The blood leaves the heart by the aorta—the largest artery springing from the left ventricle—passes through all its branches to every part of the body, and is returned by the veins to the heart, being poured into the right auricle by the venae cavae.

The blood, which leaves the heart bright red and full of oxygen, in the capillaries becomes changed in colour and quality, and returns to the heart dark and laden with impurities. The interchange is effected by the oxygen and nutritive material oozing through the thin walls of the capillaries, and the carbonic acid gas and other impurities being sucked up by them ; this causes the blood to become a purple colour ; it is by this interchange that the tissues are nourished and waste substances removed.

It is in the capillaries that the principal work of the blood is done ; they are fine hair-like vessels, which connect the arteries and veins and form a close network through nearly all the tissues, supplying every part with needful nutriment, and carrying off effete matter and material which may be reorganized and used again. They are most plentiful where blood is most needed, as in various organs. They supply the necessary substances for secretion to the glands ; they take up the products of digestion in the intestines ; in the kidneys they get rid of the waste collected from other parts ; in the lungs they give up poisonous matter and take up oxygen. Capillaries are formed by the breaking up and subdivision of arteries ; veins are formed by the union of capillaries, beginning with very small veins, which again unite to form larger ones. The blood flows more quickly in the large vessels, and is at its slowest in the capillaries.