PERIPHERAL VENOUS SYSTEM 



1079 



chamber, one would expect the true volume to exceed 

 that calculated by this method. This study therefore 

 represents some substantial support for the idea that 

 large veins represent a significant contribution to the 

 total vascular capacity. On the other hand, Knisely 

 and associates (57) have challenged the conventional 

 point of view with some data obtained from plastic 

 injections of whole rats. Using a plastic free of 

 particulate matter which flowed freely through the 

 circulation when initially injected, they obtained 

 casts of the entire vascular bed. When these casts were 

 fragmented, and the fragments sorted according to 

 caliber, over 80 per cent of the plastic was found to 

 be contained in vessels with a diameter of less than 

 200 n and only 12 per cent in vessels larger than 700 n. 

 This study suffers from the fact that veins are col- 

 lapsible, and it is not at all clear that their method 

 would have preserved a normal degree of filling of 

 the venous system. Nevertheless, such an extreme dis- 

 crepancy between the relative contribution of large 

 vessels and small vessels to total vascular capacity 

 clearly challenges the point of view that is usually 

 held. The burden of proof has been returned to the 

 proponents of the venous reservoir concept to offer 

 some more substantial documentation of their 

 hypothesis. 



Apart from the question of total capacity, however, 

 there is much better support for the thesis that the 

 venous division, together with the lesser circulation, is 

 the most variable capacity of the vascular bed. One 

 very simple observation leading to such an inference 

 is the minimal change in pressure produced by an 

 injection into the venous system as compared to the 

 pressure change produced by an injection of an equal 

 volume at the same rate into the arterial system. More 

 direct evidence on this point was presented by Green- 

 field & Paterson (39), who compared volume changes 

 in the forearm produced by venous obstruction to 

 the volume changes produced by a negative pressure 

 applied to the whole arm. In the former instance, the 

 increase in transmural vascular pressure would be 

 essentially confined to the venous side; in the suction 

 experiment, the transmural pressure of all vessels 

 should be increased equally. Yet venous occlusion 

 provided 85 per cent of the volume increase observed 

 when suction was applied to the whole arm. A similar 

 experiment was reported by Capps (16). Such data, 

 together with venous distensibility characteristics to 

 be discussed later, justify reasonable confidence in the 

 hypothesis that the venous system plays an important 

 role in contributing a reservoir of variable capacity 

 to the vascular system. 



PHYSIOLOGICAL CHARACTERISTICS OF VEINS 



Principles of Venous Hemodynamics 



The most frequent measurement made of the 

 venous system is the venous pressure. For the pur- 

 poses of our interests however, venous pressure is of 

 relatively little meaning. As competently reviewed by 

 Landis & Hortenstine (58), venous pressure can have 

 profound influence on capillary dynamics and the 

 transudation of fluid across the capillary endothelium. 

 Central venous pressure plays a key role in cardiac 

 filling and the control of cardiac output. For reasons 

 that will be developed shortly, however, venous pres- 

 sures tell very little about the venous system itself. 

 Indeed, it can be fairly stated that venous pressure 

 measurements in themselves are just as unimportant 

 to the physiologist interested in the venous system as 

 they are important to the physiologist interested in the 

 arterial system. 



Before specifically considering the principles of 

 hemodynamics underlying this statement, a word of 

 emphasis should be given in reference to the implica- 

 tion of the studies of Pappenheimer (69, 70) on the 

 capillary bed. He has lucidly argued that, under 

 steady-state conditions, the mean capillary pressure 

 must be in equilibrium with the effective osmotic 

 pressure of the plasma proteins, excluding lymph 

 flow which at best is a small fraction of total blood 

 flow. Since central venous pressure shows relatively 

 small variations under most conditions, this indicates 

 a relatively constant pressure gradient from capillaries 

 to the central veins. Furthermore, large changes 

 in blood flow may occur without significant altera- 

 tions in either plasma protein concentration or central 

 venous pressure, yielding the apparent paradox of 

 blood flow that varies widely in spite of a fixed pres- 

 sure gradient. A corollary to this is that the venous 

 smooth musculature cannot effectively control blood 

 flow by imposing a variable resistance in the venous 

 portion of the circulation. The role of the venous 

 musculature must therefore be confined to producing 

 capacity changes in the system. These capacity 

 changes can indirectly influence blood flow only 

 insofar as more effective venous return increases 

 cardiac output, or higher mean circulatory pressures 

 increase capillary transudation. 



If some of the preceding statements appear to be 

 in conflict with irrefutable principles of fluid dy- 

 namics, the reader must be reminded that the venous 

 system is a collapsible system and is therefore not 

 governed by the usual principles of fluid dynamics in 



