Homeostasis

Cells function best when conditions inside the body are kept stable. Homeostasis is the maintenance of a constant internal environment. Conditions can change, but homeostasis helps keep them within suitable limits.

One condition controlled in this way is blood glucose concentration. When it rises, the pancreas releases insulin. Insulin causes body cells to take up more glucose from the blood, reducing the blood glucose concentration and helping to keep it stable.

Negative feedback in homeostasis

An internal condition can change as the body or its surroundings change. Negative feedback helps keep that condition close to a normal target value, called its set point.

When the condition moves away from its set point, the change is detected and triggers responses that oppose the change. If the condition rises above its set point, the responses act to lower it. If it falls below its set point, the responses act to raise it.

As the condition returns towards the set point, the difference from the target becomes smaller. The stimulus for the responses therefore diminishes, so the responses decrease rather than continuing at the same strength.

Negative feedback maintains relative stability, not a condition fixed at exactly one value. The condition can vary around its set point, while changes away from it trigger responses that counteract them.

Control of blood glucose by insulin and glucagon

The hormones insulin and glucagon help keep blood glucose concentration stable. They have opposing effects on the liver: insulin promotes the storage of glucose, while glucagon promotes its release into the blood.

When blood glucose rises

When blood glucose concentration rises, the pancreas releases insulin. Insulin causes the liver to take up glucose from the blood and convert it into glycogen, a storage form of glucose. This lowers the blood glucose concentration.

When blood glucose falls

When blood glucose concentration falls, the pancreas releases glucagon. Glucagon causes the liver to break down stored glycogen into glucose and release the glucose into the blood. This raises the blood glucose concentration.

Each response counters the original change: insulin acts when blood glucose rises, while glucagon acts when it falls.

Treatment of Type 1 diabetes

In Type 1 diabetes, the body produces little or no insulin. Insulin is needed to help control blood glucose concentration, so it must be supplied as a treatment. Without enough insulin, blood glucose can rise too high.

Insulin can be supplied by regular injections or by an insulin pump, which delivers insulin into the body. Both methods provide insulin that the body cannot produce in sufficient amounts.

Monitoring blood glucose and adjusting insulin

Blood glucose is monitored to help guide insulin doses. The amount of insulin needed varies with food intake and physical activity: food can raise blood glucose, while physical activity can lower it. Monitoring helps a person adjust insulin in relation to these changes, reducing the risk of blood glucose becoming too high or too low.

Supplied insulin manages Type 1 diabetes; it does not restore the body’s own insulin production. Treatment must therefore continue to provide the insulin the body needs.

Body temperature regulation in mammals

A mammal’s internal body temperature can change when it gains or loses heat. The brain coordinates responses that oppose these changes, keeping the internal temperature relatively constant rather than allowing it to follow every change in the surroundings.

Responses when the body is too warm

Sweat glands release sweat onto the skin. As the sweat evaporates, it takes heat from the skin, cooling the body.

Arterioles supplying capillaries near the skin surface dilate (widen). This increases blood flow near the surface, so more heat is transferred from the blood to the surroundings.

Responses when the body is too cold

Shivering produces heat through rapid, repeated muscle contractions.

Arterioles supplying capillaries near the skin surface constrict (narrow). This reduces blood flow near the surface, limiting heat loss to the surroundings.

These warming and cooling responses act in opposite directions: each opposes the departure from the normal internal temperature.