CIRCULATION IN SKELETAL MUSCLE 



!359 



fig. 6. Radio-iodide clearance and blood flow in the 

 gastrocnemius: Nal 131 injected intra-arterially. Results show 

 that hypothalamic stimulation increases muscle blood flow but 

 does not alter Nal 131 clearance. O — Stimulation of the hypo- 

 thalamic vasodilator pathway; 5 trials, 4 cats. [From Hyman 

 el al. (126).] 



the stimulation of the vasodilators shifts the flow 

 from nutritional to nonnutritional (A-V shunt) 

 channels. 



Intravenous infusions of adrenaline increase the 

 rate of the blood flow through muscle without affect- 

 ing the rate of Na 24 clearance (151). This too has 

 been attributed to the opening of A-V shunts (18). 

 A-V anastomoses have been invoked to explain the 

 circulatory changes in muscle during hypothermia 

 (61) and to account for the very small A-V 2 dif- 

 ference in resting muscle (31). 



Zweifach (181 J claims that he has seen blood 

 short-circuitin g through "thoroughfare vessels" in 

 skeletal musc'e, and Redish et al. (161) have pub- 

 lished photomicrographs of A-Y shunts in human 

 skeletal muscle. However, most anatomists deny the 

 existence of A-V anastomoses in skeletal muscle 

 (43). The perfusion of skeletal muscles with fluids 

 containing minute plastic spheres shows, moreover, 

 that no sphere of 30 /* diameter or over traverses the 

 denervated gastrocnemius of the dog, though one- 

 fifth of the stream goes through vessels of 20 n diam- 

 eter. These vessels may be large capillaries (66, 159). 



Barlow et al. (32, 33), having failed to find A-V 

 shunts in muscle, have suggested another explanation 

 of the action of adrenaline on the muscle circulation. 

 They found that muscle contains two entirely sepa- 

 rate circulations, one to the skeletal muscle fibers, 

 the other to the connective tissue. According to these 

 authors adrenaline increases the rate of flow through 

 the nutritional vessels as is shown plethysmographi- 

 cally. However, it has little effect upon the rate of 

 flow in the connective tissue where, in most experi- 



ments, the Xa 24 is located, so that the rate of Na 24 

 clearance is scarcely altered. 



Folkow (93) and Mellander (149) are using the 

 following scheme for the muscle circulation. After 

 large "windkessel vessels," which transform pulsatile 

 into fairly steady flow, come "resistance vessels," 

 consisting of two variable sets — precapillary (pre- 

 dominantly the arterioles) and postcapillary (mainly 

 the small veins). These vessels determine the resist- 

 ance to flow and also affect the hydrostatic capillary 

 pressure and therefore the filtration rate. The "sphinc- 

 ter" vessels are a specialized section of the smallest 

 precapillary resistance vessels. These vessels can 

 cause intermittent closing of the capillaries and they 

 regulate the size of the capillary surface area ex- 

 posed to the blood flow and available for blood- 

 tissue fluid exchange. Then there are "capacitance 

 vessels" (mainly the veins) in which minor changes 

 in tone, too small to affect the resistance signifi- 

 cantly, will have a large effect upon the circulating 

 blood volume available for the heart. Lastly there 

 are, of course, the "exchange vessels" or true capil- 

 laries for the direct exchange of substances between 

 the blood and tissue fluids; they are devoid of smooth 

 muscle cells. 



Further work is necessary to reconcile the function 

 of the vascular bed in muscle with its structural ar- 

 rangement. 



NERVOUS CONTROL 



Skeletal muscle vessels exhibit strong intrinsic 

 basal tone and correspondingly weak nervous control. 

 Their smooth muscle is supplied by sympathetic 

 vasoconstrictor and vasodilator fibers, though the 

 belief that these act reciprocally is no longer tenable. 

 Nor is there at present any convincing evidence that 

 muscle's sensory innervation has any effect on its 

 vessels, either by antidromic impulses or by axon 

 reflexes. 



Sympathetic Vasoconstrictor Xerves 



These have been found in the cat, dog, hare, mon- 

 key, and in man, in fact in all mammals so far in- 

 vestigated [for literature see 29, 92]. The evidence 

 for their existence in animals is conclusive. It may be 

 of some interest to refer briefly to the proof of their 

 presence in man (21, 29). It is as follows. The rate 

 of the blood flow in the upper muscular parts of both 

 forearms was measured plcthysmographically and 



