1364 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



mmHg. 



FIG. 10. Results obtained with the 

 preparation shown in fig. 9. Sympathec- 

 tomy abolished the action of the carotid 

 sinuses upon the skin and muscle of the 

 hind limb. [After Folkow et al. (101).] 

 Note that an increase in saphenal blood 

 flow records downward. 



DONOR 



RECIPIENT 



SAPHENAL 

 BLOOD FLOW 



CAVAL OUTFLOW 



SIGNAL 



TIME 6OSEC 



1.5: OCCLUSION COMMON CAROTIDS 



2.4.6.8: STIMULATION RIGHT SINUS NERVE 

 3.7.9: INTRA-ARTERIAL INJECTION 0-2>ig ACh 



muscle. Folkow and others (54) recorded the effect 

 of stimulation of the carotid sinus nerve in five pa- 

 tients during block dissections of the neck performed 

 for the treatment of cancer. Stimulation at 40 per 

 sec elicited maximal effects. Mean blood pressure 

 and pulse amplitude fell promptly, there was a slight 

 increase in forearm flow, implying considerable 

 vasodilatation, which was probably of nervous origin. 

 Nevertheless it is unlikely that arterial pressure 

 changes in the carotid sinuses in man have much 

 effect on the sympathetic tone in human muscle 

 vessels for the following reasons. Stretching the ca- 

 rotid sinuses by applying subatmospheric pressure to 

 the outside of the neck causes bradycardia and fall 

 in arterial pressure — signs of stimulation of the 

 baroreceptors — but vascular resistance in the fore- 

 arm is unaltered (81). Compression of the carotid 

 arteries, followed by fall of the carotid arterial pres- 

 sure to 20 mm Hg causes tachycardia, hyperpnea, 

 and rise in brachial arterial pressure — signs of de- 

 creased baroreceptor activity — but forearm vascular 

 resistance is unaltered (163). Although strong stimula- 

 tion of the carotid sinus nerve causes reflex vaso- 

 dilatation in human muscle quite large changes in 

 the transmural pressure in the carotid sinuses do not 

 seem to have any effect on the vascular resistance 

 in human muscle. 



EFFECT OF RECEPTORS IN A LOW PRESSURE AREA IN 

 THE CARDIO-PULMONARY SYSTEM ON THE SYMPATHETIC 

 VASOCONSTRICTOR TONE IN HUMAN SKELETAL MUSCLE. 



The evidence for this important reflex is as follows 

 (39, 164, 167, 168). Raising the legs of a recumbent 

 subject increases the forearm blood flow. It does not 

 have this effect in the sympathectomized forearm. 

 The dilatation is reflex. Raising the legs after the cir- 

 culation in them has been arrested has no effect 

 upon forearm blood flow. The reflex is elicited by a 

 shift of blood from the legs into the trunk. Raising 

 the legs has scarcely any effect on arterial blood 

 pressure We have already seen that in man neither 

 stretching the carotid sinuses (81) nor reducing the 

 blood pressure in them (163) affects the tone of blood 

 vessels of the forearm — so it seems very unlikely 

 that their discharge frequency would be affected 

 by the very small change in arterial pressure which 

 follows raising the legs. On the other hand, raising 

 the legs increases the central venous pressure. It 

 seems then reasonable that rise in pressure on the 

 venous side stretches structures in the cardiopul- 

 monary system and so stimulates low pressure re- 

 ceptors which reflexly increase forearm blood flow. 

 This conclusion is supported by the finding that blood 

 flow in the normally innervated forearm increases 

 when the thoracic vessels are stretched by negative 

 pressure breathing (39). When the thoracic contents 

 are repetitively stretched by rapid alternating positive 

 and negative intrathoracic pressure changes forearm 

 blood flow is trebled or quadrupled (163). 



The low pressure receptors act reflexly by altering 

 sympathetic vasoconstrictor tone in the muscles. 

 This has been deduced from the following observa- 



