'34- 1 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 12. The right arm, with the 

 circulation off, was immersed in hot 

 water at 42.8-44.3 C at the 38th min. 

 There was no dilatation in the left 

 second (Z.2 ) or left fourth (Z.4) finger 

 until the circulation was released at the 

 76th min. [From Gibbon & Landis 

 (89)-] 



18 



16 



10 



CIRCULATION OFF 

 RT ARM IN BATH 428- 443 C 



30 



50 



MINUTES 



70 



90 



heating the trunk or both legs with radiant heat 

 caused a substantial (2-fold or 3-fold) increase in blood 

 flow through the hand, with a latency of only 10 to 

 15 sec; this was too short a time for a mechanism 

 depending on the return of hot blood to the heat- 

 regulating center. Furthermore, on heating the legs 

 the response was unaffected when cuffs around the 

 thighs were inflated to 200 mm Hg to arrest the circu- 

 lation (fig. 13). In this case the vasodilatation there- 

 fore appears to depend on afferent information 

 conveyed by nerves from the heated skin. The response 

 to heating the legs disappears after bilateral lumbar 

 sympathectomy, and in persons with unilateral 

 sympathectomy it is obtained on heating the normal 

 but not the sympathectomized leg (57). It is not yet 

 certain whether the afferent nerves traverse the 

 sympathetic ganglia, or whether sympathectomy 

 modifies the response by altering the conditions at 

 somatic nerve thermoreceptors. Stimulating the 

 intact lumbar sympathetic chain or its cut central 

 end causes vasoconstriction in the hand, but it is not 

 known whether the stimulated afferent fibers are 

 from the skin or from the viscera (58). 



As mentioned earlier, the effector side of the tem- 

 perature regulation reflex is mediated in the hand by 

 adjusting vasoconstrictor activity, and in the forearm 



mainly by adjusting vasodilator activity. The reduc- 

 tion in vasoconstrictor activity is not simultaneous 

 in all areas; the individual fingers often dilate asyn- 

 chronously, and the foot often dilates many minutes 

 after and less completely than the hand (162). The 

 vessels in warm skin dilate sooner than those in corre- 

 sponding areas of cold skin. 



Exposure of part of the body to cold causes changes 

 in the circulation in other areas by two mechanisms. 

 There is a rapid transient reflex vasoconstriction, due 

 to stimulation of afferent nerves (160), and a longer 

 lasting vasoconstriction due to cooled blood returning 

 to the heat-regulating center (160, 181). The cooling 

 effect on the temperature of the body core of exposure 

 of limbs to cold is usually restricted by local vasocon- 

 striction. This limits the quantity of cooled blood 

 returning to the core. Furthermore the efficient ar- 

 rangements for exchange of heat between arteries and 

 veins in the limbs (25, 26) reduce the cooling effect 

 of the blood. On the other hand, the temperature of 

 the blood that does return may, at the start of its 

 journey, be 30 C or more below that of the core, 

 while from a heated region the blood can hardly 

 start its return at a temperature more than 7 to 8° 

 above that of the core. Further, cold may sometimes 

 be sufficiently severe to cause cold vasodilatation, 



