. It is the most important reaction taking place within the human body and allows the creation of energy. Critical to the respiration process is the regulation of oxygen levels within the blood, which is carried out by the lungs. As respiration takes place within the body, carbon dioxide is produced and released into the blood. The level of carbon dioxide is used as an indirect measure of blood oxygen levels. Special cells in the brain detect the carbon dioxide levels in the blood, and if it is too high, nerve impulses are sent to stimulate the muscles that control breathing. The lungs then fill with air faster, increasing the amount of oxygen in the bloodstream. If carbon dioxide levels within the blood are low, the brain cells do not stimulate nerve cells, reducing the rate of breathing.
The regulation of blood-glucose levels is essential for the survival of the human body. The pancreas, a small glandular organ located close to the stomach, has a number of functions. One of the most important is the regulation of blood-sugar levels. The pancreas contains special cells known as the Islets of Langerhans that detect blood-glucose levels. If the blood-glucose levels are too high, the cells release the hormone insulin to stimulate liver, muscle and fats cells to absorb glucose from the blood and store it as glycogen, or starch. When blood sugar levels are too low, the cells release another hormone called glucagon. Glucagon acts on the liver, muscle and fat cells and stimulates them to convert glycogen to glucose, releasing it into the blood.
Water acts as an essential solvent that allows glucose, salt and other chemicals to travel throughout the body. Kidneys regulate the amount of water present in the human body. When the level of water in the bloodstream becomes too low, the hypothalamus in the brain releases a large quantity of the chemical anti-diuretic hormone, ADH. ADH travels through the blood and stimulates the kidneys to open water channels within its tubule walls, allowing water to diffuse back into nearby blood vessels and reducing the amount of water in urine. When too much water is present in the blood, smaller amounts of ADH are released. This leads the kidneys to close water channels within the tubule walls, increasing the amount of water in urine.
The body's temperature is tuned to approximately 37 degrees Celsius (98.6 Fahrenheit), which allows the body's biological enzymes to function at optimum level. When the body temperature rises, the hypothalamus sends nerve signals to sweat-producing cells in the skin. The body can sweat one to two liters of water per hour, which helps to cool the body. The skin also has tiny muscles on its surface called arrector pili. These muscles control the orientation of hairs on the skin. When the body is too hot, the muscles relax and the hairs lay flat to release heat. When the body is too cold, the arrector pili muscles contract, leading the hairs of the skin to stand up and insulate the body.