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The Circulation of the Blood.

The blood is forced by the heart through the arteries (black lines), and returns to it through the veins (dotted lines).

The impure blood flows from the arteries into the veins through tiny canals. The veins are tubes something like the arteries, and they usually lie beside the arteries. They and the arteries are like the two tracks of a great railway line, one going from the main station, the heart, the other coming back.

But the blood in the veins is not like that in the arteries. Instead of being bright red, and healthy-looking, it is dark colored. It flows more slowly, and it is full of impurities which it is carrying away from the body. The veins carry this blood to the right side of the heart, and the heart sends it to the lungs. There are many blood vessels in the lungs, and they are divided into branches running in every direction through the lung tissue. When we draw in a deep breath, we fill the lungs with fresh air. This is at once taken up by the impure blood in the branching blood vessels. The impurities are breathed out with the air that leaves our lungs, and the blood once more becomes bright red and full of new life. In this way the blood is purified. Then it is sent back to the heart, all ready to start out again through the body. The whole journey is made every two minutes.