5. The vessels which carry blood away from the heart are called arteries, and those which carry blood back to the heart are called veins.
6. The arteries and veins are connected by small tubes called capillaries.
7. The blood found in the arteries is red; that in the veins is dark blue or purple.
8. The color of the blood changes from red to blue in going through the capillaries. The change is due to the loss of oxygen.
9. In the circulation of the lungs, the blood in the arteries is blue, that in the veins, red.
10. The change from blue to red takes place while the blood is passing through the capillaries of the lungs. The change is due to the oxygen which the corpuscles of the blood take up in the lungs.
11. The pulse is caused by the beating of the heart.
12. The heart does a great deal of work every day in forcing the blood into different parts of the body.
13. Some of the white blood corpuscles escape from the blood-vessels through the thin walls of the capillaries.
14. These corpuscles return to the heart through small vessels called lymph channels or lymphatics.