13.4 Homeostasis

13.4 Homeostasis

Homeostasis is the maintenance of a relatively steady internal environment while the outside world and the animal's own activity keep changing. Two variables carry this topic: body temperature and blood glucose. Both are ordinarily held near a set point by negative feedback. A few biological events use positive feedback instead. Those events amplify a change until some other event stops them. They are not the mechanism that keeps human body temperature steady.

Negative feedback and positive feedback

In negative feedback, a move away from the set point triggers a response that opposes the move. A sensor measures the variable. A control center compares the measurement with the set point. An effector then pushes the variable back. As the variable returns, the corrective response fades. The system stabilizes instead of running away. Temperature control and glucose control both work this way in a healthy person.

Positive feedback does a different job. The response increases the change, so the process runs harder until something outside the loop interrupts it. Two examples are enough, and neither one is everyday temperature control. During labor, stretch of the cervix promotes release of oxytocin from the posterior pituitary. Oxytocin strengthens uterine contractions, and stronger contractions increase the stretch. The loop amplifies until delivery removes the stimulus. During blood clotting, activated platelets and clotting factors recruit more platelets and more clotting activity, so a small vessel injury becomes a sealed clot instead of fading after the first few molecules react. Positive feedback is real, fast, and useful for those events. It is a poor way to hold a variable near a middle value for hours, which is why it is not how body temperature is kept steady and not how blood glucose returns to normal after a meal.

Thermoregulation

Endotherms and ectotherms

Thermoregulation is the control of body temperature. Endotherms, including humans and other mammals and birds, generate most of their body heat by metabolism. Ectotherms, including most fishes, amphibians, and reptiles, depend more on the environment. A lizard can warm by basking in the sun and can cool by moving into shade. An endotherm can remain active in a cold room because its tissues release heat, but that independence costs food energy. The labels name the main heat source. They do not mean that an ectotherm has no metabolism, or that an endotherm ignores air temperature, wind, and sun.

Cooling and warming in the human body

In humans, temperature sensors and a hypothalamic control center compare blood temperature with the set point. If the blood is too warm, cooling responses turn on. Vasodilation of vessels in the skin sends more blood to the body surface, so heat leaves by radiation and by moving air. Sweating spreads water on the skin, and evaporation carries heat away. If the blood is too cool, warming responses turn on. Vasoconstriction of skin vessels reduces surface blood flow, so less heat escapes. Shivering is rhythmic involuntary contraction of skeletal muscle, and those contractions release metabolic heat.

These responses are negative feedback. Sweating removes heat, then decreases as temperature returns toward the set point. Sweating is not a positive-feedback loop that keeps heating the body.

Blood glucose

Glucose dissolved in the blood is a fuel supply for the brain and for other tissues. The pancreas keeps that supply near its set point with two hormones from the pancreatic islets, and the two hormones pull in opposite directions.

Insulin lowers the glucose level

Beta cells release insulin when blood glucose rises. A meal rich in starch or sugar is the everyday cause. Insulin lowers blood glucose. It promotes uptake of glucose into cells, especially skeletal muscle and adipose tissue, and it promotes storage. Under insulin's influence the liver builds glycogen from glucose, and adipose tissue can store surplus energy as fat. Insulin also reduces the liver's release of newly made glucose. The combined effect moves sugar out of the blood and into cells and stores. Insulin does not raise blood glucose. Any account that gives insulin the job of increasing blood sugar has the hormone backward.

Glucagon raises the glucose level

Alpha cells release glucagon when blood glucose falls, for example during a fast or prolonged exercise. Glucagon raises blood glucose mainly by promoting breakdown of liver glycogen. The liver frees glucose from that stored polymer and releases it into the blood. Glucagon is not insulin, and it is not secreted by beta cells. Skeletal muscle stores glycogen too, but muscle does not release glucose into the blood the way the liver does. For this feedback loop, connect glucagon to liver glycogen breakdown.

After a sugary meal

Consider a person who finishes a large sweet drink. Glucose is absorbed from the small intestine, and blood glucose climbs above the set point. Beta cells sense the rise and secrete insulin. Muscle and adipose tissue take more glucose in, and the liver stores more of it as glycogen. Blood glucose falls back toward the set point. As it falls, the stimulus for further insulin release fades. That taper is negative feedback, not a runaway drop organized by positive feedback. Glucagon does not surge in order to push the glucose still higher after the drink. If glucose later dips below the set point, alpha cells release glucagon, liver glycogen breaks down, and glucose rises again. The two hormones bracket the set point. Each corrects a departure in one direction.