The human endocrine system

Homeostasis and response · Hormonal coordination in humans · note 1 of 11

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The human endocrine systemSpec 4.5.3.1

In short

The endocrine system is made of glands that secrete hormones directly into the bloodstream. The blood carries each hormone to a target organ, where it produces an effect. Compared with the nervous system, its effects are slower but act for longer. The pituitary gland is a master gland: its hormones stimulate other glands to release hormones.

The endocrine system is made of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect.

Nervous system and endocrine system compared
	Nervous system	Endocrine system
Message carried by	Electrical impulses along neurones	Hormones (chemicals) in the blood
Speed of effect	Fast	Slower
How long the effect lasts	Short	Acts for longer

The pituitary gland in the brain is a ‘master gland’. It secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.

Positions of the endocrine glands
Gland	Position in the body
Pituitary gland	At the base of the brain
Thyroid	In the neck
Adrenal glands	One on top of each kidney
Pancreas	In the abdomen, just below the stomach
Ovary	In the female pelvis, one on each side of the uterus
Testes	In the male scrotum

Hormones from these glands that you need in this topic: the pituitary gland releases FSH and LH, the pancreas releases insulin, the ovaries release oestrogen and the testes release testosterone.

Higher tier

Higher tier: only on the Higher tier papers. Foundation students can skip it. What the labels mean

The thyroid gland releases thyroxine and the adrenal glands release adrenaline.

Female and male body outlines showing the positions of the pituitary gland, thyroid, adrenal glands (on top of the kidneys), pancreas, ovaries (female) and testes (male). — Positions of the main **endocrine glands**. Ovaries are found in females and testes in males.

Exam tip:

Compare with the nervous system in three words: slower, longer, chemical. Hormones are carried in the blood, so they reach every part of the body but only affect their target organ.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

What does insulin do to blood sugar?

Insulin lowers blood sugar (blood glucose concentration). The pancreas produces insulin when blood glucose is too high, for example after a meal. Insulin causes glucose to move from the blood into the cells, and in liver and muscle cells excess glucose is converted to glycogen for storage. Blood glucose then falls back to the normal level.

What is the difference between insulin and glucagon?

Higher tier Insulin lowers blood glucose and glucagon raises it. Both are made by the pancreas. Insulin is released when blood glucose is too high and makes glucose move from the blood into cells. Glucagon is released when it is too low and causes glycogen to be converted into glucose and released into the blood. They work in a negative feedback cycle.

Why are hormones called chemical messengers?

Hormones are called chemical messengers because they are chemicals secreted by glands directly into the bloodstream, which carries them to a target organ where they produce an effect. Compared with nerve impulses, hormones act more slowly but their effects last longer. They reach every part of the body but only affect their target organ.

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