Choose an activity and follow the hormone through the blood step by step. Blue and orange dots are hormones (for example insulin and glucagon), red dots are adrenaline or thyroxine, and yellow dots are a nerve impulse, so you can compare how fast nerves and hormones are.
Endocrine glands have no ducts: they release hormones directly into the blood. Every cell is reached by the blood, but only cells with the matching receptor respond, so each hormone has its own target organs. Click a gland or a colour in the key.
The blood glucose level is kept between about 4 and 7 mmol/L. Eat a meal or exercise and watch the pancreas respond. The yellow line is blood glucose; the blue line is insulin and the orange line glucagon (relative amounts). In the 3D view, blue and orange dots flow from the pancreas to the liver.
Simplified teaching model, not medical advice. 1 mmol/L ≈ 18 mg/dL.
The 3D model above shows the glands inside the body; this flat diagram is the version you draw and label in an exam. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
The body has two ways to send messages. The nervous system sends electrical impulses along neurons: very fast, short-lived and aimed at one place. The endocrine system releases hormones into the blood: slower, but the effects last longer and can reach the whole body. A hormone affects only its target organs, whose cells have receptors for it.
The hypothalamus in the brain links the two systems and controls the pituitary gland, the “master gland” whose hormones control the thyroid, the adrenal glands and the ovaries or testes. The pancreas controls blood glucose, the adrenal glands make adrenaline for “fight or flight”, the thyroid sets the metabolic rate, and the parathyroids control blood calcium.
Most hormones are controlled by negative feedback: a change in the body triggers a response that reverses the change. When blood glucose rises, insulin brings it down; when it falls, glucagon brings it back up. When thyroxine is high, the hypothalamus and pituitary release less TRH and TSH, so less thyroxine is made. This keeps conditions steady, which is called homeostasis.
Press After a meal and follow the blue insulin dots from the pancreas through the liver to a muscle. Then run A fright and compare the fast yellow nerve impulse with the slower red adrenaline. In the blood glucose lab, eat a meal with a healthy body, then switch to Type 1 diabetes and eat again: without insulin the glucose stays far above the normal band.
| Gland | Hormone | Target | Main effect |
|---|---|---|---|
| Hypothalamus | TRH, GnRH, CRH (releasing hormones); makes ADH | Pituitary gland | Tells the pituitary which hormones to release |
| Pituitary gland | TSH, FSH, LH, ACTH, growth hormone; releases ADH | Thyroid, ovaries/testes, adrenal cortex, bones and muscles, kidneys | Controls other glands; growth; water balance (ADH) |
| Thyroid gland | Thyroxine (T4) and T3 | Almost all cells | Sets metabolic rate; growth and development |
| Parathyroid glands | Parathyroid hormone (PTH) | Bones, kidneys, intestine | Raises blood calcium |
| Adrenal glands | Adrenaline; cortisol; aldosterone | Heart, liver, muscles; many cells; kidneys | Fight or flight; stress response; salt balance |
| Pancreas (islets) | Insulin; glucagon | Liver, muscles, fat cells; liver | Lowers blood glucose; raises blood glucose |
| Ovaries | Oestrogen, progesterone | Uterus, breasts, many tissues | Puberty in girls; menstrual cycle; pregnancy |
| Testes | Testosterone | Many tissues | Puberty in boys; sperm production |
| Pineal gland | Melatonin | Brain | Sleep–wake rhythm |
| Nervous system | Endocrine system | |
|---|---|---|
| Message | Electrical impulses (and chemicals at synapses) | Hormones (chemicals) |
| Carried by | Neurons | Blood |
| Speed | Very fast: milliseconds | Slower: seconds to hours |
| Effect lasts | Short | Longer (minutes to years) |
| Area affected | Precise: one muscle or gland | Widespread: every cell with the receptor |
| Example | Pulling your hand off a hot pan | Growth at puberty; control of blood glucose |
In type 1 diabetes the immune system destroys the insulin-making beta cells, so the pancreas makes little or no insulin; it is treated with insulin injections or a pump. In type 2 diabetes the body’s cells respond less to insulin (insulin resistance), and over time the pancreas may not make enough; it is linked to body weight and physical activity and is managed with diet, exercise and medicines. In both, blood glucose stays too high, which damages blood vessels and nerves over years. The glucose lab above is a simplified teaching model; it is not medical advice.
The main endocrine glands are the hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, and the ovaries (in females) or testes (in males). They release hormones directly into the blood.
Key takeaway: hypothalamus, pituitary, pineal, thyroid, parathyroids, thymus, adrenals, pancreas, ovaries or testes.A hormone is a chemical messenger made by an endocrine gland and carried in the blood to target organs. Only cells with the right receptor respond to it. Examples are insulin, adrenaline, thyroxine, oestrogen and testosterone.
Key takeaway: a chemical messenger carried in the blood to its target organs.Because its hormones control several other endocrine glands: TSH controls the thyroid, ACTH the adrenal cortex, and FSH and LH the ovaries and testes. It is itself controlled by the hypothalamus, so the hypothalamus is sometimes called the real master.
Key takeaway: its hormones switch other glands on and off.When blood glucose rises after a meal, beta cells in the pancreas release insulin, which makes liver and muscle cells take up glucose and the liver store it as glycogen. When blood glucose falls, alpha cells release glucagon, which makes the liver break glycogen down and release glucose. Together they keep glucose at about 4 to 7 mmol/L by negative feedback.
Key takeaway: insulin lowers blood glucose, glucagon raises it.Negative feedback means that a change triggers a response that reverses the change. For example, high thyroxine makes the hypothalamus and pituitary release less TRH and TSH, so the thyroid makes less thyroxine. It keeps hormone levels and body conditions steady (homeostasis).
Key takeaway: a rise switches its own production down, a fall switches it up.The nervous system sends electrical impulses along neurons: very fast, short-lived and to one precise place. The endocrine system sends hormones in the blood: slower, longer-lasting and to every cell with the right receptor. They work together; for example, a nerve signal makes the adrenal glands release adrenaline.
Key takeaway: nerves are fast and precise; hormones are slower, longer and widespread.Adrenaline is released by the adrenal glands when you are frightened or excited. It makes the heart beat faster and harder, widens the airways, sends more blood to the muscles and makes the liver release glucose, preparing the body for “fight or flight”.
Key takeaway: it prepares the body for action within seconds.In type 1 diabetes the pancreas makes little or no insulin because the beta cells have been destroyed, so insulin must be injected. In type 2 diabetes the body’s cells respond less to insulin (insulin resistance), often linked to weight and inactivity; it is managed with diet, exercise and medicines, and sometimes insulin. In both, blood glucose stays too high.
Key takeaway: type 1: no insulin made; type 2: cells respond poorly to insulin.