Page:Life in Motion.djvu/145

Rh pass into this large jar, you see how it effervesces.

The question that next arises is, Where does this carbonic acid come from? The blood, as you know, is sent out through the arteries by the force of the heart-beat; these arteries become smaller and smaller until they end in networks of fine vessels called capillaries, many of which are not wider than the one-two-thousandth of an inch; and from these capillaries the veins originate that carry the blood back to the heart. Capillaries exist in greater or less number in almost all the tissues, and it is by the blood circulating in these that the tissues are nourished. Under the pressure in these minute vessels, fluid matter oozes through their walls and bathes all the neighbouring tissues. This fluid holds in solution the matters needed for nourishing the tissues, and it also contains gases in solution. Thus the fluid is both a nutritive and respiratory medium; by it the tissues are nourished, and by it they breathe. Each little element of tissue needs oxygen, and it produces carbonic acid gas. The blood in passing through the tissues thus