VENOUS RETURN 



I IOg 



much as the resistance rises, and the arterial pressure 

 then simply forces the blood on past the resistance. 

 On the other hand, constricting the veins where they 

 empty into the heart causes the pressure in the veins 

 to rise only a few mm Hg because of the great storage 

 capacity of the veins. This small rise in venous 

 pressure is far too little to overcome the increasing 

 resistance, and, as a consequence, the venous return 

 becomes tremendously depressed. Therefore, venous 

 resistance affects venous return to the heart many 

 times as much as arteriolar or arterial resistance of 

 the same magnitude. 



EFFECT OF MEAN SYSTEMIC PRESSURE ON VENOUS 



return. Basically the mean systemic pressure is the 

 resultant of the ratio of a) the blood volume to b) the 

 ability of the circulatory system to hold blood. As 

 the blood volume increases, the mean systemic 

 pressure remains essentially zero until the blood 

 barely begins to distend the blood vessels. But, once 

 this point has passed, any further increase in blood 

 volume increases the mean systemic pressure directly 

 in proportion to the additional amount of blood 

 injected into the circulation, the mean systemic 

 pressure rising approximately 1 mm Hg for each 2 

 per cent increase in blood volume (86). Thus, it can 

 be seen that very small changes in blood volume can 

 cause relatively large changes in mean systemic 

 pressure and, as a consequence, can have a marked 

 effect on venous return unless other circulatory 

 compensations prevent this. 



Only a few measurements of mean systemic pressure 

 have ever been made, for this requires instantaneous 

 stoppage of the circulation and then rapid equilibra- 

 tion of the pressures in all segments of the systemic 

 circulation before any blood can leave or before 

 circulatory reflexes or other factors can change the 

 vascular distensibility. In our laboratory, we have 

 measured this pressure a few times by suddenly 

 constricting the aorta and pulmonary arteries (138), 

 utilizing devices implanted several weeks previously 

 in the thoracic cavity to cause the constrictions. These 

 pressure measurements showed a normal mean 

 systemic pressure of almost exactly 7 mm Hg. 



Far more measurements have been made of the 

 mean circulatory pressure (101, 105, 180, 182) than 

 of the mean systemic pressure; approximately 1000 

 such measurements have been made in our laboratory. 

 Since the mean systemic pressure is of such extreme 

 importance in determining venous return to the 

 heart, the few measurements of mean systemic 

 pressure have been compared with measurements of 



mean circulatory pressure. In all instances, the mean 

 systemic pressure has been almost identical with the 

 mean circulatory pressure except in the case of 

 extreme engorgement of the pulmonary circulation, 

 and even here the difference has been only 1 mm Hg 

 or so. Therefore, insofar as venous return from the 

 systemic circulation is concerned, one can consider 

 the mean systemic pressure and mean circulatory- 

 pressure to be almost identical. 



Measurements of mean circulatory pressure can be 

 made very easily by electrically fibrillating the heart 

 with 60-cycle current applied to needle electrodes in 

 the anterior chest wall at approximately 50 v. Studies 

 have demonstrated that all pressures of significance in 

 the measurement of mean circulatory pressure come 

 to equilibrium within only a few seconds after cardiac 

 fibrillation begins, except for the pressures of the sys- 

 temic arterial chamber and the systemic venous 

 chamber. Therefore, immediately after fibrillation of 

 the heart begins, arterial blood is pumped from a 

 catheter lying in the descending aorta and thence 

 through another catheter into the inferior vena cava. 

 After only 3 to 5 sec, the pressures in these two cham- 

 bers are brought to equilibrium and the mean circu- 

 latory pressure measured. During the next few seconds 

 the heart is defibrillated by passing 4 to 10 amperes 

 of 60-cycle alternating current at 440 v for i/[ sec 

 directly through the chest anteroposteriorly (104). 

 After 2 to 3 min of recovery, the animal returns to 

 essentially normal circulatory conditions. 



Figure 9 illustrates the typical effect on the venous 



-12 -8 -4 +4 +8 +12 +16 



RIGHT ATRIAL PRESSURE (mm Hg) 



fig. 9. Effect on the venous return curve caused by changes 

 mean systemic pressure. 



