60 HUMAN BIOLOGY 



corpuscles transport the oxygen from the lungs to the tis- 

 sues. 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 protec- 

 tion 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 aris- 

 ing 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 semi- 

 lunar 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 ali- 

 mentary 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 reenters 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. 



