210 HUMAN PHYSIOLOGY 



necessary constituents, and finally bring it back to the central 

 pump. 



The fluid, which has not passed out of the capillaries into 

 the tissues, has been deprived of many of its constituents, 

 and this withdrawal of nutrient material by the tissues is 

 made good by a certain quantity of the blood being sent 

 through the walls of the stomach and intestine (Al.C.), in 

 which the nutrient material of the food is taken up and added 

 to the blood returning to the heart. At the same time, the 

 waste materials added to the blood by the tissues are partly 

 got rid of by a certain quantity of the blood being sent 

 through the liver and kidneys (Liv. and Kid.). 



The blood is then poured back, not at once into the great 

 pump which sends it through the body, but into a subsidiary 

 pump the pulmonic heart (P.H.) by which it is pumped 

 through the lungs, there to obtain a fresh supply of oxygen, 

 and to get rid of the carbon dioxide excreted into it by the 

 tissues. Finally the blood, with its fresh supply of oxygen 

 from the lungs, and of nourishing substances from the ali- 

 mentary canal, is poured into the great systemic pump the 

 left side of the heart again to be distributed to the tissues. 



Thus the circulation is arranged so that the blood, ex- 

 hausted of its nourishing material by the tissues, is replenished 

 in the body before being again supplied to the tissues. 



The sectional area of this irrigation system varies enor- 

 mously. The aorta leaving the heart has a comparatively 

 small channel. If all the arteries of the size of the radial 

 were cut across and put together, their sectional area would 

 be many times the sectional area of the aorta. And, if all 

 the capillary vessels were cut across and placed together, the 

 sectional area. would be about 700 times that of the aorta. 



From the capillaries, the sectional area of the veins and 

 lymphatics steadily diminishes as the smaller branches join 

 with one another to form the larger veins and lymphatics : 

 but, even at the entrance to the heart, the sectional area of 

 the returning tubes, the veins, is about twice as great as that 

 of the aorta (Fig. 128, p. 271). 



The circulatory system may thus be compared to a stream 

 which flows from a narrow deep channel, the aorta, into a 

 gradually broadening bed, the greatest breadth of the channel 



