CIRCULATION THROUGH THE SKIN 



1329 



water. This follows because stirred water can convey 

 heat to or from the surface at a rate which is very 

 great indeed compared with the rate at which it can 

 be conveyed to or from the surface by even the most 

 profuse flow of blood through the tissues. If an in- 

 sulating layer is formed, by allowing the water to 

 stagnate, or by covering the skin with fabric, the 

 skin becomes a point on the temperature gradient 

 from the body core to the water. It assumes a tempera- 

 ture which depends on the ratio of the thermal 

 insulation between the body core and the skin, and 

 between the skin and the water. The thermal insula- 

 tion between the body core and the skin is highly- 

 dependent on the state of the circulation. The circu- 

 lation of the blood is the main means of transfer of 

 heat between the body core and the periphery. The 

 thermal conductivity of the skin is also highly de- 

 pendent on the rate of blood flow through it (44). 



RESPONSES OF SKIN BLOOD VESSELS TO 

 PHYSICAL DISTURBANCES 



Response of the Circulation Following 

 Periods of Arrest or Insufficiency 



reactive hyperemia. The circulation through the 

 skin is very frequently arrested by local pressure; it is, 

 for example, arrested in the sole of the foot while 

 standing and in the parts of the hand supporting a 

 heavy object. The skin is better able than most other 

 tissues to survive fairly prolonged arrest of the circu- 

 lation without permanent damage. It shows, con- 

 spicuously, the phenomenon of reactive hyperemia, 

 by which is meant the bright red flushing (51) and 

 increase in blood flow above the resting level (139) 

 when the circulation is released following obstruction. 

 This is a local change, and clearly depends upon a 

 local dilatation of the blood vessels responsible for 

 resistance to flow. The size and duration of the reac- 

 tive hyperemia are related to the duration of previous 

 arrest. Although some observations have indicated 

 that the extra blood flowing during the period of 

 hyperemia is closely similar to the amount that would 

 normally have flowed during the period of arrest 

 [debt and repayment hypothesis (142)] the corre- 

 spondence is by no means always exact (83), the debt 

 being frequently underpaid (157). Indeed, it is 

 possible in the forearm, by gradually releasing the 

 main vessel, to restore the circulation without any 

 repayment of debt (35), the blood flow never exceed- 

 ing the resting level. 



Reactive hyperemia is most readily demonstrated 

 when a limb is warm; it was found by Lewis & Grant 

 (142) to be much reduced in a cooled part. Thus 

 following arrest of the circulation for 5 min, Catch- 

 pole & Jepson (47) found average peak flows of 

 3.05 ml per 100 ml per min while the hand was 

 immersed in water at 15 C, 6.8 ml at 20 C, g.8 ml at 

 25 C, and 19.8 ml at 30 C. 



Bier (33, 34) demonstrated in 1897 that reactive 

 hyperemia is independent of nervous connections 

 with the central nervous system. While amputating 

 limbs he first divided the nerves and flesh, leaving 

 the main artery and vein intact. Occlusion of the 

 artery was followed, on release, by the usual hypere- 

 mia. Lewis & Grant (141) observed that skin which 

 had long been anesthetized, as a result of old standing 

 lesions of the main nerves, flushes uniformly with the 

 adjacent skin still possessing normal innervation. In a 

 chronically denervated and wasted forearm, the peak 

 blood flow during reactive hyperemia was found by 

 Eichna & Wilkins (73) to be 26 per cent greater, in 

 relation to the volume of the part, than in the nor- 

 mally innervated arm. Similar observations on four 

 other cases have been made by Duff & Shepherd 

 (70). The height and the duration of the reactive 

 hyperemia were found by Freeman (83) to be similar 

 in the normal and the chronically sympathectomized 

 hand. The reaction seems, therefore, to be inde- 

 pendent of all nervous elements which degenerate 

 following section of peripheral, somatic, and auto- 

 nomic nerves. 



The commonly observed rough correspondence of 

 debt and repayment has suggested that a chemical 

 substance may accumulate during circulatory arrest, 

 and act as a vasodilator. Histamine has been found 

 in the venous blood following arrest of the circulation 

 (14), but in the forearm antihistamine substances do 

 not influence the hyperemia following brief arrest, 

 though they somewhat reduce that following more 

 prolonged arrest of the circulation (6g). 



There is some evidence that the lowered pressure 

 in the resistance vessels during circulatory arrest 

 may lead to a relaxation of their muscular tissue, 

 perhaps by a local mechanism. Thus Wood et al. 

 (194) and Patterson (158) have found that reactive 

 hyperemia in the forearm is reduced if the blood 

 vessels are packed with blood, thus maintaining a 

 high transmural pressure during the period of arrest 

 of the circulation. 



Present evidence suggests that reactive hyperemia 

 depends on local chemical and physical changes, 



