Static pressure-flow relation in the total systemic vascular bed of the dog and its modification by the baroreceptor reflex.

To evaluate arterial baroreceptor reflex control of total systemic vascular resistance, we studied the relation between mean arterial blood pressure (P) and mean aortic flow (Q) before and after the reflex was abrogated. In 12 dogs with a perfusion pump interposed at the vena-caval-right atrial junction, Q was kept at 60, 80, 100, 120, and 140 ml/min kg-1 for up to 20 minutes. There was little time-dependent change in P at any of these flows. When the reflex was intact, the mean P-Q curve was only slightly convex to the pressure axis; its linearized slope was 0.55 mm Hg-min-kg/ml and its pressure axis intercept was 44 mm Hg. After section of the sinovagal nerves, administration of hexamethonium (30 mg/kg, iv) and norepinephrine (3-5 mug/kg min-1, iv), or both, the slope became 0.96 mm Hg-min-kg/ml and the pressure axis intercept was about 3 mm Hg. Similar findings were obtained in 3 closed-chest dogs in which P was servocontrolled and Q was measured. Therefore, we think that the conventional practice of calculating total peripheral resistance as P/Q and evaluating reflex control of it in terms of the changes in that value involves only a small error in the moderately sub -and supranormal flow range.