Zoom in from the whole organism to the organ system — the endocrine system — and you find a network of glands releasing chemical messengers called hormones directly into the bloodstream. Zoom further to the target organ and each hormone binds to specific receptors, triggering precise changes that keep the body's internal environment stable across timescales from seconds to months.
What is the endocrine system?
The endocrine system is a collection of glands that produce and secrete hormones — chemical messengers carried in the blood to target organs throughout the body.
Key features of hormonal communication:
- Hormones travel in the blood to their target organs (unlike nerve impulses, which travel along neurones)
- Effects are generally slower to start but longer-lasting than nervous responses
- A hormone affects only cells with the correct receptor proteins on their surface — these are the target cells
- Hormones are produced in glands and are released in small quantities
The endocrine system works alongside the nervous system to coordinate the body's responses to internal and external changes.
What are the main glands and what hormones do they produce?
| Gland | Hormone(s) produced | Main function |
|---|---|---|
| Pituitary gland (brain) | FSH, LH, ADH, growth hormone | Master gland — controls many other glands; regulates water balance and growth |
| Thyroid gland (neck) | Thyroxine | Regulates metabolic rate and energy use |
| Adrenal glands (above kidneys) | Adrenaline | Prepares the body for "fight or flight" — increases heart rate, blood glucose, breathing rate |
| Pancreas (abdomen) | Insulin, glucagon | Regulates blood glucose concentration |
| Ovaries (females) | Oestrogen, progesterone | Control the menstrual cycle and secondary sexual characteristics |
| Testes (males) | Testosterone | Controls sperm production and secondary sexual characteristics |
How is blood glucose concentration regulated?
Maintaining blood glucose within a narrow range (approximately 4–8 mmol/L) is essential — too low and cells cannot respire; too high and it causes damage to blood vessels and organs.
The pancreas detects blood glucose levels and responds using two antagonistic hormones:
When blood glucose rises (e.g. after a meal):
- The pancreas secretes insulin
- Insulin causes body cells (especially liver and muscle cells) to take up glucose from the blood
- The liver converts excess glucose to glycogen for storage (glycogenesis)
- Blood glucose falls back to normal
When blood glucose falls (e.g. after exercise or fasting):
- The pancreas secretes glucagon
- Glucagon causes the liver to break down glycogen back into glucose (glycogenolysis)
- Glucose is released into the blood
- Blood glucose rises back to normal
This is a classic example of negative feedback.
What is negative feedback and why does it matter?
Negative feedback is the mechanism by which the body detects a change from its set point and responds to reverse that change, restoring balance. It is the fundamental principle underlying most hormonal control systems.
The general pattern:
Change detected → response triggered → change reversed → response switched off
Example — blood glucose regulation:
- Blood glucose rises → insulin secreted → glucose taken up by cells → blood glucose falls → insulin secretion reduced → cycle restarts
Example — body temperature (also uses nervous system):
- Temperature rises → sweating, vasodilation → temperature falls → sweating stops
Negative feedback contrasts with positive feedback, in which the response amplifies the change (rare in biology — childbirth contractions are an example). For most homeostatic systems, negative feedback is the control mechanism.
How do hormones control the menstrual cycle?
Four hormones interact in a carefully timed cycle of approximately 28 days:
| Hormone | Source | Role in cycle |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Stimulates follicle development in the ovary; triggers oestrogen production |
| Oestrogen | Ovary (follicle) | Causes endometrium to thicken; at high concentrations triggers LH surge; inhibits FSH (negative feedback at low levels) |
| LH (luteinising hormone) | Pituitary gland | Surge triggers ovulation (release of egg) on approximately day 14 |
| Progesterone | Ovary (corpus luteum) | Maintains the thickened endometrium; inhibits FSH and LH; if no fertilisation, levels fall, triggering menstruation |
If fertilisation occurs, the embryo produces HCG, which maintains the corpus luteum and progesterone levels throughout early pregnancy, preventing menstruation.
What is the difference between hormonal and nervous control?
The endocrine (hormonal) system and the nervous system both coordinate the body's responses, but they work in very different ways:
| Feature | Nervous system | Endocrine (hormonal) system |
|---|---|---|
| Messenger | Electrical impulses + neurotransmitters | Hormones (chemical) |
| Speed of response | Very fast (milliseconds) | Slower (seconds to hours) |
| Duration of effect | Short-lived | Long-lasting |
| Target | Specific — nerve connects directly to target | General — hormone travels in blood to all cells, but only target cells respond |
| Example | Reflex arc (e.g. pulling hand away from heat) | Insulin regulating blood glucose |
The two systems often work together: adrenaline is secreted by the adrenal gland in response to a nerve signal from the brain during stress, then it prepares the body via the bloodstream.
Frequently asked questions
What is the difference between insulin and glucagon?
Insulin and glucagon are antagonistic hormones both produced by the pancreas. Insulin is released when blood glucose is too high — it stimulates cells to absorb glucose and causes the liver to convert glucose to glycogen, lowering blood glucose. Glucagon is released when blood glucose is too low — it stimulates the liver to break down glycogen back into glucose, raising blood glucose. Together they maintain blood glucose within a narrow, safe range through negative feedback.
What does adrenaline do in the body?
Adrenaline is released by the adrenal glands (above the kidneys) in response to stress, danger, or excitement — the "fight or flight" response. It increases heart rate and stroke volume (more blood to muscles), dilates airways (more oxygen intake), increases blood glucose concentration (more fuel for muscles), and diverts blood away from the digestive system to skeletal muscles. These effects prepare the body for rapid, intense physical action. Adrenaline acts quickly and its effects are short-lived — it is one of the fastest-acting hormones.
What is ADH and how does it regulate water balance?
ADH (anti-diuretic hormone) is produced by the hypothalamus and released by the pituitary gland. It controls how much water the kidneys return to the blood. When blood water concentration is too low (e.g. after sweating or not drinking enough), the hypothalamus detects this and more ADH is released. ADH causes the kidney tubules to become more permeable to water, so more water is reabsorbed into the blood and less water is lost in urine (which becomes more concentrated). When blood water concentration returns to normal, ADH secretion decreases — another example of negative feedback.
What is Type 1 diabetes and how does it relate to insulin?
Type 1 diabetes is a condition in which the pancreas produces little or no insulin, because the immune system has destroyed the insulin-producing beta cells. Without insulin, glucose cannot be taken up by cells, so blood glucose remains dangerously high after meals. People with Type 1 diabetes manage the condition by injecting insulin (calculated to match carbohydrate intake) and monitoring blood glucose levels closely. Type 2 diabetes, by contrast, is caused by cells becoming insensitive to insulin (insulin resistance) and is strongly linked to lifestyle factors including diet, physical activity, and obesity.
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