Hormones and the endocrine systemIB MYP Biology: Revision notes

A hormone is a chemical messenger. It is made by an endocrine gland, released directly into the blood, and carried around the body to its target organs, where it changes how they work.

The glands and the hormones they release make up the endocrine system. Because hormones travel in the blood, the effects are usually slower than nerve impulses but last longer.

• Pituitary gland (base of the brain): releases several hormones, including ADH, and some control other glands.
• Thyroid gland (neck): releases a hormone that controls the rate of the body's chemical reactions (metabolic rate).
• Pancreas (below the stomach): releases insulin.
• Adrenal glands (above the kidneys): release adrenaline.
• Ovaries (female): release oestrogen.
• Testes (male): release testosterone.

Insulin: after a meal the blood glucose concentration rises. The pancreas releases insulin, which makes the liver and muscle cells take up glucose, so the blood glucose concentration falls back to normal.

Adrenaline: in a frightening or exciting situation the adrenal glands release adrenaline. It increases the heart rate and breathing rate and raises the glucose available to the muscles, so more oxygen and glucose reach the muscles. This prepares the body for action, often called the 'fight or flight' response.

ADH is released by the pituitary gland when the blood becomes too concentrated (for example after sweating or too little drinking). It travels to the kidneys and makes the kidney tubules reabsorb more water. The urine becomes small in volume and concentrated. When the blood is too dilute, less ADH is released and a large volume of dilute urine is made.

Oestrogen (ovaries) and testosterone (testes) are the sex hormones. They cause the changes of puberty such as the development of secondary sexual characteristics. Oestrogen also controls the menstrual cycle and testosterone controls sperm production.

• Message: nervous uses electrical impulses; hormonal uses chemical hormones
• Travels by: nervous along neurones; hormonal in the blood
• Speed: nervous is very fast; hormonal is slower
• Duration: nervous effects are short; hormonal effects are long-lasting
• Area affected: nervous is usually one precise part; hormonal is often several organs

Nervous control suits quick responses, such as pulling away from danger. Hormonal control suits slower, longer-lasting processes, such as puberty and controlling blood glucose.