Homeostasis, << `hoh` mee uh STAY sihs, >> is a term that refers to the ability of a living organism to maintain a stable set of conditions inside its body. In an animal, for example, such internal conditions as body temperature, blood pressure, and the composition of body fluids must remain relatively stable. A change in these internal conditions could result in disease or death.
To maintain homeostasis, an organism must react to its external environment by making internal adjustments. Such adjustments are triggered by homeostatic reflexes. An example of homeostatic reflexes occurs when a person steps outside on a hot day. If the body did not adjust to the heat, body temperature would go up so high that cells, especially brain cells, would die at a massive rate. However, homeostatic reflexes help maintain a constant internal body temperature. When the heat from the sun strikes the skin, nerve endings sense that heat and send a message to the brain. The brain sends nerve impulses that cause the blood vessels in the skin to expand. The resulting increase in blood flow to the skin produces greater heat loss from the skin surface. In addition, the brain instructs the sweat glands to increase production, because the evaporation of sweat cools the skin. The brain also initiates behavioral responses that make the person want to decrease physical activity and seek a cool place.
