Homeostasis & response flashcards

Key points

• Homeostasis is keeping the body's internal conditions at the best level for cells to work, despite changes inside and outside.
• It controls blood glucose, body temperature and water levels.
• It matters because enzymes only work well in a narrow range of conditions.
• Body temperature stays near 37 °C: much hotter and enzymes denature, much colder and reactions slow down.

Key points

• Receptors are cells that detect stimuli, which are changes in the environment.
• Coordination centres, such as the brain, spinal cord and pancreas, receive and process the information.
• Effectors bring about the response: muscles contract, and glands release hormones.
• Control systems can work by nerves, by hormones, or both.

The order

Key points

• A reflex is an automatic, rapid response that does not involve the conscious part of the brain.
• Reflexes protect the body: pulling a hand off something hot, blinking, the pupil narrowing in bright light.
• The relay neurone is in the spinal cord, so the impulse turns round without waiting for the brain.

The path of the impulse

Key points

• The central nervous system (CNS) is the brain and spinal cord. It coordinates the response.
• Neurones carry information as electrical impulses: sensory neurones in from receptors, motor neurones out to effectors.
• A synapse is the junction between two neurones, with a tiny gap.
• The impulse triggers the release of a chemical, which diffuses across the gap and starts an impulse in the next neurone.

Key points

• A partner holds a ruler just above your open hand and lets go without warning.
• Catch it as fast as you can and read how far it fell: a shorter distance means a faster reaction.
• Repeat several times and find the mean.
• Change one factor only, such as drinking caffeine, and keep the hand, the starting height and the person the same.

Three catches

Key points

• A hormone is a chemical messenger made by a gland and carried in the blood to a target organ.
• The glands make up the endocrine system. The pituitary gland in the brain is the master gland, controlling many others.
• Nerves: electrical impulses that act very fast, for a short time, on one precise place.
• Hormones: slower to act, but their effects last longer and can reach more of the body.

Key points

• Adrenaline is made by the adrenal glands in times of fear or stress.
• It raises the heart rate, so more oxygen and glucose reach the brain and muscles.
• This gets the body ready for "fight or flight".
• Thyroxine, from the thyroid gland, raises the basal metabolic rate and matters for growth and development.
• Thyroxine levels are kept steady by negative feedback.

Key points

• The pancreas monitors and controls the concentration of glucose in the blood.
• When blood glucose is too high, the pancreas releases the hormone insulin.
• Insulin makes glucose move from the blood into the body's cells.
• In liver and muscle cells the extra glucose is turned into glycogen and stored.

Key points

• When blood glucose is too low, the pancreas releases the hormone glucagon.
• Glucagon makes the liver turn glycogen back into glucose and release it into the blood.
• In negative feedback, a change in a level triggers the opposite change, back to normal.
• Insulin and glucagon work this way: one lowers blood glucose when it rises, the other raises it when it falls.
• Negative here means the response opposes the change; it does not mean harmful.

Key points

• Type 1: the pancreas does not make enough insulin, so blood glucose rises too high.
• Type 1 is treated with insulin injections, timed with meals and exercise.
• Type 2: the body's cells no longer respond to the insulin the pancreas makes.
• Type 2 is treated with a carbohydrate-controlled diet and exercise. Obesity is a risk factor for it.

Key points

• At puberty, oestrogen (from the ovaries) and testosterone (from the testes) bring on the changes of puberty. Testosterone stimulates sperm production.
• FSH, from the pituitary gland, makes an egg mature in an ovary.
• Oestrogen makes the lining of the uterus build up.
• LH, from the pituitary gland, triggers the release of the egg: ovulation, about halfway through the cycle.
• Progesterone keeps the lining in place. When its level falls, the lining breaks down and the period starts.

Key points

• FSH makes an egg mature and stimulates the ovaries to make oestrogen.
• Rising oestrogen stops FSH being released and stimulates the release of LH.
• LH triggers ovulation.
• Progesterone keeps the uterus lining in place and stops both FSH and LH being released.
• When progesterone falls, the lining breaks down and FSH can rise again, starting the next cycle.

Key points

• FSH and LH can be given as a fertility drug, to help eggs mature and be released.
• In IVF, the mother is given FSH and LH so that several eggs mature.
• The eggs are collected and fertilised by the father's sperm in a laboratory.
• The fertilised eggs grow into embryos, and one or two are put into the mother's uterus.
• IVF is stressful, often does not work, and can lead to multiple births.

Key points

• The combined pill holds oestrogen and progesterone, which stop FSH being made, so no eggs mature.
• An implant, injection or skin patch releases progesterone slowly, stopping eggs maturing and being released for months or years.
• Condoms and diaphragms are barriers that stop sperm reaching an egg.
• An IUD stops an embryo implanting; spermicides kill or disable sperm; sterilisation is surgery.
• Hormonal methods are very reliable but can have side effects, and none of them stops infections.