230 



HUMAN PHYSIOLOGY 



FIG. 53. In this 

 diagram, as is usual 

 in such cases, right 

 and left are reversed. 

 This is as though the 

 observer were look- 

 ing at another sub- 

 ject. The short pul- 

 monary path is to 

 be traced from the 

 right ventricle to the 

 left auricle (P. (7.). 

 Alternative routes 

 are suggested for the 

 passage of the blood 

 through the greater 

 circulation from the 

 left ventricle to the 

 right auricle. The 

 blood which tra- 

 verses the digestive 

 tract (Z>) passes 

 through a second set 

 of capillaries in the 

 liver (L) before it 

 can return to the heart, 

 arterial supply of blood. 



smallest arteries and the finest veins 

 is through the capillaries, microscopic 

 channels with the most delicate enclos- 

 ing walls that can be imagined. A 

 capillary may not be much wider than 

 a single red corpuscle. But as these 

 vessels are the narrowest of all they 

 are likewise by far the most numerous. 



The left ventricle sends blood to' all 

 parts of the body. When it comes 

 back to the right side of the heart it 

 has parted with some of its oxygen 

 in the service of the tissues. The 

 right ventricle sends it to the lungs. 

 In these organs the corpuscles are 

 freshly charged with oxygen and some 

 carbon dioxid is discharged. The 

 blood is returned to the left side of the 

 heart and is ready to go out again to 

 sustain the activities of the various 

 systems. The arrangement is such 

 that every corpuscle which has made 

 the journey from the left side of the 

 heart to the right is compelled to pass 

 through the capillaries of the lung tis- 

 sue before it can go anywhere else. 



It is somewhat different with cold- 

 blooded animals. These have but one 

 ventricle. Blood goes out from it and 

 is sent in part to the body at large and 

 in part to the lungs or gills. Only a 

 fraction of the blood is fully oxy- 

 genated. But this fraction reenters 

 the heart and raises the average com- 

 position of the mixed blood. Thus a 

 standard is maintained which is ade- 



Note that the liver has in addition a separate 



