Mammalian hormones

Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A hormone is a chemical substance made by a gland and carried by the blood. It changes the activity of one or more specific target organs. Glands that make hormones are called endocrine glands.

You must know which gland makes which hormone. Adrenaline comes from the adrenal glands, which sit on top of the kidneys. It is released when a person is frightened, excited or in danger. It raises the blood glucose concentration and the heart rate, so more glucose and oxygen reach the muscles for respiration. The body is then ready for action.

Compared with nervous control, hormonal control is slower to act but its effects last longer.

Rules to remember

Common mistake

Do not say hormones travel along nerves or that glands send them straight to one organ. Hormones are carried in the blood all around the body, but only the target organs respond.

Worked example

A boy touches a hot iron. His hand pulls away in a fraction of a second, but his heart is still beating fast several minutes later. Which response is nervous and which is hormonal?

  1. Pulling the hand away is very fast and is over at once: this fits nervous control (a reflex).
  2. The shock also makes the adrenal glands release adrenaline into the blood.
  3. Adrenaline raises the heart rate, and it keeps acting until it is removed from the blood, so the effect lasts for minutes.

Answer: The withdrawal of the hand is nervous (fast, short-lasting). The fast heartbeat afterwards is hormonal, caused by adrenaline (slower, longer-lasting).

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