7.1Human endocrine system
The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ, where it produces an effect.
Compared to the nervous system, the effects are slower but act for longer. That comparison is asked for constantly, so hold both halves of it.
The pituitary gland in the brain is a 'master gland' which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released.
- Endocrine system
- glands which secrete chemicals called hormones directly into the bloodstream, which the blood carries to a target organ where they produce an effect
- Hormone
- a chemical secreted by a gland into the bloodstream, whose effects are slower than the nervous system but act for longer
- Pituitary gland
- a 'master gland' in the brain which secretes hormones in response to body conditions, and whose hormones act on other glands to stimulate further hormones
7.2–7.3Negative feedbackHigher tier
Higher tier only
Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for 'flight or fight'.
Thyroxine, from the thyroid gland, stimulates the basal metabolic rate — the rate of the body's chemical reactions at rest. It plays an important role in growth and development.
The difference between them is a good illustration of the endocrine system's range: adrenaline is a short, sharp response to a situation, thyroxine a steady background setting.
Thyroxine levels are controlled by negative feedback. When thyroxine in the blood falls below normal, the pituitary releases more of the hormone that stimulates the thyroid, so more thyroxine is produced. When thyroxine rises above normal, that stimulating hormone is inhibited and less thyroxine is made.
The general shape is the thing to learn, because it recurs: too high triggers what lowers it, too low triggers what raises it, and the level oscillates around a norm rather than sitting perfectly still.
Common mistake
Negative feedback does not mean a bad effect. It means the response opposes the change — negative in the sense of reversing it. Blood glucose, body temperature, water level and thyroxine are all controlled this way, so recognising the shape saves learning four separate stories.
- Negative feedback
- a control mechanism in which a level rising above normal triggers what lowers it and a level falling below normal triggers what raises it, so the level oscillates around a norm
- Adrenaline
- the hormone produced by the adrenal glands in times of fear or stress, which increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles
- Thyroxine
- the hormone from the thyroid gland that stimulates the basal metabolic rate and plays an important role in growth and development
Before you practise
- Hormones are chemicals secreted by glands, carried in the blood to a target organ — slower than nerves, but longer lasting.
- Place the glands on a diagram: pituitary, thyroid, adrenal, pancreas, ovaries and testes.
- The pituitary is the master gland: it releases hormones that act on other glands.
- Adrenalin increases heart rate, blood flow to the muscles and blood glucose, preparing the body for fight or flight.
- Thyroxine controls metabolic rate, and its own level is held steady by negative feedback — a low level triggers release, a high level suppresses it.
- Negative feedback answers need the whole loop: the change, what detects it, what responds, and the return towards normal.