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🌿 The Endocrine System

Spec 4.5.3 📗 Foundation
📖 In-Depth Theory

What is the Endocrine System?

The ENDOCRINE SYSTEM is the body's chemical communication system.
It uses HORMONES — chemical messenger molecules — to coordinate responses across the body.
Hormones are produced by ENDOCRINE GLANDS and secreted DIRECTLY INTO THE BLOODSTREAM — they have no ducts (unlike exocrine glands like salivary glands which have ducts).
The blood carries hormones to every organ in the body — but only TARGET ORGANS respond, because only they have the correct RECEPTOR PROTEINS for that hormone.
Hormonal responses are:
SLOWER to start than nervous responses (blood must carry the hormone to the target).
LONGER LASTING than nervous responses.
More WIDESPREAD — hormones reach all organs (though only targets respond).

Key Endocrine Glands and Their Hormones

PITUITARY GLAND (in the brain — below the hypothalamus):
The 'master gland' — releases hormones that control other endocrine glands.
Produces: FSH and LH (control reproduction), growth hormone, ADH (water balance).
THYROID GLAND (in the neck):
Produces THYROXINE — controls METABOLIC RATE (the speed of chemical reactions in cells).
Also involved in growth and development.
ADRENAL GLANDS (above the kidneys — one on each kidney):
Produce ADRENALINE — the 'fight or flight' hormone.
Released in response to stress, fear or excitement.
Effects: increased heart rate, increased breathing rate, pupils dilate, blood redirected to muscles.
Prepares the body for rapid physical action.
PANCREAS:
Produces INSULIN and GLUCAGON — regulate blood glucose concentration (see next subtopic).
OVARIES (females):
Produce OESTROGEN and PROGESTERONE — control the menstrual cycle and female secondary sexual characteristics.
TESTES (males):
Produce TESTOSTERONE — controls male secondary sexual characteristics and sperm production.

Nervous vs Hormonal Communication

The nervous system and endocrine system both communicate and coordinate responses, but work differently:
NERVOUS SYSTEM:
Signal type: electrical impulse along neurones.
Speed: very fast — up to 120 m/s.
Duration: short-lived (milliseconds to seconds).
Target: specific cells connected by nerve fibres.
Response: immediate and precise.
Examples: muscle contraction, reflex arc, heart rate control.
ENDOCRINE SYSTEM:
Signal type: chemical hormone in bloodstream.
Speed: slower — limited by blood flow speed.
Duration: longer lasting (minutes to hours or even days).
Target: any organ with the correct receptor.
Response: prolonged, broader.
Examples: blood glucose regulation, puberty, growth, stress response.
BOTH SYSTEMS WORK TOGETHER:
For example, during an emergency:
The NERVOUS SYSTEM detects danger and triggers an immediate response (e.g. jump back).
The ENDOCRINE SYSTEM releases adrenaline to sustain the response over the next few minutes.
⚠️ Common Mistake

Hormones travel in the BLOOD to target organs — they do NOT travel along nerves. Do not confuse the two systems. Also: not all organs respond to every hormone — only TARGET ORGANS with the correct receptor proteins respond.

📌 Key Note

Endocrine system: hormones in blood → target organs. Key glands: pituitary (master), thyroid (metabolism), adrenal (adrenaline), pancreas (insulin/glucagon), ovaries/testes (sex hormones). Slower but longer-lasting than nervous communication.

🎯 Matching Activity — Match the Gland to its Hormone and Function

Match each gland to what it produces and what that hormone does. — drag the symbols on the right to match the component names on the left.

Pituitary gland
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Thyroid gland
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Adrenal gland
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Pancreas
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Ovaries
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Testes
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Oestrogen and progesterone — menstrual cycle and female characteristics
Testosterone — male characteristics and sperm production
Adrenaline — fight or flight, increases heart rate and breathing
Insulin and glucagon — regulate blood glucose concentration
FSH, LH and growth hormone — controls other glands — 'master gland'
Thyroxine — controls metabolic rate and development
🎯 Test Yourself
Question 1 of 3
1. How do hormones reach their target organs?
2. Which hormone prepares the body for 'fight or flight'?
3. Which best describes the difference between nervous and hormonal communication?
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