320 



THE BLOOD AND ITS CIRCULATION 



by the valves, which act in the same manner as do the valves in a 

 pump. The blood is thus made to pass into the arteries upon 



the contraction of the 

 ventricle walls. 



The Course of the 

 Blood in the Body. — 

 Although the two sides 

 of the heart are separate 

 and distinct from each 

 other, yet every drop 

 of blood that passes 

 through the right heart 

 likewise passes later 

 through the left heart. 

 There are two distinct 

 systems of circulation 

 in* the body. The pul- 

 monary circulation takes 

 the blood through the 

 right auricle and ven- 

 tricle, to the lungs, and passes it back to the left auricle. This 

 is a relatively short circulation, the blood receiving in the lungs 

 its supply of oxygen, and there giving up some of its carbon 

 dioxide. The greater circulation is known as the systemic circu- 

 lation; in this system, the blood leaves the left ventricle through 

 the great dorsal aorta. A large part of the blood passes directly 

 to the muscles ; some of it goes to the nervous system, kidneys, 

 skin, and other organs of the body. It gives up its supply of 

 food and oxygen in these tissues, receives the waste products of 

 oxidation while passing through the capillaries, and returns to 



The heart is a force pump ; prove it from these 



diagrams. 



I. Circulation in a fish. G, gills ; C, capillaries oi the body. Notice the two- 



chambered heart. 



II. The circulation in a frog. L, the lungs ; C, the capillaries. Notice the hearf, 

 has three chambers. What is the condition of blood leaving the ventricle to 

 go to the cells of the body ? 



III. The circulation in man. //, head ; ^.arrns; L, lungs; *S, stomach ; Lz, liver; 

 K, kidney: S.I., small intestine; L.I., large intestine; Le, legs; 1, right 

 auricle ; 2, right ventricle ; 3, left ventricle ; 4, left auricle ; d, dorsal aorta ; 6, 

 vein to lungs. 



