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HANDBOOK OF I'lIVSK ,\ 



CIRCULATION II 



muscle vessels were dilated not only by locally 

 liberated lactic acid but also by lactic acid liberated 

 from other muscles into the general circulation. 

 Many experiments have been done on man to try 

 to explain why the sustained vasodilatation is larger 

 during intravenous infusions than it is during intra- 

 arterial ones. It is not due to rise in arterial blood 

 pressure, nervous reflexes (29, 177), histamine forma- 

 tion (153), or secretion of the pituitary (135). It is 

 probably explained by the fact that during intrave- 

 nous infusions the concentration of adrenaline re- 

 mains constant independently of the rate of flow, 

 whereas the concentration varies reciprocally with the 

 rate of flow during intra-arterial infusions. The large 

 sustained vasodilatation typical of an intravenous 

 adrenaline infusion can be obtained too during an 

 intra-arterial infusion, if the rate of infusion of the 

 hormone is gradually increased so as to keep its 

 concentration constant (143a). 



Effect of Adrenaline on the emulation in 

 Skeletal Muscle During Exercisi 



It is often stated that adrenaline dilates muscle 

 blood vessels in exercise. So far as I am aware there 

 is no evidence that it actually does so. The idea is 

 often accepted as part of Cannon's Emergency 

 Theory of the function of the autonomic nervous 

 system. So far as I can find, Cannon himself never 

 suggested it (49-52). On the other hand, why are 

 the plain muscle coats of the arteries in the skeletal 

 muscles specialized so that they are rapidly dilated 

 by adrenaline? Is this of any teleological value in the 

 cat or man? Or was it of value in some extinct an- 

 cestor? Or is it just a coincidence? 



A few minutes after the beginning of long-lasting 

 repetitive stimulation of the motor nerve to the dog's 

 gastrocnemius, the oxygen saturation of the venous 

 blood draining from the muscle sinks to its lowest 

 point to rise again later. This is because of delay in 

 the rate of opening of the vessels. If in the exercising 

 animal adrenaline secretion helped to open the ves- 

 sels, the provision of oxygen and disposal of waste 

 products would be facilitated. 



When exercise begins the blood pressure rises, 

 sympathetic impulse discharge increases, and metab- 

 olite concentration in the muscle mounts up. It 

 would be very difficult to devise an experiment in the 

 cat, dog, or human to determine the extent to which 

 the initial dilatation of the muscle vessels was in 

 fact due to the action of adrenaline. 



In man, emotional stress at the beginning of exer- 



cise may be accompanied by adrenaline secretion 

 (16). This would be expected to cause an initial 

 transient vasodilatation throughout the entire skele- 

 tal muscular system. In active muscles the vessels 

 would be rapidly dilated and the constrictor phase 

 of the initial biphasic response might well be blocked 

 b) the action of the mounting concentration of metab- 

 olites. In animal experiments the constrictor action 

 of adrenaline is blocked if muscles are active (150). 

 In other muscles which were not contracting the 

 constrictor part of the biphasic response would mani- 

 festly be of use as it would prevent useless and waste- 

 ful hyperemia. To quote from August Krogh (134), 

 "Speculations such as these, though admittedly- 

 loose, are sometimes very useful. Sooner or later an 

 opportunity offers of putting them to the test. It is, 

 of course, very gratifying to find them confirmed, but 

 generally they are even more useful when they turn 

 out to be wrong, because, in that case, they serve to 

 discover at what point the reasoning went astray and 

 to guide it back into a channel which may possibly 

 lead it onward. The problems of physiology are so 

 complicated that, to put it tersely, one cannot expect 

 to be able to reason correctly from the facts for more 

 than 5 min at a stretch." 



Apart from the beginning of exercise is the question 

 of the action of adrenaline on the vessels later on 

 (sustained vasodilator action). It is known that adren- 

 aline continues to be secreted in severe exercise in 

 man (82), but because of the very strong action of 

 metabolites its effect on the vessels would be ex- 

 pected to be negligible. This is in accordance with 

 the results of experiments. In dogs, Cannon et al. 

 (49) found that the amounts of work that dogs 

 could do to exhaustion on a treadmill was neither 

 prolonged by previous injection of adrenaline nor 

 shortened by previous adrenalectomy. In man Dorn- 

 horst & Whelan (68) found that the postexercise 

 "blood debt" was not diminished when the exercise 

 was performed during an infusion of adrenaline. 



REACTIVE HYPEREMIA 



Reactive hyperemia can be induced in skeletal 

 muscle vessels. Hilton ( 1 20) recorded it in the cat's 

 isolated gastrocnemius. Following temporary arrest 

 of the circulation through this muscle, achieved by 

 clamping the artery for 30 sec, the increase in blood 

 flow was as great as that recorded after 30 sec of 

 maximal tetanic contraction, but after ischemia the 

 flow subsided more quickly. Folkow & Lofving (97) 



