Chemical Coordination and Integration

Besides the fast nervous system, the body has a second, slower control system — the endocrine system — that coordinates activities using chemicals called hormones. A hormone is a chemical messenger, made in a gland and carried by the blood to a distant part of the body, where it controls some activity. Hormones work in very small amounts and bring about slower but longer-lasting effects than nerves.

The glands that make hormones are the endocrine glands, also called ductless glands because they pour their secretions directly into the blood (unlike exocrine glands such as salivary or sweat glands, which release their products through ducts). The nervous system and the endocrine system work together to keep the body coordinated — this combined control is the basis of the chapter's title, "integration."

The chief difference between nervous and chemical (hormonal) coordination:

• Nervous — uses electrical impulses along neurons; very fast, acts on specific points, and the effect is short-lived.
• Hormonal — uses chemicals carried in blood; slower to act, can affect many parts at once, and the effect lasts longer.

Together these two systems control growth, development, metabolism, reproduction and the body's response to stress, keeping the internal environment balanced (homeostasis).

The body has several endocrine glands, each making one or more hormones with specific jobs. The most important ones are:

• Pituitary gland — a tiny gland at the base of the brain, called the master gland because it controls many other endocrine glands. It also makes growth hormone, which controls body growth.
• Thyroid gland — in the neck; makes thyroxine, which controls the rate of metabolism. It needs iodine to work.
• Pancreas — makes insulin and glucagon, which control the level of glucose (sugar) in the blood. (The pancreas is both endocrine and exocrine.)
• Adrenal glands — on top of the kidneys; make adrenaline, the "fight-or-flight" hormone released during fear, anger or excitement (it speeds the heart and prepares the body for emergency).
• Sex glands (gonads) — the testes make testosterone (male hormone) and the ovaries make oestrogen and progesterone (female hormones); these control sexual development and reproduction.

Two examples show how precisely hormones act: insulin lowers blood sugar after a meal, while adrenaline instantly readies the body to react to danger. The amount of each hormone is carefully controlled by feedback — if there is too much or too little, the body adjusts its release to bring it back to normal.

Although a hormone travels in the blood and reaches every part of the body, it only affects certain organs — its target organs. This is because the cells of the target organ carry special receptors that exactly fit that hormone (a lock-and-key arrangement). When the hormone binds its receptor, it switches on a particular response inside the cell — for example, insulin binds liver and muscle cells and makes them take in glucose from the blood. Cells without the matching receptor simply ignore the hormone. This is why a hormone produces a specific effect even though it is carried everywhere.

Because hormones must be present in just the right amount, too much or too little of a hormone causes a disorder:

• Diabetes mellitus — too little insulin (or the body not responding to it), so blood sugar stays high; it can cause sugar in the urine and many complications. It is managed by diet, exercise and sometimes insulin injections.
• Goitre — swelling of the thyroid gland in the neck, often caused by a lack of iodine in the diet (which is why we use iodised salt).
• Dwarfism (too little growth hormone in childhood) and gigantism (too much) — disorders of body growth.
• Stress conditions linked to overactive adrenaline.

Understanding these disorders shows how finely the endocrine system is balanced. A healthy diet (including iodine and not too much sugar), regular exercise, and timely medical care help keep the hormones — and therefore the whole body — in balance.