146 



THE CIRCULATION OF THE BLOOD 



ferent arrangement. They cover the walls of the tiny 

 air sacs in the lungs so thickly that they expose to the 

 purifying and enriching action of the air a layer of blood 

 the thickness of a capillary and about eight hundred 

 square feet in extent. 



Travels of a drop of blood. Through this double 

 system of heart, arteries, capillaries and veins, we may 



trace the course of a drop 

 of blood as follows: Start- 

 ing in the left ventricle of 

 the heart, the drop of blood 

 passes through the aortic 

 valve into the aorta. It 

 quickly flows down the 

 aorta into one of its numer- 

 ous arterial branches and 

 finally reaches the capilla- 

 ries, for example, those in 

 a leg muscle. Here a cer- 

 tain amount of plasma and 

 perhaps a few of its white 

 corpuscles pass through the 

 walls of the capillaries into the spaces between the mus- 

 cle cells. Its red corpuscles give up some of their oxy- 

 gen to the muscle cells and its plasma receives carbon 

 dioxide and other waste matters. Thus changed and 

 slightly diminished by its losses, it returns by way of 

 the veins into the right auricle of the heart. It then 

 passes through the right auriculo-ventricular valve into 

 the right ventricle. From here, it is pushed by the 

 contraction of the ventricle through the pulmonary 

 artery and its branches into the capillaries of the lungs 

 which surround the air sacs. Here, some of its car- 

 bon dioxide escapes to the air and a fresh load of oxygen 



FIG. 87. Diagram of valves of veins; 

 A, valve opened by blood passing 

 forward toward heart ; B, valve closed 

 by attempted return of blood ; C, 

 vein opened to show arrangement of 

 valves. 



