Endocrine signalling: hormones coordinating the body — Biology, 14–17
Hormones carry messages through the blood, changing what target cells do. This explains slower, longer-lasting coordination such as puberty, stress and blood-glucose control.
A message that travels
A hormone is a chemical message released by one group of cells and carried, often in the blood, to particular target cells. Only cells with the matching receptor respond, rather like a key fitting one lock. The same hormone can therefore travel widely but affect only certain tissues.
Why use hormones?
Your body often needs many organs to change together, but nerve signals are not the only solution. Hormones provide a slower broadcast system for changes that may last minutes, days or years, such as growth or puberty. This idea came from studying glands whose secretions worked even when nerves could not carry the message.
Blood glucose after a meal
Imagine blood glucose rising from 5 to 8 mmol/L after a meal. Cells in the pancreas detect the rise and release insulin. Insulin binds to receptors on muscle and liver cells, causing them to take up or store more glucose, so the level moves back towards about 5 mmol/L. The message stops as the change is corrected.
The reasonable mistake
It is tempting to say that a hormone affects every cell because it travels through the whole bloodstream. That sounds reasonable: the message passes every organ. The missing step is the receptor. Without the matching receptor, a cell cannot read that hormone’s message, just as a phone cannot receive a call meant for a different number.
Where it appears
Endocrine signalling helps explain why a contraceptive pill can alter ovulation, why adrenaline changes the body quickly during danger, and why diabetes involves glucose regulation. Doctors measure hormone levels and use hormone-like medicines, but a treatment can have side effects if the same receptors occur in several tissues.
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