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Homeostasis is the ability of the body to maintain relative stability and function even though drastic changes may take place in the external environment or in one portion of the body.  Homeostasis is maintained by a series of control mechanisms, some functioning at the organ or tissue level and others centrally controlled.  The major central homeostatic controls are the nervous and endocrine systems.

We are continually challenged by physical and mental stresses, injury, and disease, any which can interfere with homeostasis.  When the body loses its homeostasis, it may plunge out of control, into dysfunction, illness, and even death.  Homeostasis at the tissue, organ, organ system, and organism levels reflects the combined and coordinated actions of many cells.  Each cell contributes to maintaining homeostasis.

To maintain homeostasis, the body reacts to an abnormal change (induced by a toxin, biological organism, or other stress) and makes certain adjustments to counter the change (a defense mechanism).  The primary components responsible for the maintenance of homeostasis are:

An example of a homeostatic mechanism can be illustrated by the body's reaction to a toxin that causes anemia and hypoxia (low tissue oxygen) (See illustration).  Erythropoiesis (production of red blood cells) is controlled primarily by the hormone, erythropoietin.  Hypoxia (the stimulus) interacts with the heme protein (the receptor) that signals the kidney to produce erythropoietin (the effector).  This, in turn, stimulates the bone marrow to increase red blood cells and hemoglobin, raising the ability of the blood to transport oxygen and thus raise the tissue oxygen levels in the blood and other tissues.  This rise in tissue oxygen levels serves to suppress further erythropoietin synthesis (feed back mechanism).  In this example, it can be seen that cells and chemicals interact to produce changes that can either perturb homeostasis or restore homeostasis.  In this example, toxins that damage the kidney can interfere with production of erythropoietin or toxins that damage the bone marrow can prevent the production of red blood cells.  This interferes with the homeostatic mechanism described resulting in anemia.

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