Year 1 · Senses, behaviour and coordination · Section 5.6

Coordination and control

Hormones and blood glucose

The body controls many processes through the nervous system and the endocrine system. The endocrine system includes hormone-producing glands and tissues such as the pituitary gland, thyroid gland, pancreatic islets, adrenal glands, ovaries and testes.

These release hormones into the blood. A hormone affects only cells with suitable receptors, called its target cells.

Controlling blood glucose

Glucose is a sugar absorbed into the blood from digested food and used as fuel by cells. Blood glucose level means its concentration in the blood. The hormones insulin and glucagon, produced in the islets of Langerhans in the pancreas, help keep it within a suitable range.

• After a meal, blood glucose rises and insulin release increases.
• Insulin encourages cells to take up glucose and promotes its storage as glycogen in the liver and muscles. Blood glucose falls.
• When blood glucose falls, glucagon release increases.
• Glucagon stimulates the liver to break down glycogen and release glucose into the blood. Blood glucose rises.

Muscle glycogen is mainly used by the muscles themselves. Glucagon acts chiefly on the liver to help raise blood glucose.

Feedback

Blood glucose control is an example of negative feedback: a change triggers responses that oppose that change.

• Rising blood glucose increases insulin release, helping lower blood glucose.
• Falling blood glucose increases glucagon release, helping raise blood glucose.

This continual adjustment limits fluctuations rather than keeping the level exactly constant.

Adrenaline

The adrenal glands, above the kidneys, release adrenaline during situations such as fear or sudden stress. Its effects are rapid and often short-lived:

• It stimulates the release of glucose from liver stores, raising blood glucose.
• It increases heart rate and helps increase breathing.
• It prepares the body for a fight-or-flight response.

Adrenaline is a clear example of a fast-acting hormone, but it is not the only hormone that can act quickly.