VENOUS RETURN 



III3 



exactly the opposite effects occur when vasomotor 

 tone is greatly reduced throughout the body by the 

 administration of nitrites (195) or ganglion-blocking 

 agents ( 1 89). Likewise, pooling of blood in the lower 

 part of the body when one stands (5, 76, 14Q-144, 

 175, 184, 196) or sequestration of blood in the limbs 

 by the application of tourniquets (59, 193) reduces 

 the venous return and cardiac output in a closely 

 similar manner. 



EFFECT OF MUSCULAR EXERCISE ON VENOUS RETURN, 

 CARDIAC OUTPUT, AND RIGHT ATRIAL PRESSURE. Figure 



14 illustrates an analysis of the circulation during 

 exercise, showing changes in both the cardiac output 

 and venous return curves. The cardiac output curve 

 is elevated as a result of a) sympathetic stimulation of 

 the heart, and b) inhibition of the vagi to the heart, 

 thus giving a cardiac output curve of a hypereffective 

 heart. 



Three different factors cause the observed altera- 

 tions in the venous return curve. First, tightening of 

 the musculature throughout the body, particularly 



4000 



-8 "4 +4 +8 + 12 +16 +20 



RIGHT ATRIAL PRESSURE (mm Hg) 



pig. 14. Effect of exercise on the venous return and cardiac 

 output curves, showing that the new curves equate at greatly 

 elevated venous returns and cardiac outputs. Also, the right 

 atrial pressures are still close to zero mm Hg. 



tightening of the abdominal musculature, causes an 

 instantaneous increase in mean systemic pressure of 

 several millimeters of mercury (100). Second, sympa- 

 thetic stimulation causes considerable increase in 

 mean systemic pressure (101). Third, the blood vessels 

 of the musculature are likely to become markedly 

 dilated, thus decreasing the resistance to blood flow 

 through the systemic circulation (12); this, in turn, 

 increases the slope of the venous return curve (87). 

 Thus, we find that in moderate exercise the cardiac 

 output and venous return may be increased to two or 

 more times normal, and the right atrial pressure will 

 still be only slightly elevated (14, 169, 174). On the 

 other hand, in severe exercise, the heart is then often 

 taxed to its limit so that the right atrial pressure rises 

 considerably as depicted by the highest equilibrium 

 point in the figure. In an animal or human being that 

 has been thoroughly trained for athletics, the cardiac 

 output curve of the heart can rise to one and one-half 

 to two times that depicted in figure 13, thus giving as 

 much as a five- to sevenfold increase in cardiac output 

 without an elevation of right atrial pressure above 

 zero. 



Here, again, it is quite evident that simultaneous 

 analysis of the function of the heart and of peripheral 

 circulatory factors is needed to ascertain the inte- 

 grated effects of exercise on venous return, cardiac 

 output, and right atrial pressure (10, 39, 50, 169, 

 177, 188). 



EFFECT OF RAPID TRANSFUSION ON VENOUS RETURN, 

 CARDIAC OUTPUT, AND RIGHT ATRIAL PRESSURE. Figure 



1 5 depicts the effects of rapidly infusing an animal 

 with whole blood. The immediate effect stems mainly 

 from an increase in mean systemic pressure (99) — in 

 this instance from 7 mm Hg to 1 1.5 mm Hg. How- 

 ever, the increased blood volume also distends the 

 vessels of the systemic circulation, thus decreasing 

 the peripheral resistance and therefore increasing 

 the "slope" of the venous return curve. In the case 

 of the heart, circulatory reflexes, especially the 

 pressoreceptor reflex, actually weaken the heart 

 because the excess blood volume tends to elevate 

 arterial pressure, thereby initiating inverse reflexes. 

 As a consequence, the cardiac output curve decreases 

 very slightly. The net result, as depicted by the 

 equilibrium point, is a moderate increase in venous 

 return and cardiac output and a very marked rise in 

 right atrial pressure, which are effects that have been 

 observed many times by many different investigators 

 (66-69, M 5> 1 83)- This figure and the previous one 

 illustrates that the relationship between right atrial 



