I I 10 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



return curve caused by altering the mean systemic 

 pressure. Note that when the mean systemic pressure 

 is increased from the normal value of 7 mm Hg up 

 to 1 4 mm Hg the curve is shifted to the right and its 

 plateau becomes approximately twice as high as 

 normal. Conversely, when the mean systemic pressure 

 is decreased to 3.5 mm Hg, which is one-half normal, 

 the curve is shifted to the left, and the plateau becomes 

 reduced to one-half normal. Since the flow of blood 

 to the heart is proportional to the pressure gradient 

 for venous return, which in turn is equal to the mean 

 systemic pressure minus the right atrial pressure, one 

 can see that any increase in mean systemic pressure 

 causes a corresponding increase in venous return at 

 any given right atrial pressure. Likewise, any decrease 

 in the mean systemic pressure will cause a cor- 

 responding decrease in venous return at all right 

 atrial pressures. 



Figure 10 depicts venous return curves in a series 

 of normal dogs and then again in the same dogs after 

 an average infusion of 200 ml of blood and also 

 after hemorrhage of 122 ml (99). Note that the 

 normal mean systemic pressure was approximately 

 7.7 mm Hg, that this rose to 1 1 .3 mm Hg in the 

 infused dog, and that it fell to 4.7 mm Hg in the 

 hemorrhaged dog. This experiment illustrates typical 

 shift of the venous return curves to the right as the 

 blood volume increases, thereby increasing the mean 

 systemic pressure. 



Changes in the distensibility of the vascular system 

 or changes in the pressure on the outside of the 

 vessels can alter the mean circulatory pressure in 

 exactly the same manner as can alterations in blood 

 volume. These changes include a) increased vaso- 



motor tone, caused either by sympathetic stimulation 

 or by infusion of sympathomimetic drugs; b) pressure 

 on the abdomen, which compresses large intra- 

 abdominal blood reservoirs; c) increased intrathoracic 

 pressure, which compresses the blood reservoirs of 

 the chest; and d) increased interstitial fluid volume, 

 which causes pressure on the outside of blood vessels 

 throughout the body. In normal circulatory ad- 

 justments the most important of these is the effect of 

 vasomotor tone on the mean systemic pressure. 



Figure 1 1 illustrates the average results from 1 1 

 typical experiments in dogs in which the degree of 

 vasomotor tone was altered from the minimal level 

 up to almost the maximal level (95). This shows the 

 typical effects one would expect when the mean 

 systemic pressure is elevated, that is, progressive 

 shift of the venous return curves to the right as the 

 vasomotor tone is increased. 



One might have expected the administration of a 

 sympathomimetic drug to cause increased resistance 

 to blood flow toward the heart as well as to increase 

 the mean circulatory pressure. This was not evident 

 from these studies, for the venous return curves did 

 not decrease in slope as the rate of epinephrine in- 

 jection was increased. On second thought, one can 

 understand why this was true. When vasomotor tone 

 is increased throughout the circulation while the blood 

 volume remains constant, pressures everywhere in 

 the circulation will tend to rise because of tightening 

 of the vessels around the blood. But, if any single 

 segment of the circulation constricts, some other 

 segment of the circulation must dilate. On the 

 average, then, for every constriction that occurs in 

 the systemic circulation following the injection of 



fig. 10. Effect of increasing or decreasing the 

 blood volume on the venous return curves. Total 

 spinal anesthesia was instituted to prevent cardio- 

 vascular reflexes during the course of the experi- 

 ment. [From Guyton et a/, (qq). 1 



Z> 

 h- 

 LU 



<r 



CO 



o 



2400 



2000 



1600 



1200 



800 



400 



TOTAL SPINAL ANESTHESIA 

 0.0005 mg EPINEPHRINE /kg /mm 

 TEN DOGS 

 AVERAGE WEIGHT =12.94 kg 



AVG ART PRESS -- 131 1 mm Hg 



INFUSION 200 

 AVG ART PRESS = 



NORMAL 



-16 -12 -8 -4 +4 +8 +12 



RIGHT ATRIAL PRESSURE (mm Hg) 



+ 16 



