Homeostasis and Response

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for normal cell function and enzyme action. In humans, important controlled conditions include blood glucose concentration, body temperature and water levels. Control systems contain receptors that detect stimuli, coordination centres that process information and effectors such as muscles or glands that produce a response.

The nervous system allows the body to detect changes and respond rapidly. Receptors detect a stimulus and electrical impulses travel along neurones to the central nervous system, which consists of the brain and spinal cord. The response pathway can be summarised as stimulus → receptor → coordinator → effector → response. The junction between two neurones is called a synapse.

Reflex actions are rapid, automatic responses that help protect the body and do not require conscious thought. A reflex arc commonly involves a sensory neurone carrying an impulse from a receptor, a relay neurone in the central nervous system and a motor neurone carrying the impulse to an effector. Reaction-time investigations can be used to study factors affecting nervous responses while controlling other variables.

The brain contains billions of interconnected neurones and different regions have different functions. GCSE Biology students should identify the cerebral cortex, cerebellum and medulla and relate them to their functions. Investigating brain function is difficult because the brain is complex and delicate, although techniques such as MRI scanning and electrical stimulation can help scientists map its activity.

The eye is a sense organ containing light-sensitive receptors. Important structures include the retina, optic nerve, sclera, cornea, iris, ciliary muscles and suspensory ligaments. The sclera is the white part of the eye. The retina detects light, while the optic nerve carries electrical impulses from the retina to the brain.

Accommodation changes the shape of the lens so that objects at different distances can be focused on the retina. For near objects, the ciliary muscles contract, suspensory ligaments loosen and the lens becomes thicker, causing stronger refraction. For distant objects, the ciliary muscles relax, suspensory ligaments tighten and the lens becomes thinner, causing weaker refraction.

Myopia is short-sightedness, where distant objects are not focused correctly on the retina, while hyperopia is long-sightedness, where near objects are not focused correctly. These defects can usually be corrected using suitable spectacle or contact lenses that refract light so the image is focused on the retina. GCSE questions may require interpretation of ray diagrams showing these defects and their correction.

Body temperature is monitored and controlled by the thermoregulatory centre in the brain, which receives information from temperature receptors in the skin and brain. When body temperature is too high, vasodilation and sweating increase heat loss. When body temperature is too low, vasoconstriction reduces heat loss, sweating stops and skeletal muscles may contract rapidly in shivering.

The endocrine system consists of glands that release hormones directly into the bloodstream. Hormonal responses are generally slower than nervous responses but their effects often last longer. Important endocrine glands include the pituitary gland, pancreas, thyroid gland, adrenal glands, ovaries and testes. The adrenal glands produce adrenaline, while the ovaries and testes produce major reproductive hormones.

Blood glucose concentration is monitored and controlled by the pancreas. When blood glucose is too high, insulin causes glucose to move from the blood into cells and promotes conversion of excess glucose to glycogen in the liver and muscles. When blood glucose is too low, glucagon causes stored glycogen to be converted back into glucose and released into the blood. Insulin and glucagon therefore act together to maintain stable blood glucose levels.

Type 1 diabetes occurs when the pancreas produces little or no insulin, resulting in uncontrolled blood glucose concentration, and is commonly treated with insulin. In Type 2 diabetes, body cells become less responsive to insulin. A carbohydrate-controlled diet and exercise can help manage Type 2 diabetes, and obesity is a recognised risk factor. Students should be able to compare Type 1 and Type 2 diabetes and interpret data showing the effect of insulin on blood glucose.

Water and ion balance are important because excessive gain or loss of water by osmosis can prevent cells from functioning properly. Water leaves the body through the lungs, skin and kidneys, while ions and urea are also lost in sweat and urine. The kidneys filter the blood and selectively reabsorb useful substances such as glucose, some ions and the required amount of water. Excess water, ions and urea are removed in urine.

ADH helps regulate water balance. When the blood becomes too concentrated, the pituitary gland releases more ADH, causing the kidney tubules to reabsorb more water into the blood and producing a smaller volume of more concentrated urine. Kidney failure may be treated using dialysis or a kidney transplant. Students should understand the basic principle of dialysis and be able to evaluate dialysis compared with transplantation.

Hormones coordinate human reproduction. Testosterone is the main male reproductive hormone and stimulates sperm production. Oestrogen is a major female reproductive hormone. In the menstrual cycle, FSH stimulates maturation of an egg, LH triggers ovulation, and oestrogen and progesterone help control and maintain the uterine lining. A gamete is a reproductive cell: sperm are male gametes and eggs are female gametes.

Fertility can be controlled or assisted using hormones. Contraceptive methods include hormonal pills, injections, implants and patches; barrier methods such as condoms and diaphragms; intrauterine devices; spermicides; abstinence during fertile periods; and sterilisation. Fertility treatment may use FSH and LH, while IVF involves stimulating egg maturation, collecting eggs, fertilising them with sperm outside the body, allowing embryos to develop and transferring one or more embryos to the uterus. Negative feedback controls hormones such as thyroxine, while adrenaline prepares the body for “fight or flight”. Plants also use hormones: auxin controls phototropism and gravitropism, gibberellins stimulate seed germination and can increase fruit size, and ethene controls fruit ripening.