The Circulatory Sysfem - 329 



fine capillary networks, and consequently the 

 walls throughout the whole arterial system 

 must stretch to accommodate each output 

 from the ventricle. Then, in the short in- 

 tervals between the beats of the heart, the 

 elastic recoil of the stretched arterial walls 

 tends to maintain the blood pressure, assur- 

 ing a continuous flow in the capillary system. 

 In feeling one's pulse, accordingly, one is 

 detecting the rapidly alternating stretching 

 and recoiling of the walls of one of the 

 larger arteries. 



Both the visceral muscle and the yellow 

 connective tissue of the wall contribute to 

 the elasticity of the arteries. With increasing 

 age, however, the arteries tend to lose their 

 natural resilience, partially because of a pro- 

 gressive hardening in the older tissues. When 

 this occurs the blood pressure rises, since the 

 heart works harder to maintain the circula- 

 tion. Such a combination of rising blood 

 pressure and more brittle arteries has serious 

 consequences if an artery breaks and deprives 

 a vital organ of its circulation. 



The Capillary Circulation. The aorta sends 

 branches to all parts of the body (except 

 the lungs), and each organ receives at least 

 one larger or smaller branch depending on 

 its size and activity. In the organ, a profuse 

 branching occurs, and this branching perme- 

 ates all the tissues. The initial branches, or 

 arterioles, are like the main artery, except 

 that they are smaller and have thinner walls. 

 Gradually, however, the outer and middle tis- 

 sue layers of the arteriole become thinner and 

 thinner, until finally only the naked endo- 

 thelium remains — and this is the wall of a 

 capillary (Fig. 17-12). The diameter of some 

 of the capillaries is so small that the blood 

 corpuscles must pass through in single file 

 (Fig. 17-13). Also the capillary network is so 

 extensive in every part of the body that one 

 can scarcely suffer the slightest cut without 

 drawing blood from the severed capillaries. 



Eventually the capillaries throughout an 

 organ begin to join each other, forming 

 larger vessels, called venules. The venules, 

 in turn, become the tributaries of the one or 



Fig. 17-12. A capillary. Note that the wall consists 

 merely of a simple endothelium. 



ENDOTHELIUM 



PLASMA 



TISSUE SPACE 

 (LYMPH) 



TISSUE CELL RED CELL 



Fig. 17-13. Internal respiration: diffusional exchange 

 of gases between tissue cells and blood. 



more veins, which drain blood from the 

 organ and return it toward the heart. 



RELATIONSHIP BETWEEN THE BLOOD, 

 LYMPH, AND TISSUE CELLS 



As may be seen in Figure 17-13, the blood 

 in an organ does not come into direct con- 

 tact with the tissue cells. Consequently mate- 

 rial exchanges between the blood and the 

 cells must involve a passage of substances 

 through the capillary walls and across the 

 lymph in the tissue spaces. Such exchanges 

 go on rapidly and continuously only in the 

 capillary parts of the blood system, where the 

 walls are sufficiently thin. 



