Q.Define the following:

Imagine a large building where one office needs to send instructions to many rooms at once. It could run a wire to each room (that is what nerves do), or it could simply release a message into the air-conditioning system so it reaches everyone, but only the rooms holding the right key actually respond. The body's second method of coordination works in this second way, and it is called the endocrine system.

The endocrine system is the body's collection of hormone-producing structures. Its central players are the endocrine glands, which have one unusual feature: they have no ducts — no little tubes to carry their product to a particular spot. For this reason they are also called ductless glands. Instead of piping their secretions somewhere, they pour them straight into the bloodstream. The chemicals they release are called hormones.

A hormone can be described as a non-nutrient chemical — not a food or fuel — that acts as a messenger between cells, and that the body needs only in very small, trace amounts. Because it is carried everywhere in the blood, you might expect a hormone to affect the whole body at once. It does not. A hormone influences only the cells that are built to respond to it, and these are called its target tissue or target organ. This is how a tiny amount of a chemical, spread through the entire body, can still produce a specific, localised effect.

A few points make the picture clearer:

• Ductless delivery. Endocrine glands secrete into the blood, not into a tube, which is why their product can reach distant parts of the body.
• Trace amounts. Hormones are powerful; only very small quantities are needed to bring about their effects.
• A whole system, not one organ. The endocrine system is scattered — it includes discrete glands such as the pituitary, thyroid, adrenals, pancreas and gonads, and also hormone-secreting cells within organs like the heart, kidney and gut.
• Simple to complex. Invertebrates manage with only a few hormones, while vertebrates such as humans rely on a large number of them to keep the body's activities in step.

Why does this matter? Together with the nervous system, the endocrine system keeps the body's many parts working as one — regulating growth, metabolism and development — but it does so more slowly and more broadly, using chemical messages rather than electrical ones. Understanding these basics is the foundation for making sense of every individual gland and hormone that follows.