HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



level at the same point, and this point represents the 

 mean systemic pressure on the right atrial pressure 

 scale and the mean pulmonary pressure on the left 

 atrial pressure scale. Thus, the mean systemic pres- 

 sure has risen from 7 mm Hg to 7.5 mm Hg, and 

 this has caused the right ventricular output to rise to 

 point B, a value about 10 per cent above normal. 

 On the other hand, the shift of blood out of the lungs 

 has decreased the mean pulmonary pressure from 

 7 mm Hg to 3.5 mm Hg, thus shifting the left ven- 

 tricular venous return curve to the right and decreas- 

 ing the left ventricular output to point B' , an output 

 40 per cent below normal. This represents a disparity 

 of outputs between the two ventricles of 2 to 1 with 

 far greater amounts of blood being pumped by the 

 right heart than by the left heart. As a consequence, 

 a major shift of blood occurs from the systemic circu- 

 lation back to the pulmonarv circulation, increasing 

 the mean pulmonary pressure and decreasing the 

 mean systemic pressure. As a result, the outputs of 

 the two sides of the heart once again become equi- 

 librated. 



Conversely, a sudden shift of blood from the sys- 

 temic circulation into the pulmonary circulation is 

 illustrated by the dashed-dot curves, showing a de- 

 crease in mean systemic pressure to 6 mm Hg and a 

 rise in mean pulmonary pressure to 14 mm Hg. The 

 net result is diminution of right ventricular output 

 by approximately 10 per cent and enhancement of 

 left ventricular output by approximately 40 per cent. 

 Here again there is almost 2 to 1 disparity between 



the outputs of the two ventricles, thus resulting in a 

 rapid shift of blood out of the lungs into the systemic 

 circulation; this shift continues until the right ven- 

 tricular output rises to equal the falling left ventricular 

 output. In this manner, the outputs of the two ven- 

 tricles once again hecorre re-equilibrated, thus ex- 

 plaining the experimental findings of many different 

 investigators that the two sides of the heart always 

 automatically re-equilibrate with each other within 

 a few heartbeats (11, 18, 129, 159). 



EFFECT OF ACUTE LEFT HEART FAILURE ON CARDIAC 

 OUTPUT, VENOUS RETURN, LEFT AND RIGHT ATRIAL 

 PRESSURES, MEAN SYSTEMIC PRESSURE, AND MEAN PUL- 

 MONARY pressure. Figure 25 illustrates the sequence 

 of events that occurs when the left ventricle suddenly 

 fails. Point A is the normal equilibrium point for the 

 right ventricle and point A' the normal equilibrium 

 point for the left ventricle. These two are in equi- 

 librium with each other. Then, suddenly, the left 

 ventricular output curve falls to less than one-half 

 normal as depicted by the lower solid curve. Instan- 

 taneously, this depressed left ventricular output curve 

 equilibrates with the normal left ventricular venous 

 return curve at point B which represents onlv 30 per 

 cent of normal output. Now a 70 per cent disparity- 

 exists between the momentary right ventricular out- 

 put and the momentary left ventricular output, this 

 causing blood to shift into the lungs from the systemic 

 circulation (138, 139). In a matter of a few heart- 

 beats the new venous return curves become the dashed 



fig. 25. Effect of sudden reduction in pumping 

 effectiveness of the left ventricle. This shows a shift 

 of both venous return curves (as illustrated by the 

 dashed curves) to the left until right ventricular 

 output falls (point C) to equal the rising left ventric- 

 ular output (point C) 



O 



z 



ID 



> 



-4 -2 + 2 + 4 +6 



RIGHT ATRIAL PRESSURE (mm Hg) 



+ 8 



+ 50 +40 +30 +20 +10 -10 

 LEFT ATRIAL PRESSURE (mm Hj) 



