CIRCULATION IN SKELETAL MUSCLE 



'377 



whether the smooth muscles of the arterial tree 

 relax simply because their oxygen is taken away 

 by the active skeletal muscle fibers. 



The experiments of Kramer and his colleagues 

 (131-133, 160) have helped a great deal to establish 

 the effect of exercise on the circulation and metab- 

 olism of muscle. The dog's gastrocnemius muscle was 

 stimulated indirectly via its nerve. The rate of its 

 venous outflow was recorded continuously by an 

 optical method in milliliters per minute as also were 

 the arterial blood pressure and the oxygen saturations 

 of both arterial and venous blood; in some experi- 

 ments blood lactate was estimated. From these, the 

 relations between work done, blood flow, oxygen 

 consumption, and lactate output were calculated. 

 Figure 22 is from an experiment of Kramer's in 

 which the sciatic nerve was stimulated maximally 

 at 310 impulses per sec for 1 sec every alternate sec. 

 The findings are relevant: 



/) At the beginning of exercise venous blood 

 oxygen tension fell abruptly to reach a "low" after 

 about 1 min. Blood flow rose exponentially to reach 

 a steady value in about 1 min. If the resting vessels 

 were opened wide by acetylcholine then oxygen 

 usage jumped up to the steady state as soon as the 

 exercise began. 



These facts may be interpreted as follows. As soon 

 as exercise begins there is an immediate demand for 

 oxvgen, the supply of blood being quite inadequate, 

 tissue oxygen tension falls to a very low level. This is 

 reflected in the low oxygen saturation of the venous 

 blood, and possibly also in the gradual relaxation 

 of the plain muscle of the arterial tree. As vasodilata- 

 tion proceeds and oxygen supply improves venous 

 oxygen saturation rises somewhat. 



2) During the steady state, blood flow, work done, 

 and rate of oxygen consumption are linearly related. 

 This is seen in figure 23. During submaximal exercises 

 the muscle gets all the oxygen it wants (or almost). 

 Opening the vessels still more with acetylcholine 

 does not increase the oxygen consumption. It is not 

 clear from these exeriments whether the blood flow 

 was linearly related to the decrease in venous blood 

 oxygen tension. It seems very significant indeed that 

 the rate of the blood flow is linearly related to the 

 rate of oxygen consumption. Further work is needed 

 to see how it is related to tissue oxygen tension. 



3) Immediately after moderate exercise extra 

 oxygen usage stops in a muscle the vessels of which 

 are opened maximally with acetylcholine. When the 

 circulation is normal, oxygen consumption and blood 

 flow rapidly subside exponentially, oxygen consump- 



z 



o 



cr 



3 



%, ARTERIAL 

 VENOUS 



-10 

 ML/MIN 



5 

 Q 



□ 

 O 



o 



MINUTES 

 fig. 22. Results obtained by Kramer and his colleagues. 

 Top: records from which blood flow, oxygen saturation, and 

 oxygen consumption were obtained. Rhythmic stimulation 

 between the vertical lines. Note the remarkable rise in the 

 venous oxygen saturation after the end of exercise. Bottom: re- 

 sults calculated from records like those shown above. [After 

 Kramer et at. (132).] 



tion falling a little more rapidly than blood flow. 

 The behavior of the venous O2 saturation is interest- 

 ing. Immediately after exercise stops, it rises tran- 

 siently to a peak and then subsides again to a low 

 level from which it recovers only very slowly. The 

 reason for the immediate postexercise peak may be 

 as follows. The demand for oxygen being soon satis- 

 fied, the arterial tree is no longer anoxic and its 

 plain muscle starts to contract. But the rate of con- 

 traction cannot keep pace with the fall off in demand 

 for O-i. Hence O2 saturation rises. It is not clear 

 why, later on, the postexercise venous blood oxygen 

 saturation subsides from the peak to a level almost 

 as low as in exercise, and from which recovery to the 

 pre-exercise level takes place only very gradually. 

 Since at this time the rate of the blood flow is de- 

 creasing, it is plain that blood flow cannot be in- 

 versely related to venous blood oxygen tension. To 

 be able to explain this odd finding would be to gain 

 much insight into the mechanism of exercise hyper- 

 emia. 



