METABOLISM AND TRANSPORT IN ANIMALS 



531 



Endothelium 



CAPILLARY 



Endothel 





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VV^ '^ ^°" necf Iv^ tissue .;/ ;/ *>^ ^^^^^^^^-i*- • 



•flv V ' Endothelium 



Connective tissue 

 Muscle layer 

 Endothelium 



Figure 385. Structure of blood vessels. The muscle layer of an artery is thicker than the 

 muscle layer of a vein of the same size; and a vein usually has a lumen of greater diameter 

 than its corresponding artery. 



arteries (arterioles) with the smallest veins 

 (venules). It has been estimated that if all 

 the capillaries in the human body were 

 placed end to end, they would form a tube 

 62,000 miles long. Since each capillary is 

 only about 1 mm. long, the total number is 

 almost inconceivable. The wall surface of 

 these capillaries is about 67,000 square feet 

 or IVi acres. Capillaries carry out the prin- 

 cipal functions of the blood, such as taking 

 in the secretions from the ductless glands, 

 absorbing oxygen from and giving up carbon 

 dioxide to the lungs, delivering waste prod- 

 ucts to the kidneys, absorbing food from 

 the digestive system, and transporting food 

 and oxygen to the tissues in exchange for 

 cellular excretions. 



The rate of circulation is so rapid as 

 almost to defy our imagination. The entire 

 blood supply passes from the heart, through 

 the body, and back again to the heart in 

 about 20 to 40 seconds. This means that the 

 5 quarts of blood in an average-sized man 

 pass through his body from 3000 to 4000 



times per day; this accounts for the rapid 

 distribution of substances that gain en- 

 trance into the blood stream. The velocity 

 of the blood flow differs in different arteries 

 and veins, depending largely on the size of 

 the vessel. In the large arteries, blood moves 

 along rapidly; in the smaller arteries and 

 capillaries, it moves more slowly. The rate 

 of movement in the veins is less than in the 

 arteries. 



Distribution of digested food 



Digested food that is absorbed by the 

 cells of the intestinal walls, and which dif- 

 fuses from them into the blood capillaries, 

 is carried in the blood stream through the 

 hepatic portal vein to the liver (Fig. 386). 

 From the liver the blood passes into the 

 hepatic vein and flows to the heart, which 

 pumps it into the general circulation. Fats 

 that enter the lymphatic vessels in the villi 

 of the intestinal wall are carried by the 

 lymph into the thoracic duct, which usually 

 empties into the venous system in the region 



