Homeostasis (ho' me-o-sta' sis) means that the human body's internal environment remains relatively constant, regardless of the conditions in the external environment. For example:
- Blood glucose concentration remains at about 0.1 %
- The pH of the blood is always near 7.4
- Blood pressure in the brachial artery averages near 120/80
- Blood temperature averages around 37°C (98.6°F)
Although we are accustomed to using the word environment to mean the external environment of the body, it is important to realize that the body's internal environment, consisting of blood and the tissue fluid that bathes the cells, is ultimately responsible for our health and well-being. The body's ability to keep the internal environment within a certain range allows humans to live in a variety of habitats, such as arctic, desert, or tropical regions.
Homeostasis is the relative constancy of the body's internal environment, which is composed of blood and the tissue fluid that bathes the cells.
Most systems of the body contribute to maintaining a constant internal environment. The digestive system takes in and digests food, providing nutrient molecules that enter the blood and replace the nutrients constantly being used up by body cells. The respiratory system adds oxygen to the blood and removes carbon dioxide. The amount of oxygen taken in and carbon dioxide given off can be increased to meet bodily needs. The chief regulators of blood composition, however, are the liver and the kidneys. They monitor the blood's chemical composition and alter it as required. Immediately after glucose enters the blood, the liver can remove the glucose for storage as glycogen. Later, glycogen can be broken down to replace the glucose used by body cells; in this way, the blood's glucose composition remains constant. The hormone insulin, secreted by the pancreas, regulates glycogen storage. The liver also removes toxic chemicals, such as ingested alcohol and drugs. These substances are converted to molecules that can be excreted by the kidneys, organs that are also under hormonal control.
All of the body's organ systems contribute to homeostasis, Some, like the respiratory, digestive, and urinary systems, remove and/or add substances to blood.
Although homeostasis is, to a degree, controlled by hormones, the nervous system has ultimate control. The brain contains centers that regulate such factors as temperature and blood pressure. Maintaining proper temperature and blood pressure levels requires a receptor that detects unacceptable levels and signals a regulatory center. If a correction is required, the regulatory center then turns on an effector. The effector brings about a response that negates the original conditions that stimulated the receptor. In the absence of suitable stimulation, the receptor no longer signals the regulatory center. Therefore, this is called a negative feedback mechanism.
A useful analogy that helps illustrate how such a negative feedback mechanism works is the control of the heating system of a house by a thermostat. This negative feedback mechanism keeps the indoor temperature within a relatively narrow range. In a similar way, body temperature in humans is controlled by a portion of the brain that functions much like a thermostat, allowing slight fluctuations within narrow limits.
The nervous and endocrine systems regulate the activities of other systems. Negative feedback is a self-regulatory mechanism by which systems and conditions of the body are controlled.
Disease represents a breakdown or upset in this normal self-regulation. When homeostasis fails, the body (or part of the body) no longer functions properly. The effects may be limited or widespread. A local disease is more or less restricted to a specific part of the body. On the other and, a systemic disease affects the entire body or involves several organ systems. Diseases may also be classified on the basis of their severity and duration. Acute diseases occur suddenly and generally last a short time. Chronic diseases tend to be less severe, develop slowly, and are long-term.
Disease represents an upset in homeostasis.
