I I 12 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



2400, 



f 2O0O- 



-4 +4 +8 +12 



RIGHT ATRIAL PRESSURE (mmHql 



fig. i 2. Equating of a normal cardiac output curve and a 

 normal venous return curve for a 12-kg dog. 



of curves. Then by plotting the two curves on the same 

 coordinates, as shown in figure 12, the equations 

 represented by the separate curves can be solved 

 (8i). Figure 12 illustrates the equating of a normal 

 venous return curve with a normal cardiac output 

 curve, as depicted for the 10-kg dog. Note that there 

 is only one single point at which the flows and the 

 pressures for the two curves are equal, and this point 

 is called the "equilibrium point." It represents the 

 solution to our graphical analysis, depicting in figure 

 12 that this particular dog at this particular time has 

 a cardiac output of 1200 ml per min, a venous return 

 also of 1200 ml per min, and a right atrial pressure 

 of — 2 mm Hg (referred to the level of the tricuspid 

 valve). 



Looking once again at figure 1 2, let us assume that 

 an extra quantity of 25 ml of blood is suddenly 

 injected into the right atrium. This would raise the 

 right atrial pressure to approximately +4 mm Hg, 

 and, as depicted by the venous return curve, the 

 elevated right atrial pressure would decrease the 

 venous return to approximately 500 ml per min. On 

 the other hand, the high right atrial pressure would 

 cause the cardiac output, as depicted by the cardiac 

 output curve, to rise to approximately 2000 ml per 

 min. Thus, a disparity of 1500 ml per min develops 

 between venous return and cardiac output so that 

 far more blood is pumped out of the heart than 

 returns to it. Therefore, within the next three to six 

 heartbeats, the right atrial pressure falls back to the 

 level of —2, thus causing the venous return to rise up 

 to 1200 ml and the cardiac output to fall to 1200 ml. 

 In other words, within a few seconds, equilibrium 

 will be re-established whenever venous return and 

 cardiac output deviate from each other (35). 



EFFECT OF SYMPATHETIC STIMULATION ON VENOUS 

 RETURN, CARDIAC OUTPUT, AND RIGHT ATRIAL PRES- 

 SURE. Using the same principles for equating venous 

 return and cardiac output curves as depicted in 

 figure 12, we can now show in figure 13 the effect of 

 strong sympathetic stimulation on venous return, 

 cardiac output, and right atrial pressure. The dashed 

 curves of the figure illustrate the normal curves. Then, 

 suddenly, sympathetic stimulation changes both the 

 venous return and cardiac output curves to the 

 respective solid curves (95). Note that the venous 

 return curve shifts far to the right and upward be- 

 cause of an increase in "mean systemic pressure," 

 and the cardiac output curve shifts upward, as is 

 characteristic of a hypereffective heart. These two 

 curves equate at an entirely new point, the new 

 equilibrium point occurring at a right atrial pressure 

 of —3 mm Hg and a cardiac output and venous 

 return of 1800 ml per min. 



This analysis of the effects of sympathetic stimula- 

 tion agrees with the typical experimental result 

 found when the sympathetics are stimulated through- 

 out the body, that is, a mild to moderate increase in 

 venous return and cardiac output and usually a 

 slight decrease in right atrial pressure (169). Almost 



E 2800 



-8 -4 +4 +8 412 



RIGHT ATRIAL PRESSURE (mm Hg) 



fig. 13. Effect of sympathetic stimulation on the venous 

 return and cardiac output curves, showing an increase in 

 cardiac output and venous return and a decrease in right 

 atrial pressure. 



