CIRCULATION IN SKELETAL MUSCLE 



[ 357 



gestion instead of by suction is also followed by con- 

 striction of the resistance vessels (157). 



Pressure-Flow Relations in Muscle Vessels 

 Deprived of Automaticity 



Folkow & Lofving (97) investigated pressure-flow 

 relations in maximally dilated muscle vessels in 

 which automaticity had been abolished by perfusing 



100 



fig. 4. Results showing that stretching of the forearm 

 vessels causes contraction. Oxygen saturations of blood samples 

 from a superficial (O) and a deep (•) forearm vein after general 

 body heating. During the time represented by the black 

 rectangle the subject's legs were passively raised. During the 

 period between the vertical lines the forearm was exposed to a 

 pressure 50 mm Hg below atmospheric. [From Blair el at. 

 (39)-] 



them with dextran-Tyrode solution. Their results are 

 summarized in figure 5. Figure 5A (continuous curve) 

 shows the pressure flow relations when the arterial 

 pressure was increased stepwise, the venous pressure 

 being maintained at zero. The curve is convex to 

 the pressure axis indicating vascular distention as 

 the pressure increased, until further distention is 

 prevented by the connective tissue and the develop- 

 ment of edema. In Figure 5A (broken line) are seen 

 the results when the mean intravascular pressure 

 was kept constant at 50 mm Hg, and the perfusion 

 pressure was increased by increasing the arterial 

 pressure above and decreasing the venous pressure 

 below the mean value. The relation between the 

 perfusion pressure and the flow is then linear. This 

 is as would be expected since the distending force, 

 and hence the resistance to flow remain constant at 

 all values of the perfusion pressure. Finally in Figure 

 5B is seen the effect of raising arterial and venous 

 pressures together by equal increments so that the 

 perfusion pressure remains constant while mean 

 pressure increases. It will be seen that the greater 

 the mean pressure the larger is the flow correspond- 

 ing to a given difference in the perfusion pressure. 

 This follows because an increase in the mean pres- 

 sure distends the vessels and decreases intravascular 

 resistance. When the mean pressure is high the ves- 

 sels are almost maximally distended and resist further 

 distention like rigid tubes. 



Critical Closing Pressure 



When the arterial supply to the cat's muscles was 

 occluded in an animal in good condition the arterial 

 pressure did not level out at a value specific for the 



FLOW ML/MIN 



% INCREASE OF FLOW 



-|=90 



*580 FjSO 



P.30 

 f=60 

 P,20 



fig. 5. Perfusion of the calf of the cat's 

 leg with dextran-Tyrode solution. Vessels 

 maximally dilated. Arterial pressure, Pa; 

 venous pressure, Pv; perfusion pressure, 



p p D Pa + Pv 



Pa — Pv = Pn; mean pressure, ' = 



2 



Pmi. For further explanation see text. 



[After Folkow & Lofving (96).] 



