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5.3.1 Human endocrine system

5.3.1 Human endocrine system

Hormones Are Chemical Messengers Carried in the Blood

Definition

Endocrine system

A system of glands that release hormones into the bloodstream to control processes in the body.

  1. The endocrine system is made of glands that release chemicals called hormones.
  2. The hormones are secreted directly into the bloodstream.
  3. The blood carries a hormone to a target organ, where it produces an effect.

A diagram showing long-distance signaling where an endocrine cell releases hormones into the bloodstream, which then travel to and affect a distant target cell.

Key Idea

Because hormones travel in the blood, a single gland can affect target organs all over the body.

Slower but Longer-Lasting Than the Nervous System

Definition

Hormone

A chemical messenger made by an endocrine gland and carried in the blood to a target organ, where it has an effect.

  1. The nervous system and the endocrine system both coordinate the body.
  2. Nerve impulses are electrical and very fast, but their effects are short-lived.
  3. Hormones travel more slowly in the blood, but their effects last much longer.
Note

Think of nerves as fast but brief, and hormones as slow but long-lasting.

A comparison of the human nervous system and endocrine system. The nervous system diagram shows the brain, spinal cord, and nerves. The endocrine system diagram shows the locations of major glands including the pituitary, thyroid, adrenal, pancreas, ovaries, and testes.

The Pituitary Gland Is the Master Gland

Definition

Pituitary gland

A master gland in the brain that releases hormones which control other endocrine glands, in response to conditions in the body.

  1. The pituitary gland in the brain is the master gland.
  2. It secretes several hormones into the blood in response to conditions in the body.
  3. Many of these hormones act on other glands, making them release their own hormones.
Example

The pituitary releases a hormone that tells the thyroid gland to release its own hormone.

The Main Endocrine Glands and Where They Are

  1. Pituitary gland: in the brain.
  2. Thyroid: in the neck.
  3. Adrenal glands: one on top of each kidney.
  4. Pancreas: below the stomach.
  5. Ovaries: in the lower abdomen in females.
  6. Testes: in males.
Self review
  • What is the endocrine system, and how does a hormone reach its target organ?
  • How do the effects of hormones compare with the effects of nerves?
  • Why is the pituitary gland called the master gland?
  • Name four endocrine glands other than the pituitary.
  • Where in the body are the adrenal glands found?
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The endocrine system is a collection of glands that produce chemical messengers called hormones. Endocrine glands secrete hormones directly into the bloodstream.

Blood carries each hormone around the body, but the hormone affects only target organs or target cells that can respond to it. This allows a gland to coordinate processes in organs far from where the hormone was released.

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What do endocrine glands release?

5.3.1 Human endocrine system Revision Guide

  1. GCSE
  2. /Biology
  3. /5.3.1 Human endocrine system

Revision notes for AQA GCSE Biology 5.3.1 Human endocrine system. Open the guide for explanations and worked examples. Written against the AQA GCSE Biology (8461) specification, so the content matches what's examinable rather than general Biology background.