BLOOD AND CIRCULATION 



555 



Figure 27.7. Variation in blood velocity and pressure in different parts of the 

 cardio\asciilar system. The velocity does not return to its original value for the 

 cross-sectional area of the veins is greater than the cross-sectional area of the arteries. 

 The blood pressure in the veins near the heart is less than atmospheric pressure 

 because of the negative pressure within the thorax. 



the blood moving in the vessels (Fig. 27.7). Blood pressure continues to 

 decrease as the blood flows through the capillaries and veins. The rate 

 of flow, however, increases as the blood passes from the capillaries to 

 the venules, and these smaller veins lead into fewer larger ones. The 

 blood is now moving into a smaller and smaller area, and, like water 

 flowing out of a lake into a narrowing river, moves faster and faster. 



Capillary Exchange. Capillaries are small and exceedingly thin- 

 walled vessels. Their diameter is about that of the blood cells, and their 

 walls consist of little more than an endothelial lining, which is con- 

 tinuous with that of the larger vessels. The capillary wall is a semi- 

 permeable membrane, and molecules that are small enough can easily 

 pass back and forth between the blood and the surrounding tissue fluid 

 (Fig. 27.8). Most substances are exchanged by simple diffusion following 

 concentration gradients. There is more glucose and oxygen in the blood 

 than in the tissue fluid, so their net movement is out of the capillaries. 

 There are more wastes and carbon dioxide in the tissue fluid, so their 

 net movement is into the capillaries. 



The exchange of water is more complicated than the exchange of 

 solutes, for its movement depends upon two opposing forces. The blood 

 pressure tends to force water out of the capillaries, whereas the osmotic 

 pressure exerted by the plasma protein molecules tends to draw water 



