How It Works
The Hormone System📮 The Endocrine System — Chemical Messages by Blood
The endocrine system is a collection of glands that produce and secrete hormones directly into the bloodstream. Hormones are chemical messengers — they travel to specific target organs and cause changes in their activity. Every cell in the body is bathed in blood, but only cells with the correct receptor proteins respond to a given hormone.
Think of it as a postal system: the gland is the post office, the bloodstream is the delivery network, the hormone is the letter, and the target organ is the address. Every cell sees the letter passing — but only the right address opens it.
| Postal Term | Endocrine Equivalent | Example |
|---|---|---|
| Post office (sender) | Endocrine gland | Pancreas, adrenal gland, thyroid |
| Letter (message) | Hormone | Insulin, adrenaline, thyroxine |
| Delivery network | Bloodstream | Circulatory system |
| Address (recipient) | Target organ/cell | Liver (for insulin), heart (for adrenaline) |
| Letterbox / key | Receptor protein on target cell | Specific shape — only the right hormone fits |
| Delivery time | Seconds to minutes (slower than nerves) | Adrenaline felt within seconds; growth hormone over months |
🔑 Key Vocabulary
Endocrine gland
A ductless gland that secretes hormones directly into the bloodstream (not through a duct/tube). Examples: pancreas, thyroid, adrenal, pituitary, ovaries, testes.
Hormone
A chemical messenger produced in one part of the body and transported in blood to a distant target organ where it causes a specific response.
Target organ
The organ or tissue that responds to a specific hormone because its cells have the correct receptor proteins for that hormone. Other organs ignore the same hormone.
Negative feedback
The control mechanism where the effect of a hormone reduces further secretion of that hormone — keeping levels within a normal range. The body's thermostat principle.
The Glands
The Post OfficesEach endocrine gland is a specialised "post office" dispatching specific chemical messages. Tap each one to open the full briefing.
📬 Key Hormones — Anterior Pituitary
- Growth hormone (GH) — stimulates growth of bones and muscles
- TSH — tells thyroid to produce thyroxine
- FSH / LH — control reproduction (ovaries/testes)
- ACTH — stimulates adrenal cortex
📬 Key Hormones — Posterior Pituitary
- ADH (antidiuretic hormone) — controls water reabsorption in kidneys
- Oxytocin — uterine contractions during childbirth; bonding
📬 Thyroxine
- Regulates basal metabolic rate (BMR) — how fast cells use energy at rest
- Essential for normal growth and brain development in children
- Requires iodine — iodine deficiency → enlarged thyroid (goitre)
- Controlled by negative feedback via TSH from pituitary
⚠️ When Things Go Wrong
- Hyperthyroidism — too much thyroxine: fast heart rate, weight loss, anxiety, heat intolerance
- Hypothyroidism — too little: fatigue, weight gain, slow heart rate, cold intolerance
- Goitre — enlarged thyroid from iodine deficiency (thyroid grows trying to produce more)
📬 Adrenaline (Adrenal Medulla)
- Released in response to stress, excitement, or danger
- Increases heart rate and breathing rate
- Diverts blood from gut to muscles
- Raises blood glucose (glycogen → glucose)
- Dilates pupils, increases alertness
- The "butterflies in your stomach" feeling
📬 Cortisol (Adrenal Cortex)
- Released in response to longer-term stress
- Raises blood glucose (protein → glucose)
- Suppresses immune response
- Aldosterone (also from cortex) — controls Na⁺ retention in kidneys
📬 Insulin — From Beta Cells
- Released when blood glucose is HIGH (e.g. after eating)
- Target: liver and muscle cells
- Causes glucose uptake by cells
- Converts glucose → glycogen (glycogenesis) for storage in liver
- Blood glucose level falls back to normal
📬 Glucagon — From Alpha Cells
- Released when blood glucose is LOW (e.g. after exercise, fasting)
- Target: liver cells
- Converts glycogen → glucose (glycogenolysis)
- Blood glucose level rises back to normal
Type 2: Body cells become resistant to insulin — glucose can't enter cells even when insulin is present. Linked to diet and lifestyle. Both result in persistently high blood glucose (hyperglycaemia) — glucose appears in urine (glycosuria), excessive thirst (polydipsia), frequent urination.
Glucose Control
Negative Feedback🍬 Blood Glucose Regulation — The Body's Sugar Balance
Blood glucose must be kept within a narrow range (approximately 4–8 mmol/L). Too high (hyperglycaemia) damages blood vessels and organs. Too low (hypoglycaemia) starves the brain of fuel — causing confusion, seizures, and unconsciousness. The pancreas constantly monitors and adjusts blood glucose using two opposing hormones.
⬆️ After a Meal — Blood Glucose Rises
⬇️ After Exercise / Fasting — Blood Glucose Falls
| Feature | Insulin | Glucagon |
|---|---|---|
| Produced by | Beta cells (β cells) | Alpha cells (α cells) |
| Released when | Blood glucose too HIGH | Blood glucose too LOW |
| Effect on liver | Glucose → glycogen (storage) | Glycogen → glucose (release) |
| Effect on blood glucose | Lowers it | Raises it |
| Antagonistic? | Yes — they oppose each other to maintain balance | |
Nervous vs Endocrine
Two Communication Systems⚖️ Two Ways the Body Communicates
The body uses two coordination systems: the nervous system (fast, electrical, short-lived) and the endocrine system (slower, chemical, long-lasting). They work together but operate very differently. Knowing their differences is a core exam skill.
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Message type | Electrical impulses + neurotransmitters | Chemical hormones |
| Transmission pathway | Neurons (nerves) | Bloodstream |
| Speed | Very fast — up to 120 m/s | Slower — seconds to minutes |
| Duration of effect | Short-lived — stops when impulse stops | Long-lasting — hours to days |
| Target | Specific — one effector | Widespread — all cells with receptors |
| Response type | Precise, localised (e.g. move one finger) | Broad, systemic (e.g. growth, puberty) |
| Examples | Reflex arc, voluntary movement, sensing pain | Puberty, blood glucose control, stress response |
🔗 Where They Connect — The Hypothalamus
The hypothalamus is the master bridge between the two systems. It is part of the brain (nervous system) but also controls the pituitary gland (endocrine system). It detects changes in blood composition and temperature via nervous signals, then triggers hormonal responses via the pituitary. This neuro-endocrine connection is why stress (a nervous experience) causes hormonal changes (cortisol, adrenaline release).
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