PERIPHERAL VENOUS SYSTEM 



I08l 



cate relationships with compensation in arterial inflow resis- 

 tance. "O" pressure refers to the hydrostatic level of the 

 peripheral vein. 



ellipse and lower resistance so as to support an in- 

 creased flow without change in the total pressure 

 gradient from capillaries to heart. 



The key relationships in this pattern of venous 

 hemodynamics are illustrated in figure 2. For veins 

 above heart level, the hydrostatic column of blood 

 descending toward the heart creates a potentially 

 subatmospheric intraluminal pressure tending to suck 

 the walls of the vein in to cause collapse. Typically, 

 this phenomenon will be most manifest at the point 

 just before the venous channel enters the thoracic 

 cavity, since in this region the negative intrathoracic 

 pressure combines with the hydrostatic forces to 

 aspirate blood from the veins. At the left of figure 2, 

 therefore, central venous pressure is indicated as 

 below atmospheric pressure; yet peripheral venous 

 pressure is maintained at a definite positive value. 

 Since under these conditions there is a significant 

 pressure gradient, an appreciable resistance exists 

 between the peripheral and the central veins. This 

 resistance is created by the state of partial collapse 

 near the central end of the channel. As the central 



venous pressure rises toward atmospheric pressure, 

 the aspiration effect causing collapse becomes pro- 

 gressively less, so that the resistance to flow progres- 

 sively lowers. Peripheral pressure, however, remains 

 unchanged. As the central venous pressure rises 

 above atmospheric pressure, there continues to be an 

 interval when the intravascular pressure remains 

 below the extravascular pressure because of the 

 existence of a positive tissue pressure in the area sur- 

 rounding the veins. 



As the central venous pressure reaches the value of 

 the extravascular tissue pressure, a dramatic altera- 

 tion occurs. The intraluminal pressure will now be 

 sufficient to prevent collapse of the vein. As a conse- 

 quence, the venous channel is distended and the more 

 typical Poiseuille relationship pertains. Neglecting 

 the minor influence of elastic distension of the veins, 

 resistance between the peripheral and central veins 

 remains constant at a relatively low value, and corre- 

 spondingly a relatively constant small pressure differ- 

 ence exists between the peripheral and the central 

 veins. Peripheral venous pressure will therefore rise 

 almost parallel with central venous pressure. 



It is to be emphasized that at all central pressures 

 below the level of tissue pressure, the peripheral 

 venous pressure remains at essentially the same level 

 as the tissue pressure. Assuming there are no changes 

 in arterial pressure or resistance factors, a constant 

 peripheral venous pressure dictates a constant 

 capillary blood flow. It is to be noted, therefore, that 

 flow remains constant in spite of the significant 

 changes in pressure gradient and resistance along the 

 venous route. Once the central venous pressure rises 

 above the tissue pressure, venous congestion occurs 

 with a rise in peripheral venous pressure and a corre- 

 sponding reduction in the arteriovenous pressure 

 gradient. This will have some influence in reducing 

 flow through the system unless compensated by other 

 changes. In actual fact, a large rise in peripheral 

 venous pressure reduces peripheral blood flow so 

 that vasodilator metabolites accumulate in the 

 tissues. The resulting compensatory dilation of the 

 arterial inflow channels will counteract the elevation 

 of peripheral venous pressure and maintain constant 

 flow, as illustrated in the dashed lines of figure 2. 



Any factors leading to a change in the extra- 

 vascular tissue pressure will produce an equivalent 

 change in the peripheral venous pressure, a corre- 

 sponding alteration in resistance, and a shift of the 

 point of inflection of the curves. Flow, however, will 

 remain unchanged. 



Students of the venous svstem must wrestle with 



