Endocrine glands release hormones

1. An endocrine gland is a gland that secretes hormones directly into the blood.
2. A hormone is a chemical messenger produced by an endocrine gland and carried in the blood to a target organ, where it causes a response.
3. A target organ is an organ containing cells that respond to a particular hormone.
4. Endocrine glands have no ducts, so their hormones enter nearby capillaries and join the circulation.

Hormones reach their target organs

1. The bloodstream carries a hormone away from the gland and around the body.
2. Hormones travel dissolved in the blood plasma, rather than inside red blood cells.

Target organs respond to hormones

1. Blood delivers each hormone to its target organ, where it produces its specific effect.
2. A hormone can circulate around the whole body, but only its target organs respond to it.

Nervous system vs Endocrine system

1. The nervous system is formed by the brain, spinal cord and nerves, and it coordinates rapid responses using electrical impulses.
2. Nervous control sends electrical impulses along neurones, whereas hormonal control sends chemical hormones through the blood.
3. Nervous responses are usually very fast because impulses travel directly along neurones to an effector.
4. Hormonal responses are usually slower because hormones must be secreted and carried through the circulation.
5. Nervous effects are usually short-lived because the impulses stop once the response is no longer required.
6. Hormonal effects can be long-lasting because hormones remain in the blood and may continue to affect target organs.
7. A nervous impulse travels to a specific effector, while a hormone circulates widely but affects only organs that respond to that hormone.
8. Nervous control is suited to immediate actions such as muscle contraction, while hormonal control is suited to longer processes such as growth, puberty and metabolic control.

Choose control by speed and duration

1. A nerve impulse is an electrical signal that travels along a neurone.
2. Touching a hot surface needs nervous control because the response must reach the arm muscles quickly and stop after the hand moves away.
3. Growth during puberty needs hormonal control because it develops slowly and continues over a long period.

Main glands and their hormones

1. The pituitary gland lies at the base of the brain and releases hormones that regulate several other endocrine glands.
2. The thyroid gland lies in the neck and secretes thyroxine.
3. The pancreas lies in the upper abdomen and secretes insulin and glucagon.
4. The adrenal glands sit above the kidneys and secrete adrenaline.
5. The ovaries lie in the pelvis and secrete oestrogen and progesterone.
6. The testes lie in the scrotum and secrete testosterone.