THE LOADING OF THE BLOOD WITH NOURISHMENT 201 



example in the body of a vein's connection with another 

 vein through capillaries. In all other cases the capillaries 

 connect veins and arteries. 



It is this arrangement of portal-vein capillaries which 

 permits the liver to act as a storehouse of carbohydrate 

 material, and to regulate the amount of that material 

 which shall enter the general circulation at any given time. 



The renal circulation. — The blood wMch is returned 

 to the left side of the heart by the pulmonarj^ veins has 

 been freed of its carbon dioxide only. Hence the blood 

 which passes into the aorta is still rich in urea and other 

 body wastes brought to the heart by the venae cavge. Much 

 of this waste is diverted into the artery branches which 

 carry blood to the kidneys (the renal arteries). In these 

 organs, by means which we shall discuss more fully imder 

 the excretory system, this waste is removed, and the 

 renal veins, therefore, return to the inferior vena cava 

 blood which is nearly free of body wastes. 



Changes in blood composition. — In passing through the 

 various systems of circulation, portal, renal, systemic, 

 and pulmonary, the blood undergoes distinct changes in 

 composition. Having in mind the parts of the body which 

 these systems supply, we may now summarize these 

 changes as follows : 



The loading of the blood with nourishment. — 1. Food 

 enters the blood at two points in its circulation. 



(a) All kinds of food, except the fats, enter the capil- 

 laries of the stomach and intestines and are collected 

 and carried to the Hver by the portal vein. In the 

 liver, the capillaries into which this vein divides 

 permit the liver cells to separate from the rest of the 



