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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



prevailing vascular tone (97), that is to say, not when 

 the muscle vessels exhibited automaticity. On the 

 contrary, the arterial pressure always fell to within 

 a few mm Hg of zero. During the occlusion basal 

 tone must have decreased, as was manifested after- 

 wards by reactive hyperemia. Observations in man 

 confirm this. After arrest of the circulation in the 

 upper arm, intrabrachial arterial pressure and pres- 

 sure in the antecubital vein fall progressively till 

 eventually intra-arterial sinks below intravenous 

 pressure. This is explained by progressive loss of 

 intravascular tone, without however the reflux of 

 venous blood, which is prevented by the action of the 

 venous valves (48). 



Using a pressure plethysmograph, Burton & 

 Yamada (48) found that the vessels of a segment of 

 the forearm did close critically after reduction of their 

 transmural pressure. About half the tissue in their 

 forearm plethysmograph must have been muscle. 

 Further work is needed on critical closing pressure in 

 healthy muscle. 



Local Temperature 



Not enough is known about the effect of local 

 temperature on the blood flow through muscle. 

 Blood flow appears to decrease progressively when the 

 cat's hind limb with paw tied off is cooled from 40 C 

 to 25 C. Further cooling is accompanied by increase 

 in flow which at 10 C generally exceeds that at 40 C 

 (155). This is not so in the limb that has been treated 

 with cyanide: blood flow diminishes as the tempera- 

 ture falls, owing to diminution in the fluidity of the 

 blood (155). 



In man the average forearm blood flow when the 

 limb was in water at 45 C was 1 7.6 ml per 100 ml per 

 min. When the water was 13 C forearm flow was 

 0.5 ml. However, these experiments tell us little 

 about the effect of local temperature on human 

 muscle blood flow (20). Forearm blood flow in- 

 creases as the temperature of the surrounding water 

 is lowered from 18 C to o C; the dilatation is mainly 

 in the muscles as it takes place after the circulation 

 in the skin has been arrested by adrenaline electro- 

 phoresis. Further work in this lield is needed. 



THE PROBLEM OF STRUCTURE AND FUNCTION 



The arrangement of blood vessels in striated 

 muscle was studied by Spalteholtz (173) and de- 

 scribed as follows by Krogh (134). "The arteries 



supplying a muscle branch freely, and between the 

 branches there are very numerous anastomoses 

 forming a primary network. Into the meshes of this 

 net small arteries are given off at regular intervals, 

 and these again anastomose freely, forming a second- 

 ary cubical net of great regularity. From the threads 

 of this network the arterioles branch off, generally 

 at right angles to the muscle fibers and at very regular 

 intervals (of about 1 mm in the warm-blooded ani- 

 mal), and these arterioles finally split up into a large 

 number of capillaries running along the muscle fibers 

 and in the main parallel to them but with numerous 

 anastomoses, forming long narrow meshes about the 

 fibers. The capillaries unite into venules intercalated 

 regularly between the arterioles, and the whole 

 system of veins reproduces and follows almost ex- 

 actly that of the arteries. All the veins down to the 

 smallest branches are provided with valves allowing 

 the blood to flow in the direction of the heart only." 



The number of capillaries per square millimeter 

 transverse section of striated muscle is related to the 

 metabolic activity of the animal. Krogh found 400 

 per mm 2 of muscle in the cod, 1350 in the horse, 2630 

 in the dog, and the number in the smallest mammal 

 he thought would be more than 4000. Assuming a 

 figure of 2000 for the number of capillaries per square 

 millimeter of human muscle he calculated that the 

 total length of all the capillaries in all the skeletal 

 muscles of a man would be equal to a distance of two 

 and a half times round the earth, and he estimated 

 that when all these capillaries were open their surface 

 area would be 6300 m-\ 



Certain experimental findings are difficult to 

 explain on the basis of the classical description of 

 skeletal muscle vessels. For example, stimulation of 

 the vasoconstrictor nerves to the dog's hind legs is 

 accompanied by decrease in muscle blood flow, by 

 decrease in oxygen consumption of the muscle and 

 surprisingly by a rise in oxygen saturation of the 

 venous blood (154). Pappenheimer thought that the 

 blood must have been directed through A-V shunts 

 whose surface area available for O a exchange was 

 small. Issekutz (127-129) came to the same conclu- 

 sion. Then, again, increase in muscle blood flow is 

 accompanied by decrease in oxygen consumption 

 during sympathetic vasodilator nerve stimulation 

 (126), but by increase in Oo consumption during 

 inhibition of sympathetic vasoconstrictor tone (169). 

 As figure 6 shows, hypothalamic stimulation is accom- 

 panied by increase in venous outflow from muscle 

 but the clearance of Xal 131 from muscle does not 

 alter. To explain such results it has been suggested 



