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.