133° HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



which may contribute in varying proportion accord- 

 ing to the circumstances. 



HYPEREMIA AFTER PROLONGED INSUFFICIENCY OF THE 



circulation. A hyperemia, with the blood flow 

 several times the normal level and lasting for some 

 weeks, is seen in the feet of some patients after the 

 relief of chronic arterial obstruction by an arterial 

 graft (86). The mechanism of this hyperemia and its 

 relationship, if any, to reactive hyperemia is not 

 yet known. 



Responses oj Skin I 'essels to Changes 

 in Transmural Pressure 



Measurements of the circulation through the finger 

 have led Burton (43) and co-workers to conclude that 

 in the resistance vessels there is an unstable equi- 

 librium between the tension in the wall and the 

 transmural pressure. If the transmural pressure falls 

 below the ''critical closing pressure,"' the value of 

 which depends on the state of activity of sympathetic 

 vasomotor nerves, the vessels close completely and 

 arrest the flow of blood. This behavior of the vessels 

 has been independently confirmed in the finger tip 

 by Roddie & Shepherd (167). 



Calorimetric measurements on the hand (53) and 

 toes (52) indicate that when the transmural pressure 

 is progressively increased beyond the normal value 

 (as by local exposure to subatmospheric pressure) the 

 resistance vessels at first are passively dilated. At 

 somewhat higher pressures they react by active con- 

 traction of their walls and may become narrower 

 than normal; this is a form of autoregulation of the 

 skin circulation, the purpose of which may be to 

 assist the antigravity defenses of the body rather than 

 to maintain constancy of the skin blood flow. 



Effect of Local Temperature on the Skin Circulation 



The effects of local temperature on the skin circula- 

 tion are of great importance, because the skin is 

 normally exposed to a greater range of temperatures 

 than any other part of the body except perhaps the 

 upper end of the alimentary canal. In the latter, 

 exposure to extremes of temperature is brief, but in 

 the skin it may be prolonged. 



A great many observations have shown that the 

 circulation through the skin is greatly influenced by 

 local temperature. The exposure of any part of the 

 body to a change of temperature probably causes 

 some alteration to the circulation in all other parts, 



40 



30 



20 



IO 



A WARM 



• COMFORTABLE 



O COLD 



15 25 35 



LOCAL TEMPERATURE C 



45 



fig. 2. The blood How through the hand measured by venous 

 occlusion plethysmography, in warm, comfortable, and cold 

 subjects, and with the hand immersed in water at various local 

 temperatures. [Data from: /) Abramson el at. (4), 2) Catchpole 

 & Jepson (47), 3) Killian & Oclassen (132), 4) Kunkel & Stead 

 (136), 5) Kunkel el at. (137), 6") Peacock (159), 7) Roddie & 

 Shepherd (166), 8) Speaman (180).] 



partly by nervous reflexes and partly by alteration in 

 temperature of the blood returning from the part to 

 the heat-regulating center. However, the effects now 

 to be described are predominantly local ones. When, 

 for example, the temperature of the water around 

 one finger or hand is altered, the changes in the 

 circulation through it are very much greater than 

 those simultaneously observed in the opposite member 

 immersed in water at a constant temperature (55, 

 166). 



LOCAL TEMPERATURES IN THE RANGE 1 3 C TO 45 C. 



Figure 2 summarizes some representative observa- 

 tions on the effect of immersion in water at tempera- 

 tures in the range 15 C to 45 C on the rate of blood 

 flow through the hand. Between the observations 

 there are differences of age, sex, and number of 

 subjects, of present and previous environmental 

 temperature, in the length of exposure to the local 

 temperature, and in the details of the venous oc- 

 clusion plethysmography technique. In general, 

 however, it may be said that the blood flow through 

 the hand is at its lowest value at about 15 C, when it 

 may be as little as 0.3 ml per 100 ml of hand per min 

 in a cold subject, and 0.9 ml in a warm one. From 



