330 TEXT-BOOK OF PHYSIOLOGY. 



in a ratio again proportional to the total area of each cross-section of the 

 stream-bed until in the venae cavae it will attain its maximal value, though 

 it will not attain its initial value in these vessels because their combined 

 sectional area is greater than that of the aorta. 



The lateral pressure will also gradually fall from the beginning of the aorta 

 to the ends of the venae cavae, though the fall will be most rapid at the pe- 

 riphery of the arteries. In the arterial system the fall of pressure will be 

 proportional to the ratio between the increase in resistance due to the narrow- 

 ing, of individual vessels, and the decrease in resistance due to the widening 

 of the stream-bed ; as the former preponderates over the latter there must be 

 an increase in resistance from the aorta to the capillaries and hence a sharper 

 fall of pressure toward the termination of the arterioles, which is very steep 

 for reasons to be stated later. In the venous system the fall of pressure will 

 continue and its rate will be proportional to the ratio between the increase in 



FIG. 153. DIAGRAM DESIGNED TO GIVE AN IDEA OF THE AGGREGATE SECTIONAL AREA OF 

 THE DIFFERENT PARTS OF THE VASCULAR SYSTEM. A. Aorta. C. Capillaries. V. Veins. The 

 transverse measurement of the shaded part may be taken as the width of the various kinds of 

 vessels, supposing them fused together. (Yeo.) 



resistance due to the narrowing of the stream-bed, and the decrease of resist- 

 ance due to the enlarging of individual vessels; as the latter preponderates 

 over the former there should be a rapid fall of pressure from the capillary 

 system to the ends of the venae cavae. This, however, is to some extent 

 prevented for the reason that the increase in velocity due to the narrowing of 

 the stream-bed increases the resistance to a relatively high value and hence 

 the pressure falls less rapidly than it otherwise would. 



The high pressure characteristic of the arterial system contrasted 



with the low pressure characteristic of the venous system determined by 



experiment cannot be accounted for alone by the resistance offered by the 



small diameter of the vessels of the capillary system. This in itself would be 



nsufficient to maintain the observed differences in pressure in the different 



