GCSE Biology Hormones & glucose revision

The endocrine system is the body’s chemical messaging network. Glands such as the pancreas, thyroid, adrenal glands and ovaries or testes secrete hormones directly into the bloodstream — no ducts, straight into the blood. The blood carries every hormone everywhere, but each hormone only acts where its target organ has the right receptors. That is how one message can reach the whole body yet change only a few targets.

At the top sits the pituitary gland in the brain, the master gland. It monitors body conditions and releases hormones that command the other glands — thyroid, adrenals, ovaries or testes — which then release their own hormones. A hierarchy of chemical memos.

Blood glucose must stay in a narrow band — too high or too low damages cells. The pancreas monitors the concentration and responds with the right hormone. After a sugary meal, blood glucose rises; the pancreas releases insulin, which makes cells take up glucose and instructs the liver and muscles to store it as glycogen — glucose falls back to normal. Between meals, glucose dips; the pancreas releases glucagon, which makes the liver convert glycogen back into glucose and release it — glucose rises back to normal.

Notice the pair: insulin lowers, glucagon raises. Students swap them constantly — fix the order with “insulin invests (stores); glucagon gives back (releases)”.

Type 1 diabetes: the pancreas produces little or no insulin, so glucose cannot be moved out of the blood and concentrations climb dangerously — untreated, this causes death. It usually appears in childhood and is treated with insulin therapy: injections before meals for life, plus limiting simple carbohydrates and taking regular exercise to smooth glucose levels.

Type 2 diabetes: the pancreas still makes insulin, but the body’s cells stop responding to it — insulin resistance. It usually develops later in life, and obesity is a major risk factor. Treatment is lifestyle-based: a carbohydrate-controlled diet and an exercise programme. A screening tool for obesity is the body mass index: BMI = mass (kg) ÷ height² (m²).

Water and ion levels must stay within narrow limits — cells swell or shrink otherwise. The kidneys do the balancing: they filter the blood, then reabsorb all of the sugar, plus as much water and as many ions as the body needs. Whatever is left — excess water, excess ions and urea (the nitrogen waste from excess protein) — leaves as urine, stored in the bladder.

How much water is reabsorbed is set by the hormone ADH (antidiuretic hormone). Receptors in the brain detect the water concentration of the blood. On a hot, sweaty day the blood concentrates: the pituitary gland releases more ADH, which makes the kidney tubules more permeable so more water is reabsorbed — urine becomes small and concentrated. Drink a litre of water and ADH release drops, so less is reabsorbed and urine is plentiful and pale. Receptors → hormone → kidney response, reversing the change: negative feedback again.

When kidneys fail, waste builds up in the blood. Dialysis does the kidney’s job artificially: the patient is connected to a machine several times a week, and their blood is filtered — urea, excess ions and excess water removed, glucose and needed ions restored. It keeps people alive but means lifelong regular sessions, a carefully controlled diet between sessions, and inconvenience.

A kidney transplant replaces the failed kidneys with a healthy donor kidney — a long-term cure with no further dialysis. The catch: the donor must be a tissue match, the operation is major surgery, and the immune system will attack ("reject") the new kidney unless the patient takes immunosuppressant drugs for life, which leave them vulnerable to infections. Donor kidneys are also scarce. A balanced evaluation weighs freedom and cure against lifelong medication and scarcity.