The endocrine system is composed of glands which secrete chemicals called…

The idea

The endocrine system is a set of ductless glands that secrete hormones straight into the blood. Blood plasma carries each hormone throughout the body, but an effect occurs only at a target organ whose cells have receptors that bind that hormone. For example, the adrenal glands secrete adrenaline into the blood; adrenaline reaches the heart, where it increases heart rate, and the liver, where it stimulates glycogen breakdown to release glucose. Because transport is by mass flow of blood and cells must be reached and respond, hormonal effects begin more slowly than nerve impulses. However, hormones persist in the blood and are removed gradually, so their effects act for longer and can be widespread. This contrasts with the nervous system, where electrical impulses travel along neurones and produce rapid, short-lived, precisely targeted responses.

What earns the marks

• Endocrine glands are ductless and secrete hormones directly into the bloodstream.
• Blood plasma transports hormones around the body to target organs.
• A target organ has cells with receptors that the hormone binds to, producing a specific effect.
• Hormonal effects are slower in onset than nervous effects because hormones must travel in the blood and reach target cells.
• Hormonal effects act for longer than nervous effects because hormones persist in the blood and are broken down gradually.
• A valid example is adrenaline from the adrenal glands increasing heart rate and blood glucose concentration.

Common mistakes and how to put them right

• Stating that hormones are carried by nerves.

  Put it right by saying hormones are secreted into the bloodstream and carried in the plasma.

• Writing that hormones affect every cell equally.

  Put it right by explaining that only target organs with complementary receptors respond.

• Confusing slower onset with weaker effect.

  Put it right by stating that hormonal effects are slower to start but often longer lasting and can be widespread.

Exam tips

• Use a three-part chain: gland secretes hormone, blood carries it, target organ responds.
• For comparison questions, pair each difference with a reason, such as slower because transport is by blood rather than along neurones.
• Name one gland, its hormone and its target organ effect to make the description concrete.