CIRCULATION IN SKELETAL MUSCLE 



'355 



4-0 - 



20 



10 



IE 



fig. 3. Diagrammatic representation of changes in blood flow 

 in the calf muscles of the human leg during strong rhythmic 

 contraction. [From Barcroft & Dornhorst (19).] 



resting value. At speeds of from 4 to 8 mph there was 

 no further increase in the size of the peak post- 

 exercise flow, but the flow returned to the resting rate 

 more and more slowly as the speed increased. 



These opening paragraphs recall the circulatory 

 changes in muscle that take place during the per- 

 formance of its most important function — namely, 

 contraction. The mechanism of the hyperemia of 

 exercise is not yet understood and it is the most 

 important problem in this field. Besides dealing with 

 the hyperemia of exercise this article must refer to 

 many other matters. For example, we shall have to 

 deal with the basal tone of the vessels, and with their 

 nervous regulation, their responses to adrenaline, and 

 so forth. It will be convenient to refer first to these 

 general matters, and afterward, with such knowledge 

 as a background, to return to the central problem of 

 the hyperemia of exercise. 



is not curtailed by previous arrest of the circulation 

 in the thigh. That is to say, the circulation in the calf 

 is of no functional significance in tiptoe standing, an 

 observation in accord with the fact that the gastroc- 

 nemius soleus arrests its own circulation when stand- 

 ing on tiptoe (28). During this exercise intramuscular 

 pressure in the calf does not exceed about 50 mm Hg 

 (1 19), so that it seems likely that the blood supply to 

 the muscle is stopped by nipping of its vessels. 



The circulation in the calf muscles behaves quite 

 differently during weak sustained contraction. Then 

 there is marked hyperemia. The effect of the vaso- 

 dilatation predominates (28). The behavior of the 

 circulation during the sustained contraction of other 

 human muscles also depends upon the force of their 

 contraction and the extent to which vasodilatation 

 overcomes the effect of mechanical compression (67, 

 141, 142, 145). 



When human muscles contract rhythmically, each 

 strong contraction checks the hyperemia (19). This 

 is shown in figure 3. In running, blood flow through 

 the calf must be intermittent; free flow through 

 widely dilated vessels when the muscles are relaxed 

 must alternate with partial or perhaps complete 

 arrest of the circulation during contraction. 



Black (36) has investigated the effect on the post- 

 exercise blood flow of walking at different speeds 

 from 1 to 8 mph. The subject wore a light celluloid 

 plethysmograph on his calf. The distance covered was 

 130 yards. Up to 4 mph the size of the immediate 

 peak postexercise blood flow was directly propor- 

 tional to the speed. The flow returned rapidly to its 



BASAL TONE 



Skeletal muscle vessels exhibit a very pronounced 

 basal tone. In this respect they differ from the vessels 

 of the skin, or at any rate from the A-V anastomoses 

 in the skin. Lofving & Mellander (143) found that 

 the resistance to flow in acutely denervated cat 

 muscles can be decreased by 80 to 85 per cent by the 

 close arterial injection of supramaximal amounts of 

 acetylcholine or ATP; the resistance in the denervated 

 paw can only be decreased by 20 to 50 per cent. 



The action of a circulating vasoconstrictor sub- 

 stance has often been invoked to explain the strong 

 basal tone in muscle vessels. If this were so then con- 

 strictor substances such as noradrenaline, adrenaline, 

 serotonin, angiotonin, and vasopressin should act 

 more powerfully in muscle, where basal tone is strong, 

 than in the skin where basal tone is weak. However 

 Lofving & Mellander (143) have shown that many 

 constrictor substances act more powerfully on the 

 skin vessels of the paw than on skeletal muscle vessels. 

 They concluded that the basal tone in muscle vessels 

 cannot be due to the action of adrenaline, noradren- 

 aline, serotonin, angiotonin, or vasopressin since 

 muscle vessels did not respond more sensitively to 

 any of these agents. 



Human muscle vessels, too, exhibit strong basal 

 tone. Vascular resistance in the normal forearm is 

 about the same as that in the chronically sym- 

 pathectomized forearm (73) and in both it decreases 

 to about one-tenth in severe exercise (113). If the 

 smooth muscle coats of these vessels were to stop 



