l82 



BIOLOGY AND HUMAN LIFE 



with a large vein that brings blood gathered from the capillaries 

 of the lungs. The opening between the receiving chamber and 

 the pumping chamber is guarded by a set of valves that prevents 

 the blood from flowing back when the ventricle contracts. An- 

 other set of valves prevents the blood from flowing back from the 

 aorta into the ventricle when the latter expands again. The left 



heart thus pumps blood re- 

 ceived from the capillaries 

 of the lungs into arteries 

 all over the body. 



The right heart receives 

 blood into its auricle from 

 two large veins, and passes 

 it into the ventricle, or 

 pumping chamber. The 

 auricle and the ventricle 

 on the right side are con- 

 nected, as are the corre- 

 sponding chambers on the 

 left side, with a valve that 

 prevents the back-flow of 

 blood when the ventricle 

 contracts. The right ven- 

 tricle pumps blood into a 

 large artery {pulmonary 

 artery) that carries it to 

 the capillaries of the lungs. The right heart pumps blood re- 

 ceived from all over the body to the capillaries of the lungs. 

 152. The double circulation. The stream of blood makes two 

 circuits: (i) from the heart (left) through arteries, capillaries, 

 and veins of the body and back to the heart ; (2) from the heart 

 (right) through arteries, capillaries, and veins of the lungs and 

 back to the heart (see Fig. 96). The blood-stream may be 

 traced from any point and back to the start only by passing 

 through the two sides of the heart, through the pulmonary, or 

 lung, circuit, and through the systemic, or body, circuit. Thus, 



Fig. 95. Diagram of the human heart 



aa, receiving chambers, or auricles; bb, pump- 

 ing chambers, or ventricles; cc, main veins, 

 bringing blood to the heart; dd, main arteries, 

 carrying blood away from the heart; //, valves, 

 preventing back-flow from arteries to ven- 

 tricles; between a and b, valves preventing 

 back-flow from ventricles to auricles 



