Homeostasis, under control.

Part 1What homeostasis means

Homeostasis is the regulation of the internal conditions of a cell or organism to keep them roughly constant, in response to internal and external change. Your cells only work well within narrow limits, so the body holds conditions steady.

In humans, homeostasis keeps three things under control: blood glucose concentration, body temperature and water levels. Keeping them stable means enzymes and other cell reactions can work at their best.

These automatic control systems may involve nervous responses or chemical (hormonal) responses. Either way, the order is the same: a receptor detects the change, a coordination centre decides what to do, and an effector carries it out.

Part 1The CNS and neurones

The nervous system lets you react to your surroundings and coordinate your behaviour. Information travels as electrical impulses along cells called neurones.

The central nervous system (CNS) is the brain and spinal cord. The basic pathway is: a stimulus is detected by a receptor; an impulse passes along a sensory neurone to the CNS; the CNS coordinates a response; an impulse passes along a motor neurone to an effector (a muscle or gland), which carries out the response.

Where two neurones meet there's a small gap called a synapse. The electrical impulse can't jump the gap, so a chemical (a neurotransmitter) is released, diffuses across the gap, and sets off a new electrical impulse in the next neurone. This chemical step is why a synapse slows the signal very slightly.

Part 2Reflexes

Reflex actions are automatic and rapid — they do not involve the conscious part of the brain. This makes them fast, and they're important because they protect you from danger (like pulling your hand off a hot pan).

The pathway is the reflex arc: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector → response. Because the signal goes through the spinal cord and not the conscious brain, it's quicker.

Part 1Chemical messengers

The endocrine system is made of glands that secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ, where it produces an effect.

Compared with the nervous system, hormonal effects are slower but generally act for longer. Nervous responses are fast and short-lived; hormonal responses are slower to start but more lasting.

The pituitary gland is called the master gland because it secretes several hormones into the blood in response to body conditions. Many of these hormones act on other glands, telling them to release the hormones the body needs.

Part 1Insulin lowers blood glucose

The blood glucose concentration is monitored and controlled by the pancreas. After you eat, glucose from digested food is absorbed into the blood and the concentration rises.

If blood glucose is too high, the pancreas secretes the hormone insulin. Insulin causes glucose to move from the blood into the liver and muscle cells. In the liver and muscles, excess glucose is stored as glycogen. This lowers the blood glucose concentration back to normal.

Part 2Diabetes

Type 1 diabetes is a disorder where the pancreas fails to produce enough insulin, so blood glucose can rise dangerously high. It is normally controlled with insulin injections.

Type 2 diabetes is where the body cells no longer respond properly to insulin. Obesity is a risk factor. It is commonly treated with a carbohydrate-controlled diet and an exercise regime.

Part 1Puberty & the main sex hormones

During puberty, reproductive hormones cause secondary sex characteristics to develop. Testosterone is the main male reproductive hormone, produced by the testes; it stimulates sperm production. Oestrogen is the main female reproductive hormone, produced by the ovaries.

At puberty, eggs begin to mature and one is released about every 28 days — this is ovulation. The monthly release of an egg and the build-up and breakdown of the uterus lining is the menstrual cycle.

Part 1Hormonal methods

Fertility can be controlled by a variety of methods of contraception. Hormonal methods use hormones to stop eggs maturing or being released.

Oral contraceptives (the "pill") contain hormones such as oestrogen and/or progesterone. Oestrogen, taken every day, can inhibit FSH so that no eggs mature. Progesterone reduces fertility too — for example by causing the production of thick mucus that stops sperm reaching an egg.

Part 2Non-hormonal methods

Non-hormonal methods don't change the menstrual cycle — they physically stop the sperm and egg meeting.

These include barrier methods such as condoms and diaphragms, which stop sperm reaching an egg (a condom also helps prevent the spread of sexually transmitted infections). Intrauterine devices (IUDs) are placed in the uterus to prevent implantation; some also release a hormone. Spermicides kill or disable sperm. Surgical sterilisation cuts or ties the tubes that carry sperm or eggs. Abstinence — not having intercourse, especially around ovulation — also avoids pregnancy.

Part 1Fertility drugs & IVF

If a woman doesn't produce enough FSH, eggs may not mature, so she can't get pregnant. FSH and LH can be given as a fertility drug to stimulate eggs to mature and be released — this can help her become pregnant naturally.

IVF (in vitro fertilisation) is used when other treatments don't work. The steps are:

Part 2Evaluating IVF

IVF is a good example of where you're asked to evaluate a treatment — give the benefits, the drawbacks, and a justified conclusion.