The Endocrine System

This lesson introduces the endocrine system — a network of glands that produce hormones to coordinate and control processes throughout the body. Understanding the difference between the nervous system and the endocrine system is a key requirement of Edexcel GCSE Biology (1BI0) Topic 7.

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What Is the Endocrine System?

The endocrine system is a collection of glands that produce chemicals called hormones. These hormones are released directly into the bloodstream and are carried to target organs, where they produce an effect.

Key definitions:

• Gland — an organ that produces and secretes a substance (in this case, hormones).
• Hormone — a chemical messenger produced by a gland and transported in the blood to a target organ, where it triggers a specific response.
• Target organ — the organ or tissue on which a hormone acts. Target organs have specific receptor molecules on their cell surfaces that the hormone binds to.

Exam Tip: If asked to define a hormone, always include three elements: (1) it is a chemical messenger, (2) it is carried in the blood, and (3) it acts on a target organ. Missing any of these can lose marks.

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Nervous System vs Endocrine System

The body has two major communication systems. The nervous system uses electrical impulses along neurones, while the endocrine system uses chemical hormones in the blood. Both coordinate body functions, but they work in very different ways.

Feature	Nervous System	Endocrine System
Signal type	Electrical impulses	Chemical hormones
Transmission route	Along neurones (nerve cells)	Via the bloodstream
Speed	Very fast (milliseconds)	Slower (seconds to minutes to reach target)
Duration of effect	Short-lived; stops when impulse stops	Longer-lasting; can persist for hours, days or longer
Area affected	Very precise; targets a specific effector	Can be widespread; affects any organ with the right receptors
Example	Pulling hand away from a hot object (reflex)	Growth, puberty, blood glucose regulation

Exam Tip: A common question asks you to compare the nervous and endocrine systems. Learn this table thoroughly — use the mnemonic STDA (Signal, Transmission, Duration, Area) to remember the four key comparison points.

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The Pituitary Gland — The Master Gland

The pituitary gland is a small, pea-sized gland located at the base of the brain. It is often called the "master gland" because it produces hormones that control many other glands in the body.

Hormones produced by the pituitary gland include:

Hormone	Target / Function
FSH (follicle-stimulating hormone)	Stimulates egg maturation in the ovaries; stimulates oestrogen production
LH (luteinising hormone)	Triggers ovulation (release of an egg from the ovary)
ADH (antidiuretic hormone)	Controls water reabsorption in the kidneys
Growth hormone	Stimulates growth of bones and tissues
TSH (thyroid-stimulating hormone)	Stimulates the thyroid gland to produce thyroxine

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Key Glands and Their Hormones

The endocrine system contains many glands located throughout the body. You need to know the location and function of each for the Edexcel specification.

Gland	Location	Hormone(s) Produced	Key Function
Pituitary gland	Base of the brain	FSH, LH, ADH, growth hormone, TSH	Controls other glands; master gland
Thyroid gland	In the neck, around the trachea	Thyroxine	Controls the basal metabolic rate (the speed of chemical reactions in the body)
Adrenal glands	On top of each kidney	Adrenaline	Prepares the body for fight or flight
Pancreas	Below the stomach	Insulin and glucagon	Regulates blood glucose concentration
Ovaries (females)	In the pelvic region	Oestrogen and progesterone	Control the menstrual cycle and female secondary sexual characteristics
Testes (males)	In the scrotum	Testosterone	Controls male secondary sexual characteristics (e.g. deepening voice, muscle development, body hair)

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How Hormones Work

When a gland releases a hormone:

1. The hormone enters the bloodstream.
2. It travels around the entire body in the blood.
3. It only affects target organs — those that have the correct receptor molecules on their cell surface.
4. The hormone binds to the receptor, triggering a response inside the target cell.

Because hormones travel in the blood, they take longer to act than nerve impulses. However, their effects tend to last much longer.

Exam Tip: Remember that hormones are not "used up" instantly. They circulate until they are broken down by the liver, which is why their effects can last for hours or even longer.

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The Endocrine System — Diagram Overview

Imagine the body from top to bottom. The key glands are positioned as follows:

• Pituitary gland — base of the brain (centre of the head)
• Thyroid gland — in the neck, wrapped around the front of the trachea
• Adrenal glands — one sitting on top of each kidney, in the middle of the back
• Pancreas — behind the stomach, in the upper abdomen
• Ovaries — in the lower abdomen (females only)
• Testes — in the scrotum, outside the body (males only)

Exam Tip: In the exam you may be asked to label glands on a diagram of the human body. Make sure you can identify each gland by its location. A common mistake is confusing the adrenal glands (on the kidneys) with the kidneys themselves.

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Summary

• The endocrine system is a network of glands that produce hormones.
• Hormones are chemical messengers carried in the blood to target organs.
• The pituitary gland is the master gland and controls many other glands.
• The nervous system is fast but short-lived; the endocrine system is slower but longer-lasting.
• You must know the key glands (pituitary, thyroid, adrenal, pancreas, ovaries, testes), their hormones, and their functions.

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Key Terms

Term	Definition
Endocrine system	A collection of glands that produce hormones
Hormone	A chemical messenger carried in the blood to a target organ
Target organ	The organ where a hormone has its effect
Pituitary gland	The master gland at the base of the brain
Gland	An organ that produces and releases hormones or other substances