Body Temperature Regulation
Consider that you are exercising in the hot sun. When your body temperature increases, you have a homeostatic mechanism to lower it:
- Receptors in the skin detect the increased temperature. They send the information by nerve impulses to temperature centers in the hypothalamus.
- The hypothalamus integrates the information, and sends nerve impulses to the capillaries in the skin and to the sweat glands.
- The blood vessels in the skin become wider, which diverts more blood flow to the skin. The increased blood flow transfers heat, which is lost to the air through the skin. The sweat glands secrete sweat, which evaporates and cools the skin.
- The body temperature decreases to near normal. This continues until you leave the sunny area and stop exercising. The body temperature then returns to normal, and you stop sweating.
Note that this is a negative feedback mechanism.
Conversely, if you go out on a cold day, your body temperature decreases.
- Receptors in the skin detect the decreased temperature. They send the information by nerve impulses to temperature centers in the hypothalamus.
- The hypothalamus integrates the information, and sends nerve impulses to the capillaries in the skin and to skeletal muscles.
- The blood vessels in the skin become narrower, which reduces blood flow to the skin. The decreased blood flow reduces heat lost to the air through the skin. The muscles contract rapidly (shiver), which produces heat.
- These actions increase the body temperature.
Gas exchange in leaves
Homeostatic mechanisms are not limited to animals. Plants have them too.
Here's one example. There are openings in leaves called stoma (plural: stomata). The stoma have guard cells around them. Gases like carbon dioxide, oxygen, and water vapor enter and exit the leaf through these openings. Here's how the mechanism works:
During the day there is enough sunlight for photosynthesis. Photosynthesis in the guard cells causes them to accumulate potassium ions and water (by osmosis). The guard cells swell and open the stoma. As a result, gases can flow into and out of the leaf. Conversely, at night photosynthesis stops. The guard cells remove potassium and water (by osmosis). The guard cells shrink and close the stoma. Subsequently, no gases can move into or out of the leaf.
Images from Microsoft Word 2010.