1 1 1 8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



would be lost and the venous return curves would 

 proceed toward the left. This would continue until 

 the cardiac output should fall to 700 ml per min, 

 which would be the equilibrium point for the three 

 different curves. The method by which this third 

 curve, the level required for normal renal function, 

 can be lowered is by administration of a diuretic or 

 by drastic reduction of fluid and salt intake. Under 

 either of these conditions the kidneys can then put 

 out an amount of fluid equal to the daily intake de- 

 spite the fact that the cardiac output is greatly re- 

 duced. Thus, the animal is kept from dying as a result 

 of decompensation, and over a period of time its 

 heart can perhaps recover or at least its life can be 

 saved by continual adherence to a strict diuretic and 

 fluid regimen. 



ANALYSIS OF EFFECTS RESULTING FROM CHANGES IN 



vascular resistance. The simplified graphical ap- 

 proach can also be used to analyze the effect of vascu- 

 lar resistance changes resulting from several different 

 causes. For instance, the effects of anemia and poly- 

 cythemia have been studied (102), and the results of 

 the analyses are in accord with the experimental 

 findings in these conditions (60, 109, 164, 176). Sec- 

 ond, an analysis for A-V fistulae (104) also accords 

 with the often repeated findings in these conditions 

 (43, 116, 140, 148, 173). Third, a graphical analysis 

 showing a very detrimental effect on venous return 

 of increasing venous resistance (87) is in accord with 

 many studies which have demonstrated that relatively 

 slight venous constriction can cause either shock in 

 the acute situation (63) or peripheral congestion in 

 the chronic preparation (26). 



a more complex graphical analysis of 

 venous return, ventricular outputs, 

 and atrial pressures 



The simplified graphical analysis which has just 

 been presented is ordinarily quite satisfactory for 

 analyzing the effects of most circulatory stresses on 

 the circulation except when the stresses involve an 

 imbalance between the left and right hearts. Such 

 imbalances can result from unilateral cardiac failure 

 or unilateral excess load on the heart. To determine 

 the effects of these unilateral disturbances on the cir- 

 culation, we now need to analyze the functions of all 

 the four different segments illustrated in figure 1, 

 all at the same time, that is, of the systemic circulation, 

 the right heart, the pulmonary circulation, and the 



left heart. Then all of these must be equated against 

 each other. To do this we can proceed by pointing 

 out, first, that the analyses of right heart venous re- 

 turn and right ventricular output are approximately 

 the same as the analyses for the entire heart-lung 

 segment. Therefore, right heart function, for practical 

 purposes, can be depicted by the usual simplified 

 analysis which has already been presented. 



Also, the analyses for the pulmonary circulation 

 and the left heart obey identically the same principles 

 as those already discussed for the systemic circulation 

 and right heart, though the quantitative values are 

 entirely different. Figure 21 illustrates a typical analy- 

 sis of venous return in the pulmonary circulation 

 and also of the ventricular output of the left heart. 

 Note that the left atrial pressure scale has been greatly 

 shortened in relation to the right atrial pressure scale 

 used in previous figures. The reason for this is that 

 in later figures we will wish to use this analysis for 

 the left heart in association with analyses for the 

 right heart. When blood is shifted from the systemic 

 circulation to the pulmonary circulation, the mean 

 pulmonary pressure rises approximately 7 mm Hg 

 for each 1 mm Hg fall in mean systemic pressure 

 (138, 139, 146). Therefore, in the following equating 

 procedures the scale for right atrial pressure will in 

 all instances be seven times as great as the scale for 

 left atrial pressure. 



2800-, 



-10 +10 +20 +30 +40 n 

 LEFT ATRIAL PRESSURE (mm Hg) 



■50 



fig. 21. Analysis of pulmonary venous return to the left 

 heart and output from the left ventricle, illustrating the 

 equating of these with each other in the steady -state condition. 



