Hormones in animals

"Why do we need hormones if we have nerves?" Nerve impulses reach only the cells that nerves are connected to, and a nerve cell needs a little time to reset before sending the next impulse. Hormones are chemicals released into the blood, so they reach every cell and their effect lasts. They are slower, but steady.

Imagine a dog chasing you. Your heart races, you breathe fast, and you suddenly have the strength to run. That is adrenaline, released by the adrenal glands. It sends more blood to the muscles and speeds up the heart and breathing, getting the body ready for an emergency.

Each gland, its hormone, and what it does:

What goes wrong without them:

• The thyroid needs iodine to make thyroxin. Without enough iodine, the neck swells: goitre. That is why we use iodised salt: check the packet at home.
• Too little growth hormone in childhood causes dwarfism.
• Too little insulin makes blood sugar rise: diabetes.

The hypothalamus in the brain sends releasing factors that tell the pituitary when to release its hormones.

The right amount of hormone is kept by feedback. Think of a water-tank alarm that switches off the motor when the tank is full. When blood sugar rises, the pancreas releases more insulin; as the sugar comes down, insulin release is reduced. The change itself switches the response off.