372 . THE BLOOD AND ITS CIRCULATION 



dilation in the body. The pulmonary circulation takes the blood 

 through the right auricle and ventricle, to the lungs, and passes it 

 back to the left auricle. This is a relatively short circulation, the 

 blood receiving in the lungs its supply of oxygen, and there giving up 

 some of its carbon dioxide. The greater circulation is known as 

 the systemic circulation; in this system, the blood leaves the left 

 ventricle through the great dorsal aorta. A large part of the 

 blood passes directly to the muscles ; some of it goes to the nervous 

 system, kidneys, skin, and other organs of the body. It gives up 

 its supply of food and oxygen in these tissues, receives the waste 



Capillaries 



Capillaries 



Diagram of the circulation of blood in a 

 mammal. 



products of oxidation while 

 passing through the capillaries, 

 and returns to the right auricle 

 through two large vessels known 

 as the venae cavce. It requires 

 from twenty to thirty seconds 

 only for the blood to make 

 the complete circulation from 

 the ventricle back again to the 

 starting point. This means 

 that the entire volume of blood 

 in the human body passes three 

 or four thousand times a day 

 through the various organs of 

 the body. 1 



Portal Circulation. Some of 

 the blood, on its return to the 

 heart, passes by an indirect path 

 to the walls of the food tube and 

 to its glands. From there it passes 

 with its load of absorbed food to 

 the liver. Here the vein which 

 carries the blood (called the portal 

 vein) breaks up into capillaries 

 around the cells of the liver. We 

 have already learned that the liver 

 is a great storehouse of animal 

 sugar called glycogen. This gly co- 

 gen is a food that may be easily 



1 See Hough and Sedgwick, The Human Mechanism, page 136. 



