The total amount of body fluid is not large, for the spaces among the cells are in most cases extremely tiny; it follows that with all the millions of cells absorbing food materials and oxygen from this fluid and discharging waste materials into it the time will soon come when no more food or oxygen will be left to be absorbed and there will be no more capacity for holding waste substances. If this state of affairs were actually to happen, metabolism would come to an end and death would be the result; evidently there must be some means of keeping the body fluids constantly renewed in respect to the things which the cells need for their metabolism, and constantly drained of the waste substances which the cells pour out. The way in which this renewal is accomplished is simple; part of the body fluid is separated off from the rest in a system of pipes, known to us as the blood vessels, and this part is kept in motion; at intervals along the system are stations at which the moving fluid can exchange substances with the fluid which actually comes in [Illustration: DIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND KIDNEYS] contact with the cells; thus the stationary fluid can obtain from the moving fluid the materials which the cells, in turn, are constantly withdrawing from it, and can pass on to the moving fluid the products with which the cells are continuously charging it. All that is necessary to complete the successful operation of the system is to have additional stations at which the moving fluid can obtain supplies of food materials and of oxygen, and stations where it can get rid of the wastes which it accumulates from the stationary fluid, and there must be a pump by which the moving fluid is kept in motion. We are familiar with the moving fluid under the name of blood; the system of pipes in which it moves are the blood vessels; the pump which keeps it in motion is the heart; the various supply stations include the digestive organs, the lungs, and the kidneys. In later chapters the operation of all these stations will be described in detail. The present outline has been given to show in a general way how the problem of metabolism is handled in highly organized bodies in which the individual cells have no direct access to food or oxygen supplies. CHAPTER VI THE SUPPORTING FRAMEWORK Since protoplasm is so very soft and fragile it must be supported in all animals and plants except the very tiniest. The nature of the supporting framework has a great deal to do with both the form and the working of the body, so it is desirable that we become familiar with it before trying to go further in the examination of the living protoplasm itself.