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HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



100 



1 2 3 



Minutes 



fig. 2i. Results showing that after circulatory arrest 

 (vertical lines) the oxygen saturation in the venous blood rises to 

 above the initial value. Oxygen consumption (open circles) 

 returns to the resting level more quickly than the blood flow 

 (filled circles). [From McNeil (148).] 



arterial pressure. The resting rate of flow, post- 

 ishemic peak and "area under curve" or "blood 

 debt" were all much reduced, but this was not so 

 for the exponential rate of restoration of the flow. 

 They concluded that the flow could not depend on 

 the local concentration of some metabolite, the re- 

 moval of which depended critically on the rate of the 

 blood flow. They say "the facts were compatible with 

 the concept of a metabolite diffusing out of the tissues 

 with a concentration gradient which effectively 

 limits its rate of removal when the blood flow is 

 above some small value, or of a metabolite oxidized 

 at a rate dependent on its concentration and inde- 

 pendent of the local oxygen tension when this ex- 

 ceeds some low figure." The significance of this 

 finding is not yet known. Another relevant experi- 

 ment was performed by Blair et al. (41). They studied 

 the blood flow in the forearm after 5 min circulatory 

 arrest. Coincident with the release of the circulation 

 the brachial artery was compressed digitally for 5 

 min to prevent the blood flow from rising above the 

 resting level. Release of the artery was not followed 

 by any reactive hyperemia. They concluded that it 

 was not necessary to have an increase in blood flow 

 alter circulatory arrest to "repay" the "debt" in- 



curred during this procedure. It would be interesting 

 to compare the oxygen debt incurred during the 

 circulatory arrest with the subsequent oxygen repay- 

 ment. So far as I am aware this has not been done, 

 because of technical difficulty. [However, see (180).] 

 It is certainly worth recalling that reactive hy- 

 peremia in totally denervated forearms is just as 

 great as in normal ones (72). Indeed, owing to the 

 withering of the muscles the response per 100 ml is 

 far greater in the denervated forearm. Can reactive 

 hyperemia be due to ischemia of the plain muscle of 

 the arterial tree? 



EXERCISE HYPEREMIA 



Active muscles must get oxygen from the air. To 

 this end total ventilation increases, heart rate is 

 speeded up, and muscle blood vessels dilate. We 

 still do not know for certain the mechanism of any 

 of these responses. The hyperpnea that we can see 

 and the tachycardia that we can feel and record have 

 attracted more attention than the deeply hidden 

 dilatation of the muscle vessels. We still seem a long 

 way from understanding the cause of the hyperemia 

 of exercise. I recall some sentences of my father's. 

 "Let us then jot down such information as is forth- 

 coming in the hope that the points at issue may be 

 taken up one by one by future workers, and that one 

 day systematic work may be done on the subject. 

 I say 'jot down' rather than 'put together' because 

 to make any sort of story from such unsatisfactory 

 material would be quite unwarrantable" (30). 



Let us first "jot down" some points about the 

 hypothesis that exercise hyperemia is caused by 

 anoxia of the vascular tree. It is certainlv worth 

 noting that the rate at which muscle blood vessels 

 open may be as fast after simple arrest of the circu- 

 lation as during exercise. Eichna & Wilkins (77) 

 found that the peak forearm flow after 5 min of simple 

 circulatory arrest was as large as that after 5 min 

 combined circulatory arrest and rhythmic exercise. 

 Dornhorst & Whelan (68) recorded a peak flow of 

 about 20 ml per 100 ml calf per min after 2 min 

 ischemia; after 2 min rhythmic exercise the same post- 

 exercise peak flow was recorded. Hilton (120) noted 

 that the flow from the cat's gastrocnemius after 30 

 sec ischemia was the same as that after 30 sec exercise. 

 True, the mechanisms of ischemia and exercise are 

 not really comparable, but further work is necessary 

 to see if ischemia opens the vessels as fast alone as 

 when combined with exercise. If so, we must ask 



