The impure or venous blood which is poured into the right auricle by the venae cavae passes into the right ventricle, and leaves the heart by the pulmonary artery, which divides in two, one part passing into the right lung, the other into the left. They go on dividing and subdividing till the capillaries permeate every part of the lungs and surround the air cells. It is here that the blood gives off carbonic acid gas and all impurities, and oxygen is absorbed by the red corpuscles from the freshly-breathed air. This renewed blood is collected by small veins, which unite to form larger ones, till they give rise to the pulmonary veins, two from each lung, which carry the blood to the left auricle. In this circulation it is the impure blood which leaves the heart, and purified blood which is poured into it ; the capillaries give up carbonic acid and absorb oxygen, the arteries carry bad blood, and the veins carry good, all of which is quite the reverse of the greater circulation. Bloodvessels springing from the heart are always called arteries, and the vessels pouring into the heart always veins. These two circulations, starting from the ventricles and returning to the auricles, are always going on simultaneously ; blood is poured into the auricles from the venae cavae and pulmonary veins, the auricles dilate to receive the blood, then contract, the tricuspid and mitral valves open, the ventricles dilate to receive the blood, then contract, the valves between them and the auricles close, and the semilunar valves open, and the blood is forced into the great arteries.

The semilunar valves at the entrances to these arteries prevent the blood from flowing back.

If the finger be placed on an artery, say, at the wrist or temple, an intermittent expansion of the artery will be felt ; this is caused by the contraction of the heart, and constitutes what is called 'the pulse.' In the first few years of life the child's pulse averages 100 beats a minute ; about fourteen or fifteen it has fallen to eighty, and in the man to seventy-two. It varies according to position, being slower when lying or sitting than when standing. It is increased by movement.