Role of the veins in the carotid sinus reflex

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Abstract
Reflexly mediated changes in venous tone in the limbs of dogs caused by changes in activity of the carotid sinus baroreceptors were measured by following the changes in venous pressure in a hindlimb whose circulation had been arrested temporarily. The response of the resistance vessels was measured by studying the change in perfusion pressure when the hindlimb was perfused at constant flow or by noting the changes in aortic blood pressure. The results were contrasted with the effect on these vessels of electrical stimulation of the lumbar sympathetic chain. Decreases of 150 mm Hg in sinus pressure caused responses in the resistance vessels equivalent to sympathetic chain stimulation of 6-7 impulses/sec and an increase in venous tone equivalent to only 0.5 impulse/sec. The effect of these small changes of venous tone was studied by measuring venous return, cardiac output, and aortic blood pressure. The increase in venous return during maximal sinus hypotension was 3.5 ml/kg. Preventing this blood from reaching the heart and so keeping cardiac output constant did not affect the magnitude of the aortic blood pressure response.