154 T^he Lije Story of the Fish 



forces the blood through the arteries and veins. The human 

 heart has two series of chambers, a right auricle and ventricle, 

 and a left auricle and ventricle. The right set receives used 

 blood from the body, blood which has given up its oxygen 

 and has absorbed waste products — so-called "venous blood" 

 — and forces it through the pulmonary artery to the lungs 

 where it is purified. It returns to the left auricle, goes thence 



Organs 



Fish Human 



Figure 22. BLOOD SYSTEMS 



to the left ventricle, and here receives the impulse which 

 starts it on its journey around the body. The right side of 

 the heart deals only with venous blood, the left side only 

 with pure blood. 



The fish's heart lies close behind its mouth, and contains 

 only one series of chambers. Impure or venous blood enters 

 the base of the heart, and is forced out at the forward end 

 to go directly to the gills. Here it is purified, and then it 

 continues, forced by the pressure of the blood behind it, 

 to the head and other parts of the animal. The fish's heart 

 deals only with venous blood. 



In the human, the stream of pure blood divides after it 

 has received its impulse from the left side of the heart. In 

 the fish, this division takes place only after the blood has 

 received its oxygen supply from the gills. The purpose of 



