Nervous vs endocrine system

The body has two control systems. The nervous system uses fast electrical impulses; the endocrine system uses chemical hormones carried in the blood to target organs.

• Nervous: very fast, acts for a short time, very precise (targeted area).
• Endocrine: slower, but effects can be long-lasting and spread over the whole body.

Hormones are made by glands. Key ones: the pituitary (the "master gland"), thyroid (thyroxine), adrenal (adrenaline), pancreas (insulin & glucagon), and the ovaries (oestrogen) / testes (testosterone).

Watch out: hormones travel in the blood, not along nerves. That's why they act more slowly but can affect many organs at once.

How do hormones travel?

Why would your body use a slow messenger?

Adrenaline & thyroxine

Adrenaline is released by the adrenal glands in times of fear or stress (the "fight or flight" response). It increases heart rate and blood pressure, increases blood flow to the muscles, and raises blood glucose by making the liver change glycogen into glucose.

Thyroxine, from the thyroid gland, controls the body's metabolic rate. Its level is kept steady by negative feedback: low thyroxine → the hypothalamus releases TRH → the pituitary releases TSH → the thyroid makes thyroxine. Normal thyroxine inhibits TRH and TSH release:

Key idea: negative feedback always reverses the change. Too high → the body lowers it; too low → the body raises it.

Fight or flight

How does the body keep time without a clock?

The menstrual cycle

Four hormones control the roughly 28-day cycle. Learn what each one does:

• FSH (follicle-stimulating hormone) — from the pituitary; makes an egg mature and stimulates the ovaries to release oestrogen.
• Oestrogen — from the ovaries; rebuilds the uterus lining and triggers a surge of LH.
• LH (luteinising hormone) — from the pituitary; causes ovulation (release of the egg) at about day 14.
• Progesterone — from the ovaries; maintains the uterus lining. When it falls, the lining breaks down (a period).

Which gland makes it?

Tap a hormone, then tap the gland that releases it.

How does the body keep time without a clock?

Contraception & assisted reproduction

Contraception prevents pregnancy in two broad ways:

• Hormonal — e.g. the pill contains oestrogen and/or progesterone, which stop FSH being made, so no egg matures. Also injections, implants and patches.
• Barrier — e.g. condoms and diaphragms physically stop sperm reaching the egg (condoms also reduce STI spread).

To help people become pregnant:

• Clomifene — a drug that increases FSH and LH, stimulating ovulation.
• IVF (in-vitro fertilisation) — FSH and LH stimulate egg maturation; eggs are collected and fertilised by sperm in a lab; embryos are inserted into the uterus.

Evaluate IVF: it gives infertile couples a chance of a baby, but is emotionally/physically stressful, has low success rates, and can lead to risky multiple births.

How does the pill work?

What keeps your blood glucose steady?

Controlling blood glucose

The pancreas monitors and controls blood glucose using two hormones that act on the liver:

• Glucose too high → pancreas releases insulin → liver converts glucose to glycogen for storage → glucose falls.
• Glucose too low → pancreas releases glucagon → liver converts glycogen back to glucose → glucose rises.

Type 1 diabetes: the pancreas produces too little insulin; treated with insulin injections. Type 2 diabetes: body cells stop responding to insulin; linked to obesity; managed by diet and exercise.

Your turn — BMI & Type 2 risk

What keeps your blood glucose steady?

Thermoregulation

Body temperature is kept near 37 °C (the best temperature for enzymes). The hypothalamus in the brain monitors it and the skin responds:

• Too hot → sweating (evaporation cools the skin) and vasodilation — blood vessels near the skin surface widen, so more heat is lost.
• Too cold → shivering (muscle contraction releases heat), hairs stand up, and vasoconstriction — vessels narrow, so less heat is lost.

Misconception: in vasodilation/vasoconstriction the blood vessels themselves widen or narrow — the capillaries do not move to the surface. "Vasodilation = dumps heat" is a handy memory hook.

Cooling down

The kidney & osmoregulation

The kidneys control the water and ion content of the blood (osmoregulation) and remove urea (the waste from breaking down excess amino acids in the liver). Each kidney contains millions of tiny tubes called nephrons:

• Filtration (in the glomerulus and Bowman's capsule) — high pressure forces water, glucose, ions and urea out of the blood; big things (proteins, blood cells) stay behind.
• Selective reabsorption — all the glucose, the water the body needs, and useful ions are taken back into the blood.
• The remaining water, ions and urea leave as urine.

Higher tier — ADH: when the blood is too concentrated (not enough water), the pituitary releases more ADH, making the collecting duct more permeable so more water is reabsorbed and urine is more concentrated. This is negative feedback.

Kidney failure: dialysis filters the blood by machine several times a week; a transplant is a longer-term fix but risks rejection and needs a matching donor.

Reabsorbed or not?

Match each role to the hormone

Tap a role on the left, then its matching hormone on the right.

The big ideas to know

You've covered the key ideas in Edexcel Topic 7 — animal coordination, control and homeostasis. Press Finish to see your score.