There are all kinds of big roads and highways connecting big cities so that food and other necessary materials can reach them. But what about the small towns and little villages? They need food and supplies, too.

The little roads and byways that reach the small towns and villages are like the capillaries in the human body. As we know, blood is pumped out of the heart to all parts of the body and the big vessels through which this blood flows are known as the arteries.

But far from the heart, in all parts of the body, these big vessels become tiny hairlike branches called capillaries. A capillary is 50 times thinner than the thinnest human hair! In fact, the average diameter of a capillary is about 0.008 millimeters. It is so thin that blood corpuscles pass through a capillary in single file, which means blood passes through the capillaries very slowly.

About 700 capillaries could be packed into the space occupied by a pin. Each capillary is about 0.5 millimeters long. Since the purpose of capillaries is to bring and take away needed substances to and from every part of the body, you can imagine how many millions upon millions of capillaries there are in the human body.

What happens when blood flows through a single capillary (which takes about a second) ? The blood does not leave the capillary. But the wall of the capillary is very thin; it consists of only a single layer of cells. Through this wall, the blood gives up its oxygen to the surrounding tissues. In return, it receives the carbon dioxide which the tissues around the capillary have given up.

At the same time, other substances which supply nourishment to the tissues pass from the blood, and waste products enter the capillary. Eventually, the blood and the materials it has picked up are returned to the heart by way of the veins.