1078 



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CIRCULATION II 



the early phase of the reversal. While a good part of 

 this resistance is undoubtedly attributable to the 

 valves, the situation contrasts with that observed 

 in healthy veins in the extremities where valves 

 present infinite resistance to retrograde flow until 

 very high pressures are reached. 



The functional contribution of venous valves 

 should be clearly defined. In the idealized circulatory 

 scheme with continuous venous flow, the valves must 

 necessarily remain open and hence make no func- 

 tional contribution. With intermittency of flow, due, 

 for example, to intermittency of flow in the peripheral 

 bed, the valves would tend to close during the inter- 

 vals of flow cessation. Nevertheless, we must reject 

 the view that this "breaking up" of the venous 

 column into segments relieves the dependent parts 

 from the hydrostatic load of a continuous fluid 

 column. The hydrostatic gradient is inherent in the 

 hydraulics of the system; energetically the valves 

 cannot contribute to the need for adequate pressure 

 energy to overcome the hydrostatic barrier between 

 dependent parts and the heart. In the system as a 

 whole, intermittent flow must preserve the same mean 

 pressure gradient as is required of continuous flow. 



Valves make their functional contribution by 

 translating extramurally applied forces into flow 

 energy. When an external force compresses a fluid- 

 filled vessel, local intramural pressure will rise and 

 tend to drive the blood in both directions from the 

 point of compression. The actual flow which will 

 occur in the two possible directions will be a function 

 of the pressure gradients and resistances in the alter- 

 nate directions. The resistance to retrograde flow 

 toward the capillary bed is far higher than the re- 

 sistance to forward flow toward the heart, and the 

 pressure gradient, which normally favors central 

 return, would very strongly favor central flow the 

 moment the retrograde flow combined with con- 

 tinuing capillary drainage to build up peripheral 

 venous pressure. Thus it should be appreciated that a 

 '"milking" action of intermittent venous compression 

 will effectively propel blood toward the heart even 

 in the complete absence of valves. Valves, however, 

 can greatly increase the efficiency of this process by 

 producing an almost immediate rise of retrograde 

 resistance to infinitv. 



The importance of this process is most clearly 

 demonstrated by the dramatic relief from orthostatic 

 hypotension which is produced by movements of the 

 legs and their associated compressing forces on the 

 leg veins. Walking movements are so effective in pro- 

 pelling flow up the venous channels that they can 



restore adequate venous return to the heart even 

 when vasomotor tone has been completely abolished 

 by sympatholytic drugs (71). Direct measurements 

 have demonstrated that the sequential compression 

 of venous segments during walking milks blood up the 

 legs efficiently enough to reduce the pressure in the 

 uncompressed veins of the ankle to less than one 

 quarter of the hydrostatic gradient from the ankle to 

 the heart (74). There is no question that such extra- 

 vascular forces constitute a significant "booster pump" 

 (49) for maintaining the circulation. The idea cham- 

 pioned by Henderson (48), that muscular activity 

 acting in conjunction with the venous valves was the 

 primary "venopressor" mechanism, should therefore 

 not be dismissed lightly, even though this mechanism 

 does not appear to play quite such a comprehensive 

 role in maintaining venous return as Henderson 

 claimed (90). 



Venous Capacity 



Because of the relatively large caliber of veins, and 

 also the conspicuous venous sinusoids which occur in 

 some organs, it is commonly supposed that the capaci- 

 tative function of the vascular bed resides dominantly 

 in the venous system. The full functional significance 

 of this concept will be developed in the next chapter. 

 Our concern at the moment is confined to examining 

 the evidence underlying this basic assumption. 



Widely quoted data in support of this venous reser- 

 voir concept are those published by Green (37) calcu- 

 lated from an analysis of the intestinal vascular bed 

 of the dog reported by Mall. These data picture some 

 70 per cent of the vascular capacity to reside in the 

 venous system with 62 per cent in veins greater than 

 1 mm in diameter. Landis & Hortenstine (58) carried 

 out a very similar calculation based upon the in- 

 testinal data of Schleiser, which yielded a value of 

 75 per cent of the vascular volume within the venous 

 system, 50 per cent of the total being found in veins 

 greater than 1 mm in diameter. The Landis calcula- 

 tion did not include the venae cavae, inclusion of 

 which would have increased the percentage of 

 volume in the large veins. 



It is distressing, however, when one realizes how 

 little direct evidence there is to support these esti- 

 mates. By measuring the mean transit time for dye 

 passage between the femoral vein and the right 

 atrium, Milnor & Bertrand (65) were able to calcu- 

 late a volume between these two sites which averaged 

 18 per cent of the total blood volume. Since the in- 

 ferior vena caval system is a notoriously poor mixing 



