VENOUS RETURN 



I IO7 



The second method for establishing the normal 

 venous return curve has been to determine different 

 points along the curve intermittently by suddenly 

 elevating the right atrial pressure and making venous 

 return measurements within the next 5 to 7 sec 

 before circulatory reflexes can take place. Then the 

 circulation is returned to normal, and after a reason- 

 able control period another intermittent measure- 

 ment is made. 



The venous return curves recorded by these two 

 different procedures have been identical. Further- 

 more, venous return curves have been recorded in 

 closed-chest animals in which a special occluding 

 system has been surgically placed around the pul- 

 monary artery so that the pulmonary artery could be 

 occluded to any desired degree (97). Then, using 

 especially the intermittent procedure, points along 

 the venous return curve were established. The results 

 agree with the measurements established when using 

 the above two procedures. 



The venous return curve of figure 5 is the average 

 curve, recorded in approximately 100 separate dogs 

 anesthetized with sodium pentobarbital, and then 

 extrapolated on a weight basis to the 12-kg dog. 

 Several features of this curve deserve special comment. 



First, when the right atrial pressure becomes more 

 negative than o to — 4 mm Hg, a further increase in 

 the negativity of the right atrial pressure does not 

 cause a further increase in venous return. In other 

 words, the venous return curve reaches a '"plateau." 

 The cause of this effect is the well-known collapse 

 factor in veins (88, 112). One can actually see the 

 veins entering the thoracic cavity begin to collapse 

 when the right atrial pressure becomes negative with 

 respect to atmospheric pressure. Furthermore, 

 measurements in the veins immediatelv bevond the 

 collapsed points show that these veins all have ap- 

 proximately o mm Hg pressure in them regardless of 

 how low the right atrial pressure falls. Thus, the 

 collapse factor effectively sets the venous pressure of 

 the blood leaving the systemic circulation almost 

 exactly at o. 



The second important point in relation to the 

 venous return curve is that elevation of the right 

 atrial pressure above o causes a very rapid decrease 

 in return of blood from the systemic circulation 

 (98). On the average, for each mm Hg rise in pressure 

 above o, the venous return decreases 14 per cent, and 

 it reaches zero when the right atrial pressure has 

 risen to approximately +7 mm Hg in "areflex" dogs. 



The third important point is that when venous 

 return reaches zero, the right atrial pressure at this 



level is equal to the mean systemic pressure (98). The 

 mean systemic pressure is the pressure in the systemic 

 circulation that is measured if the root of the aorta 

 and the large systemic veins entering the heart are 

 suddenly occluded and all pressures in the systemic 

 circulation are brought instantaneously to equilib- 

 rium. That is, when blood flow ceases absolutelv 

 in the systemic circulation, the pressures in all its 

 segments become equal. Therefore, the right atrial 

 pressure becomes equal to the pressure everywhere 

 in the systemic vessels. This equilibrium pressure is 

 the mean systemic pressure. 



The fourth point of major significance in relation 

 to the venous return curve is the almost complete 

 linearity of the venous return curve in the range 

 between o right atrial pressure and the mean systemic 

 pressure level. That is, the venous return is approxi- 

 mately proportional to the difference between mean 

 systemic pressure and right atrial pressure (Pms - 

 Pra). This difference is called the "pressure gradient 

 for venous return" (81), and it is an important 

 concept in establishing the forces that lead to the flow 

 of blood toward the heart. This will be seen below, 

 especially in relation to alterations in systemic 

 resistances, for when there is no pressure gradient for 

 venous return, there will be no venous return to the 

 heart regardless of the changes in systemic resistances. 



EFFECT OF PERIPHERAL RESISTANCE ON VENOUS RETURN. 



Figure 6 illustrates the effect on the venous return 



2400 



2000- 



1600 



CE 



\- 

 UJ 

 CC 



w 



z> 

 o 



z 



UJ 



> 



1200 



800 



400- 



+4 +8 



RIGHT ATRIAL PRESSURE (mm Hg) 



fig. 6. Effect on the venous return curve of changing the 

 peripheral resistance. Note that the mean systemic pressure 

 remains constant at approximately 7 mm Hg. 



