no8 



HANDBOOK OF PHYSIOLOOV 



CIRCULATION II 



fig. 7. Effect on the venous return curse of suddenly open- 

 ing large bilateral femoral A-V fistulae. 



curve of changing the systemic resistance from normal 

 (87). Note that the venous return is exactly zero in 

 the case of each of these three curves when the right 

 atrial pressure is equal to the mean systemic pressure. 

 That is, when there is no pressure gradient for venous 

 return, there is likewise no flow toward the heart. 

 Yet, when the right atrial pressure falls to some value 

 below the mean systemic pressure, then a pressure 

 gradient does exist for forcing blood toward the 

 heart, and the return of blood is inversely propor- 

 tional to the resistance. The greater the resistance, 

 the less is the return of blood to the heart, and the 

 less the resistance, the greater is the venous return. 

 Thus, figure 6 shows the normal venous return curve, 

 a venous return curve in which the resistances 

 throughout the systemic circulation are approximately 

 one-half normal, and a venous return curve in which 

 the resistances are approximately two times normal. 

 Figure 7 illustrates a typical experiment in which 

 peripheral resistance was suddenly changed while 

 all other conditions of the circulation were kept as 

 nearly constant as possible (103). In this instance two 

 large femoral A-V fistulae were suddenly opened so 

 that the total peripheral resistance was decreased to 

 approximately 60 per cent of the control value. 

 Circulatory pressures remained exactly constant. 

 Note that the study depicts precisely the same effects 

 as those illustrated in the previous figure but this 

 time showing a typical and actual experimental 

 study 



It should not be supposed, however, that increasing 

 the resistance to blood flow in the arteries affects 

 venous return equally as much as increasing the 

 resistance in the veins. Indeed, for a given increase 

 in venous resistance, the venous return decreases 

 approximately eight times as much as when the 

 arterial resistance is increased the same amount. 

 This was illustrated by a comparative study in which 

 arterial resistance was increased by injecting micro- 

 spheres into the arterial system and venous resistance 

 was increased by progressive occlusion of all the large 

 veins emptying into the right atrium (87). Figure 8 

 illustrates the difference between these two effects, 

 the upper curve showing that the total peripheral 

 resistance could be increased by arterial embolization 

 to as much as 400 to 500 per cent of control values 

 before the venous return decreased a great amount. 

 On the other hand, increasing the total peripheral 

 resistance only 30 per cent by the method of venous 

 compression decreased the venous return to one-half 

 normal. 



The cause of this difference between venous re- 

 sistance and arterial resistance is that the arterial 

 system proximal to the arterioles has very little 

 capacitance (DV/DP) in relation to the total capa- 

 citance of the s\stemic circulation proximal to the 

 venous constriction at the outflow of the veins into 

 the heart (89). Because of the small storage ability of 

 the arteries for blood, increasing the resistance at 

 the arterioles elevates the arterial pressure almost as 



100 



° 80- 



60- 



C40- 



S20i 

 o 



^9 °"er,ol resistance 



100 200 300 400 500 



TOTAL PERIPHERAL RESISTANCE (% of control value) 



fig. 8. Effect on venous return of increasing the total 

 peripheral resistance when the resistance is increased in three 

 different ways : 1 ) by injecting microspheres into the arteries 

 to increase arterial resistance, 2) by constricting the inflow- 

 veins to the heart, and 3) by a combination of these two 

 procedures. [From Guyton et til. (87).] 



