936 HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



RESISTANCE VESSELS 



Pressure-Flow Relations in I 'ascular Beds 



methods. Pressure-flow relations in vascular beds 

 have been studied by two principal methods. In one, 

 the bed was perfused by a constant flow pump at 

 various flow rates while the artery to vein pressure 

 differences were recorded. More usually inflow or 

 outflow from the vascular bed was recorded while the 

 perfusion pressure was varied by using a constant 

 pressure pump or by varying the degree of com- 

 pression of the artery supplying the vascular bed. In 

 most cases, the arterial and venous pressures were 

 recorded together with the flow. Varying perfusion 

 pressure by altering the degree of compression of the 

 supplying artery has the advantage of providing a 

 more nearly physiological situation with a minimum 

 of complicating equipment, but does not allow 

 exploration of the pressure-flow relationships above 

 the animal's existing mean arterial pressure. Flow 

 has been measured by orifice meters, rotameters, 



40 80 

 PERFUSION PRESSURE 



120 160 

 N METERED ARTERY 



fig. 2. Relationship between perfusion pressure and arterial 

 inflow. I — curve representing the direct relationship between 

 arterial pressure and flow through the cognate bed when clamps 

 were applied to the arterial anastomotic channels as at point C 

 in fig. 1. II — artifactual curve obtained if clamp C remains 

 open when the arterial pressure supply to the cognate bed is 

 varied. Note that at perfusion pressures higher than that existing 

 in the collateral artery, the metered flow is greater than the 

 flow through the cognate bed, and at pressures a little below 

 that in the collateral artery, the inflow is less than that through 

 the cognate bed while at some lower perfusion pressure back- 

 flow is recorded. Ill — artifactual curve obtained if clamps are 

 applied to the arterial supply to the collateral bed as at point 

 B in fig. 1 . Note that while no backflow is obtained, the metered 

 inflow is greater than the flow through the cognate bed at all 

 levels of pressure, and the magnitude of the error increases with 

 the perfusion pressure. [Modified from Green el at. (41)-] 



Cognate bed- 



Venous pressure 

 gauge - 



anastomotic channel 



Outflow 

 meter 



-Collateral bed 



-Collateral vein 



s Venous pressure 

 gauge 



fig. 1 . Diagram of collateral communications between a 

 metered (cognate) capillary bed and collateral bed. Inflow 

 meter — meter used to measure flow through the metered or 

 cognate bed; outflow meter — meter used to measure the outflow 

 from the metered or cognate bed; anastomotic channels — com- 

 munications between cognate bed and collateral bed on the 

 arterial and venous sides, respectively, of the metered or cognate 

 bed. A — clamp used to occlude the inflow to the metered bed 

 when perfusing it with fluid at various pressures; B — position 

 where clamp might be placed to occlude the arterial supply to a 

 collateral bed ; C and D — positions where clamps must be placed 

 in order to occlude the communications between cognate and 

 collateral beds; E — point of occlusion to prevent outflow from 

 a collateral bed. [Modified from Green el al. (41).] 



electromagnetic flowmeters, drop recorders, and 

 Gaddum-type ordinate recorders (20, 27-29, 46). 



In studying pressure-flow relationships in intact 

 vascular beds, it is necessary that all anastomotic 

 communications with collateral vascular beds be 

 occluded. If they are not, then, during measurement 

 of inflow at perfusion pressures above the animal's 

 mean arterial pressure, part of the perfusion fluid will 

 leak across the anastomotic channels from the cog- 

 nate into collateral vascular beds (fig. i-a), giving 

 an inflow which is higher than the true flow through 

 the cognate bed (fig. 2— II). Similarly, at perfusion 

 pressures below the animal's mean arterial pressure, 

 blood will leak across the communicating channels 

 from the animal's collateral arteries to the metered 

 (cognate) vascular bed (fig. i-b); as a result the 

 metered inflow will be less than the actual flow 

 through the cognate bed. At some lower pressure 

 inflow in the metered artery will cease and, at still 

 lower pressures, backflow from the artery will be 

 recorded, thus giving an entirely false picture of the 



