210 THE HUMAN BODY. 



left halves of the heart are represented at different points 

 in the vascular system. Such a diagram makes it clear that 

 the heart is really two pumps working side by side, and 

 each engaged in forcing blood to the other. Starting from 

 the left auricle, la, and following the flow, we trace it 

 through the left ventricle, and along the branches of 

 the aorta into the systemic capillaries, sc; thence it 

 passes back through the systemic veins, vc. Reaching 

 the right auricle, ra, it is sent into the right ventricle, TV, 

 and thence through the pulmonary artery, pa, to the lung 

 capillaries, pc, from which the pulmonary veins, pv, carry 

 it to the left auricle, which drives it into the left ventricle, 

 lv, and this again into the aorta. 



Arterial and Venous Blood. The blood when flowing 

 in the pulmonary capillaries gives up carbon dioxide (a 

 waste product which it has gathered in its flow through 

 the other organs) to the air, and receives oxygen from it; 

 since its coloring matter (haemoglobin) forms a scarlet 

 compound with oxygen, the blood which flows to the left 

 auricle through the pulmonary veins is of a bright red 

 color. This color it maintains until it reaches the sys- 

 temic capillaries, but in these it loses much oxygen to 

 the surrounding tissues, and gains much carbon dioxide 

 from them. But the blood-coloring matter which has lost 

 its oxygen has a dark purple-black color, and since this 

 unoxidized or " reduced " haemoglobin is now in excess, the 



Why are we justified in diagrammatically representing the heart 

 as made of two separated parts? Starting from the left auricle, 

 describe the course of the blood until it returns there. 



What does the blood give up in the pulmonary capillaries? What 

 does it receive? Why is it bright red when it enters the left auricle? 

 How far in its course does it keep this color? What gases does the 

 blood gain and lose in the systemic capillaries? 



