Homeostasis and control systems, the nervous system and reflexes, the structure and function of the brain and eye (separate sciences), hormonal coordination by the endocrine system, the control of blood glucose, and human reproduction hormones.

What this topic is asking

AQA wants you to define homeostasis, describe the nervous system and the reflex arc, explain hormonal coordination by the endocrine system, describe how blood glucose is controlled and what diabetes is, and describe the hormones in human reproduction.

Homeostasis and the nervous system

All control systems, whether nervous or hormonal, share three components: receptors (cells that detect a change in the environment, the stimulus), coordination centres (such as the brain, spinal cord or pancreas, which receive and process information) and effectors (muscles that contract or glands that secrete a hormone) that bring about a response that restores the optimum level. This is negative feedback: the response reverses the change that triggered it, keeping the internal condition near a steady set point.

The nervous system sends fast electrical impulses along neurones. A reflex is rapid and automatic and does not involve the conscious areas of the brain, which is why reflexes (such as the knee-jerk or pupil reflex) protect the body before you have time to think.

Hormonal coordination

The endocrine system is made of glands that secrete hormones (chemical messengers) directly into the blood, which carries them to target organs. Compared with nervous responses, hormonal responses are slower to start but longer lasting and more general. The pituitary gland in the brain is the master gland: it secretes hormones that act on other glands to release further hormones, coordinating the whole system.

Blood glucose is controlled by the pancreas through two hormones working in opposition. When glucose is too high, the pancreas releases insulin, which makes body cells take up glucose and makes the liver and muscles convert excess glucose into glycogen for storage, so the level falls. When glucose is too low, the pancreas releases glucagon, which makes the liver convert glycogen back into glucose and release it, so the level rises. This pair is a classic negative-feedback loop. In type 1 diabetes the pancreas produces too little or no insulin, blood glucose can rise dangerously, and it is treated with insulin injections matched to diet and exercise. Type 2 diabetes is where body cells stop responding properly to insulin; it is linked to obesity and is usually controlled first by a carbohydrate-controlled diet and exercise.

Human reproduction

During puberty, testosterone (from the testes) and oestrogen (from the ovaries) trigger the secondary sexual characteristics and the production of gametes. The menstrual cycle is controlled by four hormones: FSH (from the pituitary) causes an egg to mature in the ovary and stimulates oestrogen release; oestrogen repairs and thickens the uterus lining and triggers a surge of LH; LH (from the pituitary) causes ovulation (release of the egg) around day 14; and progesterone (from the empty follicle) maintains the uterus lining, with its fall triggering menstruation if no egg is fertilised. Hormonal contraceptives use oestrogen or progesterone to prevent the maturation or release of eggs, while fertility treatment such as IVF uses FSH and LH to stimulate the maturation of several eggs for collection.