RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



95 1 



Effects of Changes of Perfusion Pressure and of Venous 

 Pressure on Large Artery and Vein, and 

 Distal Small Vessel Pressures 



In general, when perfusion pressure is varied while 

 other factors remain constant, pressure in the small 

 vessels parallels that in the large vessels (fig. 21). 



200- 



150- 



100' 

 80 



60- 

 40 



20 



10 



150 H 



100 

 80 



® 





ADRENERGIC 



I 





5; 



<0 <o 



tr 







I 



/ 



fig. 20. Bar graph of responses to stimulation of sympathetic 

 constrictor nerve supply (A) and of sympathetic dilator nerves 

 (B). The ordinate values give the flow at the moment of maxi- 

 mal response to the stimulation, expressed as per cent of the 

 control flow. See legend to figs. 1 2 and 1 9 for further explana- 

 tion. [Reproduced from (45).] 



PRESSURE - mmHg 

 80 120 160 



Elevation of an intestinal large vein outflow pres- 

 sure leads to a progressive increase in the venous 

 volume (yielding a measurable venous compliance), 

 to a decrease in the venous resistance, to a progressive 

 elevation of capillary pressure, and to a slower change 

 in gut volume due to capillary nitration (67). 



Effects of Extrinsic Agents on Segmental Resistance 



Decrease in C0 2 tension and hydrogen ion concen- 

 tration, produced by hyperventilation, induced dila- 

 tion of the arteries with concomitant constriction of 

 small vessels in intact forelegs. Opposite changes in 

 both vessel segments were induced by increased CO2 

 tension and hydrogen ion concentration. Serotonin 

 decreased small artery pressure markedly associated 

 with an increase in small vein pressure and no change 

 in large artery and vein pressure or total resistance 

 [Haddy (52)]. These findings suggest that serotonin 

 exerts its effects predominantly on vessels larger than 

 the arterioles and venules. 



Intra-arterial infusion of epinephrine into the 

 dog's paw caused the large artery to small vein 

 pressure difference to increase as flow decreased (fig. 

 21C). Simultaneously, the small vein to large vein 

 pressure difference decreased with flow in about the 

 same degree as that which had been noted during con- 

 trol studies in which perfusion pressure had been 

 lowered progressively (fig. 21 A). Subsequently, large 

 artery to small vein pressure difference decreased, and 

 flow rose, but small vein to large vein pressure differ- 



30 



20 



10 



ART PRESS. 



40 80 120 160 



- PERIPH VENOUS PRESS mmHg 



Control 



Infusion of Epinephrine -Art. Press 



Constant at 200 mmHg (1/jg/mm ) 



o- Control 



Cs - During Infusion 



o- Post Infusion 



fig. 21. Plots of relationship 

 of blood flow in the dog's paw 

 to pressure. A — flow vs. the 

 difference between distal small 

 vein and large vein pressure; 

 B — flow vs. the large artery to 

 large vein pressure difference; 

 C — flow vs. large artery to distal 

 small vein pressure. Heavy lines 

 — successive responses to an in- 

 tra-arterial infusion of 1 Mg/min 

 of epinephrine while the perfu- 

 sion pressure was held constant 

 at 200 mm Hg. Abscissal values 

 for C = abscissal values for B 

 minus abscissal values for A. Dis- 

 tal small vein pressures were re- 

 corded from an 0.5-mm poly- 

 ethylene catheter inserted into a 

 superficial vein and passed as far 

 distally as it would go. 



