There is no transportation system in any city that can compare in efficiency with the circulatory system of the body.

If you will imagine two systems of pipes, one large and one small, both meeting at a central pumping station, you’ll have an idea of the circulatory system. The smaller system of pipes goes from the heart to the lungs and back. The larger one goes from the heart to the various other parts of the body.

These pipes are called arteries, veins, and capillaries. Arteries are blood vessels in which blood is going away from the heart. In veins, the blood is coming back to the heart. In general, arteries are carrying pure blood to various parts of the body; and the veins are bringing back blood loaded with waste products. The capillaries are tiny vessels for conducting blood from arteries to veins. The pumping station is the heart.

Arteries lie deep in the tissues, except at the wrist, over the instep, at the temple, and along the sides of the neck. At any of these places, the pulse can be felt, and a doctor can get an idea of the condition of the arteries.

The largest arteries have valves where they leave the heart; These vessels are made of many elastic muscles which can expand and contract. Arterial blood is bright red in color and moves through the arteries in spurts.

Veins lie closer to the surface of the skin; the blood in them is darker and flows more evenly; and they have valves at intervals all along their course.