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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



the intra-abdominal pressure. In other words, for 

 blood to flow through a vein as it returns toward the 

 heart, the pressure inside the vein must be greater 

 than the pressure applied to the outside of the vein. 

 If the abdominal pressure is 25 mm Hg, then the 

 pressure in all the lower veins of the body that feed 

 blood through the abdominal cavity, including the 

 leg veins, must be greater than the 25 mm Hg intra- 

 abdominal pressure. Likewise, if a bone or some other 

 structure presses against a vein with a pressure of 

 10 mm Hg, the pressure in the vein beyond that point 

 must rise above 10 mm Hg to force blood past the 

 compression point. These are simple hydrodynamic 

 principles. 



EFFECT OF CENTRAL PULSATION ON VENOUS RETURN. 



Probably the most extensively studied factor that has 

 been considered to affect local venous flow is central 

 pulsation. There are two different types of central 

 pulsation which can affect blood flow to the heart. 

 These are /) increases and decreases in venous pres- 

 sure resulting from the contractions of the heart it- 

 self (6, 21, 30, 124, 185), and 2) increases and de- 

 creases in central venous pressure resulting from 

 respiration (1, 28, 32, 33, 62, 130, 149, 187). All studies 

 that have ever been reported on phasic blood flow 

 from the peripheral veins to the heart have demon- 

 strated that the flow of blood toward the heart in- 

 creases greatly during the negative phases of the 

 central pressure pulses. Then, during the positive 

 phases, blood flow becomes markedly reduced and 

 can even flow backward from the right atrium into 

 the veins. A very significant inflow of blood into the 

 right atrium occurs during inspiration for two differ- 

 ent reasons: First, movement of the diaphragm down- 

 ward decreases the intrathoracic and right atrial 

 pressures slightly, which helps to move blood toward 

 the heart. Second, and much more important, down- 

 ward depression of the diaphragm compresses the 

 veins of the abdomen, thus forcing large quantities 

 of blood toward the heart. Brecher and his colleagues 

 (31) have recently been foremost among a long line 

 of investigators, extending back a hundred years, in 

 pointing out the phasic flow of blood to the heart 

 caused by central pressure pulsations. 



Still more important to our present discussion, 

 however, is not whether or not blood flows into the 

 heart in greater amount during the negative phase 

 than the positive phase but, instead, whether or not 

 central pulsation on the average aids venous return. 

 Different investigators in the past have gone so far 

 as to state that central pulsations are among the most 



important of all the factors tending to return blood 

 to the heart, while others have gone so far as to state 

 that, if anything, central pulsations are harmful to 

 the venous return rather than beneficial. Brecher's 

 monograph on venous return presents very admirably 

 the first point of view (31 ). On the other hand, studies 

 from our own laboratories during the past year have 

 indicated that central pulsations on the average 

 (though not during the negative phases of the pulsa- 

 tions) cause considerable diminution of venous return 

 rather than enhancement (91 ). For this reason, it 

 would be impossible for the author to present any 

 arguments in favor of the importance of central pul- 

 sations in returning blood to the heart. Therefore, 

 the reader is referred to Brecher's thorough mono- 

 graph for this point of view. 



The basis for our belief that central pulsations are 

 harmful rather than helpful, on the average, to venous 

 return is depicted in figure 29. This shows the typical 

 venous return curve, and it shews by means of the 

 horizontal sine waves the central pulsation excursions, 

 varying in this instance between the values —6 and 

 + 2 mm Hg. The figure then shows by the vertical 

 pulsations the effects of these pressure changes on 

 venous return as would be predicted from the venous 

 return curve. Note that venous return is considerably 

 depressed during the positive phase of the pulsatile 

 cycle. On the other hand, venous return is only slightly 

 increased during the negative phase. Therefore, the 

 average venous return is decreased approximately 

 10 per cent as a result of the central venous pulsation. 



To test this premise experimentally a cannula was 

 inserted in the wall of the right atrium, and varying 

 quantities of blood were injected and removed from 

 the right atrium at frequencies varying between 60 



fig. 29. Effect of central pulsations on venous return, 

 illustrating a rectification phenomenon that causes depressed 

 venous return when central pulsations occur. 



