Homeostasis is the regulation of the internal conditions of a cell or organism to maintain a stable environment in response to internal and external changes. In humans this keeps conditions such as blood glucose concentration, body temperature and water levels at the right values for enzymes and cells to work well. Control is achieved by two coordinating systems: the fast, electrical nervous system and the slower, chemical endocrine system. This topic covers how these systems detect changes, respond to them and use negative feedback to correct deviations, and finishes with how plants respond to their surroundings using hormones.
All control systems, whether nervous or hormonal, share the same components:
Conditions controlled by homeostasis include blood glucose concentration, internal body temperature and water levels. Keeping these stable provides the best conditions for enzyme action and all cell functions.
The nervous system lets humans react to their surroundings and coordinate behaviour. Information from receptors passes as electrical impulses along neurones. The pathway is: stimulus → receptor → sensory neurone → coordinator (CNS) → motor neurone → effector → response.
The central nervous system (CNS) is the brain and spinal cord. Reflex actions are automatic and rapid; they do not involve the conscious part of the brain, which makes them protective (for example pulling a hand off a hot object). In a reflex arc, the impulse passes from a sensory neurone, across a synapse to a relay neurone in the spinal cord, across another synapse to a motor neurone, then to the effector. At a synapse the impulse is carried across the tiny gap by a chemical (neurotransmitter) that diffuses across and triggers a new impulse in the next neurone.
The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions:
Neuroscientists have mapped these regions by studying patients with brain damage, by electrically stimulating different parts of the brain, and by using MRI scanning techniques. The brain is complex and delicate, which makes investigating and treating brain disorders very difficult.
The eye is a sense organ containing receptors sensitive to light intensity and colour. Key structures: the cornea and lens refract (bend) light onto the retina; the retina contains light receptors; the optic nerve carries impulses to the brain; the iris controls how much light enters; the ciliary muscles and suspensory ligaments change lens shape.
Accommodation is changing the lens shape to focus on near or distant objects. To focus on a near object the ciliary muscles contract, suspensory ligaments loosen, and the lens becomes thicker and more curved. To focus on a distant object the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is pulled thin so it refracts light less strongly.
Defects: myopia (short sightedness) and hyperopia (long sightedness) occur when the lens cannot focus the image on the retina. They are usually treated with spectacle lenses (concave for myopia, convex for hyperopia), and also with contact lenses, laser surgery to reshape the cornea, or replacement lens surgery.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain, which contains receptors sensitive to the temperature of the blood. The skin also contains temperature receptors that send impulses to the centre.
If body temperature is too high, blood vessels near the skin surface dilate (vasodilation) so more blood flows near the surface and more heat is lost, and sweat glands release sweat which cools the body as it evaporates. If body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops, and skeletal muscles contract rapidly (shivering) which requires respiration, releasing some energy as heat to warm the body.
The endocrine system is composed 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 responses are usually slower but act for longer.
The pituitary gland in the brain is the ‘master gland’: it secretes several hormones in response to body conditions, and many of these act on other glands to release further hormones that cause effects. Other important glands include the thyroid, adrenal gland, pancreas, ovaries (in females) and testes (in males).
Blood glucose concentration is monitored and controlled by the pancreas. If blood glucose is too high, the pancreas produces the hormone insulin, which causes glucose to move from the blood into cells. In the liver and muscles excess glucose is converted to glycogen for storage.
If blood glucose is too low, the pancreas produces the hormone glucagon (Higher tier). This causes glycogen to be converted back into glucose and released into the blood. Insulin and glucagon act as opposites in a negative feedback cycle.
Type 1 diabetes is a disorder in which the pancreas fails to produce enough insulin, causing uncontrolled high blood glucose. It is normally treated with insulin injections. Type 2 diabetes is where body cells no longer respond to insulin; obesity is a risk factor. It is treated with a carbohydrate-controlled diet and exercise.
During puberty reproductive hormones cause secondary sex characteristics to develop. Testosterone is the main male hormone, produced by the testes, and stimulates sperm production. Oestrogen is the main female hormone, produced by the ovaries.
The menstrual cycle is controlled by four hormones. FSH (from the pituitary) causes an egg to mature in an ovary. Oestrogen (from the ovaries) causes the uterus lining to build up and inhibits FSH. LH (from the pituitary) stimulates the release of the egg (ovulation) at around day 14. Progesterone (from the ovaries) maintains the uterus lining during the second half of the cycle; when its level falls the lining breaks down.
Fertility can be controlled by hormonal and non-hormonal methods of contraception. Hormonal methods include oral contraceptives containing hormones to inhibit FSH so that no eggs mature, and injections, implants or skin patches of slow-release progesterone. Non-hormonal methods include barrier methods such as condoms and diaphragms, intrauterine devices, spermicides, abstaining from intercourse near ovulation, and surgical sterilisation.
Higher tier: Hormones can be used to treat infertility. FSH and LH can be given in a ‘fertility drug’ to stimulate eggs to mature. In IVF (in vitro fertilisation), a woman is given FSH and LH to stimulate the maturation of several eggs, which are collected and fertilised by sperm in the laboratory; the fertilised eggs develop into embryos and one or two are inserted into the uterus. IVF can be emotionally and physically stressful, has low success rates, and can lead to multiple births which are risky for mother and babies.
Negative feedback (Higher tier): When a level in the body rises above or falls below the normal range, this is detected and a corrective mechanism is triggered to bring it back towards normal. For example, thyroxine from the thyroid (controlled by the pituitary hormone TSH) regulates the metabolic rate, and adrenaline from the adrenal glands prepares the body for ‘fight or flight’ by increasing heart rate.
Plant hormones (biology only): Plants respond to stimuli with growth responses called tropisms. A positive phototropism is growth towards light (shoots) and gravitropism (geotropism) is growth in response to gravity (roots grow down, shoots grow up). These are controlled by the hormone auxin, which is made at shoot tips and controls growth by stimulating cell elongation; it becomes unevenly distributed in response to light and gravity. Other plant hormones are gibberellins (initiate seed germination) and ethene (controls cell division and fruit ripening). Auxins are used as weed killers and rooting powders, gibberellins to end seed dormancy and promote flowering, and ethene to control fruit ripening during storage and transport.
Practise exam-style questions on this topic.