(1) What are hormones? Explaining what is the function of hormones?

A hormone can be defined as a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs

Hormones are produced in the endocrine gland system and are transported by the blood to their target cells, tissues or organs

Many process within the body are coordinated and controlled by chemical substances called hormones.

You can think of hormones as chemical messengers sent around the bloodstream.

Hormones are often quite large organic molecules.

Hormones and the nervous system send information around the body.

Hormones, being directly released into the blood, are quite rapidly carried to all parts of the body BUT only affect the function of particular cells, tissue or organs - the 'targets''.

Hormones, in acting as 'chemical messages', trigger particular biochemical reactions in various types of tissue and organs.

Hormones control functions in cells, tissue and organs that need constant adjustment and their effect is relatively long-lasting compared to eg the nervous responses of a reflex arc.

The activated cells are called 'target cells' and have a chemical receptor that responds to the hormone - the hormones work on effectors.

You should appreciate that the nervous system and hormones enable us to respond to external changes and also help us to control conditions inside our bodies.

BUT, unlike the nervous system, the hormone response times are slower e.g. minutes or hours, an exception is adrenaline , which is the fastest acting hormone response.

Reminder: Homeostasis the function of self-regulating processes by which biological systems (organisms) tend to maintain physical and chemical stability while adjusting to conditions that are optimal for survival i.e. counteracting unfavourable conditions.

These process can involve hormones secreted from the glands of the endocrine system.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key points about hormones and how they relate to homeostasis

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Hormones and Homeostasis – GCSE/IGCSE Biology Revision Notes

1. What Are Hormones?

• Hormones are chemical messengers secreted directly into the bloodstream by endocrine glands.

• They travel in the blood to target organs, where they produce specific effects.

• Compared to the nervous system, hormonal responses are slower but longer lasting.

Key Properties:

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2. Major Endocrine Glands and Hormones

Gland	Hormone Produced	Function
Pituitary ("master gland")	ADH, FSH, LH, TSH	Controls other glands, regulates growth
Pancreas	Insulin and Glucagon	Controls blood glucose levels
Thyroid	Thyroxine	Regulates metabolism
Adrenal	Adrenaline	Triggers "fight or flight" response
Ovaries (female)	Oestrogen, Progesterone	Regulate menstrual cycle, secondary sexual characteristics
Testes (male)	Testosterone	Controls sperm production, male traits

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3. Hormones and Homeostasis

Homeostasis refers to the maintenance of a constant internal environment, such as:

• Body temperature

• Blood glucose concentration

• Water balance

Hormones play a central role in:

a. Blood Glucose Regulation

• Controlled by insulin and glucagon from the pancreas.

• Insulin lowers blood glucose by increasing uptake into cells and converting excess to glycogen in the liver.

• Glucagon raises blood glucose by triggering the liver to break down glycogen into glucose.

b. Water Balance

• Regulated by ADH (anti-diuretic hormone) from the pituitary gland.

• ADH increases the permeability of kidney tubules, so more water is reabsorbed into the blood when the body is dehydrated.

c. Temperature Regulation

• Involves the hypothalamus, which detects temperature changes and coordinates responses such as:

  • Sweating or vasodilation to cool down.

  • Shivering or vasoconstriction to warm up.

  • Although nerves are involved, hormones like thyroxine also help regulate basal metabolic rate.

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4. Why Is Hormonal Control Important for Homeostasis?

• Hormones ensure internal conditions remain stable, vital for enzyme activity and cell function.

• Failure in hormonal regulation can lead to serious conditions, e.g.:

  • Diabetes mellitus – caused by lack of insulin or insulin resistance.

  • Thyroid disorders – affect metabolism and energy balance.

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Summary of learning objectives and key words or phrases

Be able to describe and explain what hormones are and what do they do?

Be able to define and describe the function of hormones.

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