Role of the venous system in baroreceptor-mediated reflexes in man

Although baroreceptor stimulation produced by marked alterations in arterial pressure has been shown to produce reflex changes in venous tone in animals, the effects on venous tone in man of altering arterial pressure within the physiologic range have not been clear. In six subjects, venous tone did not change when mean arterial pressure was raised by 25-40 mm Hg, although heart rate fell reflexly by 40%. Venous tone remained constant in 10 subjects when arterial pressure was lowered. This contrasted to the sustained rise in forearm vascular resistance and the persistent tachycardia that occurred. However, 12 subjects continued to respond to these interventions by transient venoconstriction. To eliminate possible emotional influences on venous tone due to the experimental intervention, venous responses were studied before and during general anesthesia in five of these subjects. In contrast to the response before anesthesia, an equivalent fall in arterial pressure during anesthesia no longer evoked a venoconstrictor response. Venous reactivity and the baroreceptor reflex arc remained intact during anesthesia, since venous tone always rose after a deep inspiration, and tachycardia always accompanied the fall in arterial pressure. It is concluded that changes in arterial pressure in the physiologic range in man do not induce measurable reflex alterations in venous tone, and that the increases sometimes seen with decreases in arterial pressure appear to be due to extraneous psychic factors.