Homeostasis

On a freezing morning and on a hot afternoon, the temperature inside your body is nearly the same, about 37 degrees Celsius (98.6 degrees Fahrenheit). How does your body hold steady while the world around it swings?

The answer is homeostasis: the body's ability to keep its internal conditions, such as temperature, blood sugar, and water balance, within the narrow range that cells need to work. Homeostasis does not mean nothing ever changes. Conditions drift a little, and the body brings them back toward a set point.

Most of this works through negative feedback loops, which have three parts. A receptor detects a change. A control center, often the brain, compares it with the set point. An effector, such as a muscle or gland, acts to reverse the change. A home thermostat does the same: the thermometer is the receptor, the controller is the control center, and the furnace or air conditioner is the effector. "Negative" does not mean bad. It means the response works against the change.

Body temperature shows the loop. When you overheat, receptors tell the brain, and the brain signals your skin. Blood vessels near the surface widen to release heat, and sweat glands release sweat, which cools you as it evaporates. When you get cold, the vessels near the surface narrow to lose less heat, and your muscles shiver, which releases heat. Each response pushes temperature back toward the set point. Once the temperature is back in range, the receptors stop signaling and the responses fade, so the loop switches itself off, just as a thermostat shuts off the furnace when the room warms up.

Blood sugar is controlled the same way, by two hormones from the pancreas. After a meal, blood sugar rises, and the pancreas releases insulin, which helps cells take in sugar and the liver store it, so blood sugar falls. If blood sugar drops too low, the pancreas releases glucagon, which makes the liver release stored sugar, so blood sugar rises. In diabetes this loop does not work properly, and blood sugar can stay too high or too low.

Notice how many systems you have studied are involved: nervous, endocrine, muscular, circulatory, and integumentary. Homeostasis is what systems working together actually accomplish. Next lesson: choices that help keep your body healthy.