Cells work properly only within a narrow range of conditions, and external conditions vary. Homeostasis is the set of mechanisms that keep internal conditions roughly constant anyway. Two systems do the work: the nervous system, which is fast and short-lived, and the endocrine system, which is slower and longer-lasting.
Homeostasis is the regulation of internal conditions to keep them roughly constant, in response to both internal and external change. It matters because enzymes and cells function only within narrow limits. The conditions controlled in humans are blood glucose concentration, body temperature and water levels.
The control systems studied at GCSE, nervous and hormonal, all include the same three parts:
Homeostatic control systems can be described in terms of receptors, coordination centres and effectors.
The nervous system carries electrical impulses along neurones, which makes it very fast and its effects short-lived. Information travels from a receptor along a sensory neurone to the central nervous system, meaning the brain and spinal cord, and out along a motor neurone to an effector.
Neurones do not touch. At a synapse the impulse arrives at the end of one neurone and triggers the release of a chemical, which diffuses across the tiny gap and sets off a new impulse in the next neurone. Transmission across a synapse is slower than conduction along a neurone.
A reflex is automatic and does not involve the conscious part of the brain, which is what makes it fast enough to be protective, such as pulling a hand away from something hot before the pain is noticed. The reflex arc runs: stimulus → receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector → response.
The brain is made of billions of interconnected neurones and controls complex behaviour. Four regions are named at GCSE:
Neuroscientists have mapped the regions of the brain in three main ways: by studying patients with brain damage and seeing which abilities were lost, by stimulating parts of the brain electrically and observing the effect, and by using functional MRI scans to identify regions that become more active during a particular task.
Treating damage is much harder than mapping it. The brain is extremely complex and delicate, it is enclosed in the skull so access is difficult, and the tissue is easily damaged further by surgery. Drugs often cannot reach the brain because of the blood-brain barrier, and nerve tissue does not readily repair itself.
The eye is a sense organ containing receptors sensitive to light intensity and colour. Light passes through the transparent cornea, which does most of the focusing, then through the pupil, whose size is controlled by the muscles of the iris, and through the lens onto the retina, where the receptor cells are. Impulses travel to the brain along the optic nerve.
Accommodation is the process of changing the shape of the lens to focus on objects at different distances. For a near object the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more strongly refracting. For a distant object the ciliary muscles relax, the ligaments are pulled tight, and the lens is stretched thin.
Two common defects: in myopia (short sight) light is brought to a focus in front of the retina and a concave lens corrects it; in hyperopia (long sight) light would focus behind the retina and a convex lens corrects it. Contact lenses, laser surgery and replacement lenses are alternatives to spectacles.
Human body temperature is held close to 37 °C because human cells and enzymes function effectively only within a narrow temperature range. The monitoring centre is the hypothalamus, which contains receptors sensitive to the temperature of the blood; receptors in the skin send information about the surroundings.
If the body is too hot, sweat is produced and evaporates from the skin, taking energy with it, and the blood vessels supplying the skin capillaries dilate, which is vasodilation, so more blood flows near the surface and more energy is transferred from the body to the surroundings.
If the body is too cold, sweating stops, the vessels constrict, which is vasoconstriction, so less energy is lost, and skeletal muscles contract rapidly in shivering, which requires respiration and releases energy that warms the body.
Blood vessels do not move up and down in the skin. It is their diameter, and therefore the volume of blood flowing through the surface capillaries, that changes.
The endocrine system is made of glands that secrete hormones directly into the bloodstream. The blood carries them to target organs, so the effects are slower to arrive than nervous impulses but last much longer.
The pituitary gland, in the brain, is often described at GCSE as the ‘master gland’, because it secretes several hormones in response to body conditions and many of those act on other glands to make them release their own hormones.
The glands to know are the pituitary, the thyroid (thyroxine), the adrenal glands (adrenaline), the pancreas (insulin and glucagon), and the ovaries and testes (oestrogen and testosterone).
Blood glucose concentration is monitored and controlled by the pancreas. When it rises too high, the pancreas secretes insulin, which causes glucose to move from the blood into liver and muscle cells, where excess is stored as glycogen. When it falls too low, the pancreas secretes glucagon, which causes the liver to convert glycogen back into glucose and release it.
The two hormones work as an opposing pair. Glycogen is the store; glucagon is the hormone that releases it.
In type 1 diabetes the pancreas produces little or no insulin, so blood glucose can rise dangerously high; it is treated with insulin injections. In type 2 diabetes the body's cells stop responding properly to insulin; obesity is a risk factor, and it can often be managed with a carbohydrate-controlled diet and more physical activity, though some people also need medication.
Water is lost from the lungs when breathing out, from the skin in sweat, and from the kidneys in urine. Water loss in urine is the route adjusted specifically to regulate the body's water balance. If body cells lose or gain too much water by osmosis they do not function efficiently.
Excess amino acids cannot be stored. They are broken down in the liver by deamination to form ammonia, which is toxic, and the liver immediately converts it to urea for the kidneys to excrete.
The kidney works by filtration and selective reabsorption. Blood is filtered under pressure at the glomerulus, and useful substances are selectively reabsorbed into the blood: all of the glucose in a healthy person, together with appropriate amounts of ions and water. What remains, namely urea together with excess ions and water, is released as urine.
ADH controls the last step. It is released from the pituitary when the blood is too concentrated, and it makes the collecting ducts more permeable, so more water is reabsorbed and the urine is more concentrated. This is a negative feedback loop.
Kidney failure can be treated by dialysis, in which blood flows past a partially permeable membrane against a dialysis fluid. Urea and excess mineral ions move from the blood into the fluid, and excess water can also be removed. The fluid contains normal concentrations of useful substances such as glucose, so those are not lost from the blood. A transplant is the alternative: it restores continuous function rather than requiring regular sessions, but donor organs are scarce and the recipient must take immunosuppressant drugs to prevent rejection.
Negative feedback is the general pattern behind homeostasis: when a level rises above normal a mechanism acts to bring it down, and when it falls below normal a mechanism acts to bring it up. The response opposes the change, returning the condition towards its normal range whenever it moves away from it.
Thyroxine is one example. It is released by the thyroid, stimulates the basal metabolic rate and is needed for growth and development. When the level in the blood rises, the release of the pituitary hormone that stimulates the thyroid is inhibited, so less thyroxine is produced.
Adrenaline is released by the adrenal glands in stressful or frightening situations. It increases heart rate and helps deliver more oxygen and glucose to the brain and muscles for the fight-or-flight response. It is not controlled by negative feedback.
At puberty reproductive hormones cause secondary sex characteristics to develop: testosterone from the testes in males, oestrogen from the ovaries in females.
Four hormones control the menstrual cycle:
FSH causes an egg to mature; LH triggers its release.
Hormonal methods work by preventing an egg maturing or being released. Oral contraceptives contain hormones that inhibit FSH. Implants, injections and patches contain synthetic hormones such as progestogens, sometimes combined with oestrogen, and can prevent ovulation, thicken cervical mucus and alter the uterus lining.
Non-hormonal methods work physically or chemically. Condoms and diaphragms are barriers that stop sperm reaching the egg, and condoms also reduce the transmission of sexually transmitted infections. Spermicides kill or disable sperm. Copper intrauterine devices are non-hormonal and reduce the chance of fertilisation. Hormonal intrauterine systems release a progestogen and also reduce the chance of pregnancy. Natural family-planning methods attempt to avoid intercourse during the fertile period, but they are less reliable because the timing of ovulation can vary. Sterilisation is a permanent surgical method.
Contraceptive methods can be classified as hormonal or non-hormonal.
Where a woman has too little FSH to mature her eggs, FSH and LH can be given as a fertility drug so that she ovulates and may become pregnant naturally.
In vitro fertilisation (IVF) goes further. The patient is given FSH and LH to make several eggs mature. The eggs are collected and fertilised with sperm in a laboratory dish. The fertilised eggs develop into small embryos, and one or two are inserted into the uterus.
IVF allows some people who could not otherwise conceive to have a child. Against that, the process is emotionally and physically stressful, the success rate is not high, and it can lead to multiple births, which carry risks for both the patient and the babies. Some people also object to the fate of the unused embryos.
Plants coordinate growth with hormones too. A tropism is a growth response to a directional stimulus: phototropism is growth in response to light, gravitropism (or geotropism) is growth in response to gravity.
Auxin is the hormone responsible, and it works by unequal distribution. In a shoot tip lit from one side, auxin accumulates on the shaded side, where it makes the cells elongate more, so the shoot bends towards the light. Shoots therefore grow towards light and against gravity. Roots show positive gravitropism and grow in the direction of gravity.
Two other hormones are named: gibberellins, which end seed dormancy and promote flowering, and ethene, which controls fruit ripening and, on the higher tier, is also described as controlling cell division.
Each of the three plant hormones has commercial uses:
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