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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



spinal cord above the level of the sympathetic outflow 

 to the hands (91), and in these cases the response 

 appears to be a purely spinal reflex. At least some of 

 the afferent fibers must enter the cord below the 

 second thoracic roots. The receptors and the precise 

 nature of the effective stimulus have not been identi- 

 fied. The reflex may be responsible for the reduction 

 in the blood flow through the hands in hyperventila- 

 tion (2), but its functional significance is unknown. 



RESPONSE TO DISTENTION OF THE BLADDER. Distention 



of the bladder causes a constriction of the blood 

 vessels of the skin, and elsewhere, by a spinal reflex. 

 The response was first described by Guttman & 

 Whitteridge (111) in patients with complete trans- 

 verse section of the spinal cord and in whom the 

 isolated cord was undamaged. With sections above 

 the level of the second lumbar outflow there was 

 constriction in the skin of the feet and legs; with high 

 section there was also constriction in the hands. The 

 response may be sufficient to raise the arterial pres- 

 sure, and to lead to consequent reflex adjustments in 

 that part of the circulation innervated by the brain 

 and upper part of the spinal cord. It is important 

 when making observations on the skin circulation 

 to start with the subject's bladder empty, and to re- 

 empty it before it becomes uncomfortably full. 



hypoglycemia. The blood flow is increased through 

 the hand and the forearm in insulin hypoglycemia 

 (3). The increase in the forearm is partly in the skin, 

 and mediated by an active vasodilator mechanism 

 (11), probably associated with sweating. Injection of 

 an adequate dose of atropine into the brachial artery 

 reduces the blood flow through the forearm, but does 

 not affect that through the hand (13). 



posture. Changes in body posture cause complex 

 changes and reactions in the circulation, but if the 

 inclination to the horizontal of the observed limb re- 

 mains unchanged the net effect is that the circulation 

 through the hands (27) and fingers and toes (131 ) is 

 little changed. By contrast, as a single arm or leg is 

 raised above the horizontal, the posture of all other 

 parts remaining unchanged, the rate of blood flow 

 through the digits is progressively diminished (174, 

 191) and in all dependent positions the blood flow is 

 slightly increased (174). The digital pulse volume is 

 greater in the raised limb, and less in the dependent 

 limb, than it is in a horizontal limb (96). This is a strik- 

 ing example of the way in which, under some circum- 

 stances, blood flow and pulse volume may even 



change in opposite directions, although in other 

 circumstances their changes may correspond closelv 

 (42). 



RESPONSES TO BARORECEPTOR STIMULATION. Present 



evidence suggests that the blood vessels of the skin 

 are largely, and perhaps entirely, excused from par- 

 ticipation in baroreceptor reflexes. 



Low pressure baroreceptor s. Unidentified low pressure 

 baroreceptors within the thorax can be stimulated 

 by raising the legs of a recumbent subject and allow- 

 ing part of the blood they contain to flow into the 

 central venous pool, or by breathing through a narrow 

 tube which restricts air flow and causes intrathoracic 

 pressure transients of +30 to — 20 mm Hg to be set 

 up. Such stimulation causes a reflex dilatation, 

 brought about by reduction in vasoconstrictor nerve 

 activity, in the blood vessels of the voluntary muscle 

 of the forearm, but no change in the resistance to 

 flow through the hand or through the skin of the 

 forearm (173). 



Systemic arterial baroreceptors. The decisive animal 

 experiments, in which observations have been made 

 on a perfused isolated innervated limb, while the 

 baroreceptors have been stimulated in various ways 

 (sec 119), have dealt either with whole limbs or 

 skinned limbs. There do not appear to have been 

 anv decisive experiments dealing with the skin as 

 such. In the human, in experiments in which bilateral 

 arterial compression caused increases in heart rate 

 and arterial blood pressure (and was thereby shown 

 to affect the baroreceptors), the vascular resistance 

 through the hand remained unchanged (168). 



ACTION OF HUMORAL AGENTS ON THE 

 BLOOD VESSELS OF THE SKIN 



The action of drugs on the skin has been recently 

 reviewed by Herxheimer (118). Only substances of 

 ph\ siological importance will be considered here. 

 Their direct local action is best tested by a steady 

 intraarterial infusion. The dose is adjusted to the 

 volume of tissue to which it will be distributed, and 

 it is usually so small that any returning to the general 

 circulation causes a negligible disturbance of arterial 

 pressure and of the blood vessels elsewhere. By measur- 

 ing the blood flow in the contralateral limb as well 

 .is in the infused limb general disturbances of the 

 circulation can be detected and can be allowed for 

 since these normally affect the limbs symmetrically. 



The effect of humoral agents released into the 



