216 



THE HUMAN BODY. 



resented at different points in the vascular system. Such 

 an arrangement makes it clear that the heart is really two 

 pumps working side by side, and each engaged in forcing 

 the blood to the other. Starting from the left auricle, la, 

 and following the flow we trace it through the left ventri- 

 cle and along the branches of the 

 aorta into the systemic capillaries, sc; 

 from thence it passes back through 

 the systemic veins, vc. Reaching 

 the right auricle, ra, it is sent into 

 the right ventricle, rv, and thence 

 through the pulmonary artery, pa, 

 to the lung capillaries, pc, from 

 which the pulmonary veins, pv, car- 

 ry 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 pulmo- 

 nary capillaries gives up carbon diox- 

 ide to the air and receives oxygen 

 from it; and since its coloring mat- 

 ter (haemoglobin) forms a scarlet 



FIG. 84. -Diagram of the compound with OXV^en, it flows to 

 blood vascular system, show- * - fe . 



ing that it forms a single the left auricle through the pulmo- 



closed circuit with two pumps . , . , , 



in it, consisting of the right nary veins of a bright red color. This 



and left halves of the heart, - , j. ^rt *A i 



which are represented sepa- Color it maintains Until it reaches 



the systemic capillaries, but in these 

 ifc loses m <* o^gen to the rarronnd- 

 S fcisSU6S and S ains m ^ h carbon 

 pul " dioxide from them. But the blood 

 coloring matter which has lost its 

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

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

 blood returns to the heart by the venae cavag of a dark 

 purple-red color. This color it keeps until it reaches the 

 lungs, when the reduced haemoglobin becomes again oxi- 

 dized. The bright red blood, rich in oxygen and poor in 

 carbon dioxide, is known as "arterial blood" and the dark 



