CIRCULATION 



357 



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 0^'^^^^=^^^^^'^^ 



volume of blood in the 



human body passes 



three or four thousand 



times a day through the 



various organs of the 



body.^ 



Portal Circulation. 

 — Some of the blood, on its 

 return to the heart, passes 

 by an indirect path through 

 the spleen, pancreas, and 

 other organs of the body 

 cavity, to the liver. Here 

 the vein which carries the 

 blood (called the portal 

 vein) breaks up into capil- 

 laries around the cells of 

 the liver. We have already 

 learned that the liver is a 

 great storehouse of animal 

 sugar called glycogen. This 

 glycogen is a food that 

 may be easily oxidized to 

 release energy, and is stored 

 for that purpose . The sugar 

 that becomes glycogen is 

 carried to the liver directly 

 from the walls of the stom- 

 ach and intestine, where it 

 has been absorbed from the 

 food there contained. From 



the liver, blood passes directly to the right auricle. The fortal system, 

 as it is called, is the only part of the circulation where the blood passes 

 through two sets of capillaries. 



Changes in Blood within the Body. — We have already seen that 

 blood loses much of the carbon dioxide it has taken from the tissues, 



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



Diagram of the course of the blood in the circulation. 



