=The Use of Each Part of the Blood.=--The _plasma_ keeps the blood in a liquid state, so that it may flow readily; the plasma also transports the food that has been eaten and digested, and carries carbon dioxid to the lungs and other waste material to the kidneys. The _red corpuscles_ transport the oxygen from the lungs to the tissues. The _white corpuscles_ devour and destroy irritating particles, such as drugs, poisons, and germs. They are of great importance in purifying the blood and as a protection against disease. One is shown in Fig. 60. =The sounds of the heart beat= may be heard by applying the ear to the chest. They are two, a _long, dull_ sound and a _short, clear_ one. The first comes from the vibration of the bicuspid valve together with an unexplained tone arising from large contracting muscles, in this case the walls of the ventricles. The second, or short, clear sound, is produced by the sudden closing and vibration of the semilunar valves. =Changes in the Composition of the Blood as it passes through the Various Organs.=--When the blood is forced out by the heart, part of it goes to the stomach and intestines through arteries which divide into capillaries. These capillaries _absorb all kinds of food_ from the alimentary canal except the fats (see p. 64), and unite to form the portal vein, which takes the absorbed food to the liver. In the liver some of the _impurities of the blood are burned up_ and changed into bile. The blood, purified and laden with food, is carried from the liver to the heart, where it reënters the general blood stream. The blood flow from the food tube through portal vein and liver to the heart, as just described, is called the _Portal circulation_. _Renal circulation._ Two branches from the aorta carry blood to the _kidneys_. There the _urea_ and _a large amount of water_ are taken out, and the purified blood is emptied into the large vein that leads up to the heart. _Pulmonary circulation_ (Fig. 67). This is the circulation through the lungs. During this circulation _carbon dioxid gas is removed_ from the blood and _oxygen is added_ to it. [Illustration: FIG. 62.--BLOOD CLOT separated from serum.] Some impurities and a large amount of water escape from the blood as it passes through the skin.