IGCSE Biology / Chemical coordination
The endocrine system revision notes
Edexcel International GCSE Biology (4BI1), chapter 7: Chemical coordination, specification point 2.94, 2.95B. Free lesson.
Watch the video lesson · IGCSE Biology past papers
Hormones are chemical messengers carried in the blood from glands to target organs. Knowing where the main glands are and what their hormones do helps explain how the endocrine system coordinates body functions.
What a hormone is
A hormone is a chemical messenger. It is made in a gland and carried in the blood, so it can reach all parts of the body. It acts on a target organ, which is the organ that responds to it. Hormones travel in the blood, unlike signals that travel along nerves.
Where the main endocrine glands are
The endocrine glands release hormones into the blood. Each gland makes particular hormones, which have particular effects. The pituitary is at the base of the brain. It makes ADH, FSH and LH. The thyroid is in the neck. It makes thyroxine. The adrenal glands sit above the kidneys, one on each kidney. They make adrenaline. The pancreas is behind the stomach. It makes insulin and glucagon. The ovaries make oestrogen and progesterone. The testes make testosterone.
What the named hormones do
The effects of hormones depend on the target organs they act on. Several hormones control important body processes, such as blood glucose, metabolism and the reproductive cycle. Insulin comes from the pancreas and lowers blood glucose. It causes the liver to convert glucose into glycogen for storage. Glucagon also comes from the pancreas and raises blood glucose. It causes the liver to break glycogen down into glucose. Adrenaline comes from the adrenal glands and prepares the body for action. It increases heart rate and breathing rate, and causes more glucose to be released.…
Why only a target organ responds
Although a hormone travels to many organs in the blood, it only affects organs with the right receptors. A receptor has a complementary shape to its hormone, so the hormone can bind to it. Without the matching receptor, a cell does not respond. For example, only organs with receptors complementary to adrenaline respond to it.