210 



III! INIIKNAl. LNVIUONMHNT OI rill BODY 



Pari III 



uppr/t 



VENA - 



AORTA 



Fig. 12.15. Scheme of the main circulation of the human blood. The vessels 

 carrying well oxygenated blood are in outline; those containing blood poor in 

 oxygen are in heavy black. The vena cava and the aorta actually course along the 

 mid dorsal line of the body but have been pulled aside for labeling. Stomach and 

 right kidney are omitted for space. ( Reprinted from The Machinery of the Human 

 Body by Carlson and Johnson, by permission of The University of Chicago Press. 

 Copyright 1948.) 



blood in them stops it. The second important part of the circuit is to the liver. 

 This important organ receives both arterial and venous blood; the latter repre- 

 sented by the hepatic-portal vein is the unique feature (Fig. 12.15). It carries 

 food-laden blood from the small intestine and blood from the spleen and 

 pancreas directly to the liver and there breaks up into the hepatic-portal sys- 

 tem of innumerable capillaries that eventually converge into the hepatic veins. 

 These carry blood into the postcaval vein and on to the right auricle. 



In the systemic as in the pulmonary circuit the flow is from larger to smaller 

 arteries on into the capillaries; thence it goes into larger and larger veins and 

 on until it empties into the heart. 



Whatever the region of the body, blood stays longest in the capillaries and 

 is there constantly engaged in exchanges with the surrounding cells and fluids 

 (Fig. 12.18). In the capillaries it distributes the supplies for metabolism, foods 

 and oxygen, and receives the products of metabolism, carbon dioxide and 

 nitrogenous by-products. These are but the high points in the complex capillary 

 cell and tissue fluid exchange. 



The complexity of the internal aquatic environment of the body results 

 from the liquid that penetrates through capillary walls. This has never been so 



