CIRCULATION THROUGH THE SKIN 1 33 1 



15 C to 29 C there is a modest rise, and from 29 C 

 to 35 C a faster rise in flow with temperature (180); 

 35 C to 37 C is the highest temperature to which the 

 hand is normally warmed by the body's own heat. 

 At local temperatures in the range 25 C to 35 C, the 

 level of blood flow is greatly influenced by the heat- 

 regulating mechanism of the body, and the observed 

 values are distributed over a wide range. With further 

 rise in local temperature from 35 C to 45 C, there is 

 a steep increase in flow, to a maximum of about 

 35 ml per 100 ml per min (4); Peacock (159) found 

 in 12 women an average of 36.0, and a range of 30.8 

 to 41.0; Kunkel & Stead (136) in 18 subjects at 43 C 

 found an average of 32 and a range of 18.7 to 54.4. 

 Even at high local temperatures the heat-regulating 

 mechanism still exerts an influence, for if the subject 

 is generally warmed the blood flow through the hand 

 at 44 C increases to about 56 ml per 100 ml per min, 

 individual observations of over 70 ml per 100 ml per 

 min having been recorded (166). Most people find 

 immersion in stirred water hotter than 45 C to be 

 painful or intolerable. 



In the foot, the effect of immersion in water at 

 various temperatures is very similar to that in the 

 hand, but the blood flow per unit volume of tissue is 

 generally about 50 per cent, and per unit of surface 

 area about 75 per cent (136), of that in the hand. 

 Allwood & Burry (12) report average blood flows in 

 four subjects ranging from 0.2 ml per 100 ml per 

 min at 15 C to 16.5 ml at 44 C, and these seem typi- 

 cal. Thus in the range 43 C to 45 C flows have been 

 reported of 14.8 in one subject (132); 16.3 with a 

 range 11.1 to 20.9 in 33 male feet; and 18.7 with a 

 range 13.4 to 25.9 in 15 female feet at 43 C, go per 

 cent of the observations falling between 13 and 20 

 (136); 15.2 in one subject (4) and 20.5 in 33 subjects 

 (191). The high blood flow with local heating prob- 

 ably has a useful protective effect. By conducting 

 heat away from the tissues it reduces the temperature 

 below the surface, and the likelihood of thermal 

 damage. A hand immersed in stirred water at 45 C 

 becomes painful if the circulation is arrested. 



It takes time for the blood flow through an ex- 

 tremity to settle after a change of the temperature 

 of the water in which it is immersed. Figure 3 shows 

 blood flows after immersing the feet in water at 

 various temperatures. The delay may be partly 

 explained by the time needed for the internal tissues 

 to reach a new equilibrium temperature. Once estab- 

 lished the blood flow through the hand and fingers 

 is well maintained after immersion for as long as 2 

 hours at 41 C (8). 



40 60 



Time (min) 



fig. 3. Foot blood flow plotted against time of immersion 

 during experiments at seven different temperatures. Each 

 point represents the average blood flow over 5 min. [From 

 Allwood & Burry (12).] 



The local effect of temperature is usually very- 

 similar to normal in chronically sympathectomized 

 hands (83), but an anomalous response has been 

 reported in one case with a reduction in the blood 

 flow through a sympathectomized hand on raising 

 the temperature to 41 C (7). The response after 

 chronic total denervation also appears to be similar 

 to normal at local temperatures above 18 C (62). 

 The vessels supposedly respond directly, but some 

 recent evidence suggests that a local nervous pathway 

 may assist. Irradiating the proximal half of the fore- 

 arm with infrared rays causes a vasodilatation which 

 spreads to the nonirradiated distal half; the spread 

 is prevented by a cutaneous nerve block at the 

 junction of the two halves of the forearm, and it is 

 unaffected by sympathectomy, or by nerve block at 

 the elbow (59). 



LOCAL TEMPERATURE IN THE RANGE O C TO 15 C: 



cold vasodilatation. Lewis (140) observed that 

 following exposure to low temperature the tempera- 

 ture of the skin rose above its former resting level. 

 For example, following cooling for 15 min at 7 C, 

 the temperature of the skin of the index finger rose to 

 above 28 C, while that of the nonimmersed third 

 finger remained at 19 C, the subject being in a room 

 at 17.8 C to 19. 1 C. The temperature of the index 

 finger was at its maximum 1 1 min after cooling 

 ended, and was raised for about 50 min. 



Further observations showed that the vasodilata- 

 tion started while the finger was exposed to cold. 

 Figure 4 shows Lewis's experiment in which the Ri 



