1328 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



between the plates is recorded (1 14). The instrument 

 does not, of course, distinguish between the effect 

 of blood circulating through the local blood vessels 

 of the skin, and blood flowing through nearby veins 

 draining distal regions. The results cannot be quan- 

 titatively translated into measurements of blood 

 flow, but the method retains its sensitivity over the 

 wide ranges of flow for which measurement of skin 

 temperature is of little help. 



The rate of clearance of radiosodium from an in- 

 jection site (130) probably depends on the rate of 

 blood flow through those vesse's which nourish the 

 tissues, and is probably little affected by the rate of 

 blood flow through, for example, arteriovenous 

 anastomoses. 



The capillary loops of the nail fold are among the 

 most easily visualized in the living body, and they 

 have been much observed ( 1 75). 



total cutaneous blood flow. This quantity has 

 not been measured with precision, but in a warm 

 subject it is a considerable fraction of the cardiac 

 output. Hardy & Soderstrom (112) by a study of deep 

 and superficial temperature and heat exchange ar- 

 rived at a blood flow through the skin of 278 ml per 

 m 2 of body surface per min in a nude subject at rest 

 at an environmental temperature of 35 C. Behnke & 

 Willmon (29) measuring helium absorption through 

 the skin under similar conditions arrived at a figure 

 of 230 ml per m 2 per min. During generalized maxi- 

 mum cutaneous vasodilatation the total blood flow- 

 is presumably very much greater. Assuming, for 

 example, a mean thickness of 1.2 mm, and a maxi- 

 mum flow of 180 ml per 100 ml skin per min, which 

 has been reported in digits and inferred in the fore- 

 arm, the flow would be 1200 ml per m' 2 per min. 

 Another estimate, based on skin conductance, is 

 2000 ml per m 2 per min (1 15). 



Color of the Skin 



The color of the skin due to tissue pigment is re- 

 vealed by expelling the blood by local pressure. The 

 additional color, due to circulating pigment, de- 

 pends on the quantity, quality, and distribution of 

 this pigment in the skin and subcutaneous vessels. 

 It is not dependent on the rate of blood flow (139). 

 Although it often happens that the skin contains more 

 blood when the flow is fast than when it is slow, the 

 amount of blood contained in the tissue and the rate 

 at which blood flows through the tissue by no means 

 run parallel to each other (54). Thus, the intensity 



of the color indicates the amount of pigment present, 

 and how near the surface are the vessels containing 

 it. The hue is determined by the proportions of the 

 various hemoglobin derivatives (oxy-, reduced, met-, 

 carboxy- etc.) present. 



Temperature of the Skin 



The temperature of the skin in air depends partly 

 on the rate of blood flow through it. It depends on 

 the temperature at which the arterial blood arrives; 

 that of the blood in the radial artery may be as low 

 as 21.5 C in a subject who is not feeling unduly 

 cold (25). It depends also on the rate of blood flow 

 through both distal and subjacent tissues, on the 

 activity of nearby muscle (101), on the rate of evap- 

 oration of sweat, on the temperature, humidity, 

 motion, and pressure of the surrounding air, and on 

 the exchange of radiant heat with the environment. 

 It is clear, therefore, that there can be no simple rela- 

 tionship between the temperature of the skin and the 

 rate of blood flow through it. The simplest relation- 

 ship between the two quantities is probably found in 

 the digits, examined in still air at a comfortable 

 temperature. If the circulation is arrested, the fingers 

 cool until their temperature settles near that of the 

 air. With the circulation fully opened up, the tem- 

 perature of the skin of the fingers comes to within 

 about 1 C and that of the toes to within about 3 C of 

 the temperature of the mouth. Between these ex- 

 tremes the relationship between flow and temperature 

 is by no means linear. For example, in a room at 

 22 C, the temperature of the fingers may rise to 34 C 

 when the blood flow is one quarter of the maximum, 

 and to 36 C when the maximum is attained (55). In 

 a room at 20.5 C, a skin temperature of 24 C corre- 

 sponded with a blood flow through the toes of 3 ml of 

 blood per 100 ml of toe per min; 29 C, with 10 ml, 

 and 32 C, with more than 30 ml. Even flows of 70 ml 

 do not cause the temperature to reach 34 C (75). 

 This does not mean that the higher ranges of blood 

 flow are always wastefully employed by the body, 

 for in colder air, or in moving air, the difference in 

 temperature to which the skin is raised by, and the 

 difference in heat dissipation at, one quarter of the 

 maximum flow and the maximum flow may be very 

 considerable. 



By far the greatest variations in skin temperature 

 are found in the extremities, particularly the hands 

 and feet in man, and the ears in the rabbit. 



The temperature at the surface of the skin in 

 thoroughly stirred water is essentially that of the 



