Chap. 12 



CIRCULATION AND TRANSPORTATION BODY FLUIDS 



221 



f...^. 



Endothelium 



'■:'^.:^ 



>)^>^- Connective »'«"«^5;^^^^,^^W 

 ''■^-■f- Muscle layer . I'^^t^Y l^^^^Wt^ 



Endothelium' 



CAPILLARY 



Endothelium 



VEIN 



ARTERY 



Connective tissue 

 Muscle layer 

 Endothelium 



Fig. 12.20. Three types of blood vessels. In the muscle layer of an artery the 

 contraction of the heart is actually continued (pulse); in the thinner muscle layer 

 of a vein it is not. In the cavity of an artery blood is under more pressure than 

 in the larger cavity of a vein of the same size. The walls of capillaries, but one cell 

 thick, make the income and outgo of substances easy. (Courtesy, Hegner and 

 Stiles: College Zoology, ed. 6. New York, The Macmillan Co., 1951.) 



usually taken on the radial artery at the wrist; its rate is identical with the 

 heartbeat, in adults about 70 times per minute. 



Capillaries. When blood enters the capillaries, it encounters a network 

 whose combined caliber is greater than that of the artery from which it came, 

 thus bringing it under less pressure. It drags along their walls and runs against 

 the forkings of their branches. Its energy of motion is continually dissipated 

 in the heat of friction (Chap. 2). At the arterial end where pressure is 

 higher, water is pushed out of the capillaries into the tissue fluid; at the 

 venous end where pressure is lower, water from the tissue fluid is taken back 

 into the capillaries. Thus the water content of the plasma is kept constant and 

 that in the tissue fluid is continually refreshed. 



Veins. Nearer the surface of the body than arteries, veins are thin-walled, 

 extensible, and the larger ones are provided with valves that prevent backward 

 flow (Fig. 12.21 ). Blood is pushed through the veins by the pressure of more 

 blood coming from the capillaries, by the movements of skeletal muscles, and 

 by the motions of the body in breathing. Most veins are surrounded by skeletal 

 muscles; when they contract the veins are collapsed; when they relax, the 

 veins refill and the blood continues flowing toward the heart. This "milking" 

 motion helps the venous flow of blood just as it does the flow of lymph. It is 



