Endocrine system: hormones

The nervous system is not your body's only messaging network. A second system sends messages with chemicals. This is the endocrine system, and its messengers are called hormones.

A hormone is a chemical made by a gland and released into the bloodstream. The blood carries it all around the body, but a hormone only affects its target cells, the cells that have the matching receptors. A receptor works like a lock, and the hormone is the key. Cells without the right receptor ignore the hormone. Hormones are powerful in tiny amounts, so a very small quantity can change how many cells behave.

You may already know some hormones. Insulin helps control the amount of sugar in your blood. Adrenaline (also called epinephrine) is released when you are frightened or excited; it raises your heart rate and gets your body ready for action. Growth hormone helps children and teens grow. Thyroid hormones set how fast your cells use energy. Estrogen and testosterone guide the changes of puberty. Melatonin helps signal when it is time to sleep. Hormones do far more than control puberty; they affect growth, energy, mood, sleep, and your response to stress.

How is this different from the nervous system? Nerve messages are electrical impulses sent along neurons to specific cells, and they are very fast and brief. They act in milliseconds, like a phone call to one person. Hormone messages travel by blood to many targets at once, so they take longer, seconds to hours or more, but the effects can last much longer, like a radio announcement to everyone listening. The two systems cooperate constantly. When you see something frightening, the nervous system reacts at once and signals the adrenal glands to release adrenaline, which keeps your body on high alert for a while.

The endocrine system is also controlled by feedback. Often, when a hormone has done its job, the change it caused signals the gland to make less, which keeps levels steady. You will see how this fits into homeostasis at the end of the unit.

A common mistake is thinking hormones are only about puberty or only about moods. Another is thinking they work like nerves. They are slower, they travel in blood, and they affect only cells with the right receptor. Next lesson: the glands that make them.