Observations on the Regulation of Arterial Blood Pressure in Unanesthetized Dogs

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Abstract
Aortic pressure rises as a dog's head is raised and falls as it is lowered. Systemic pressure change was 49%±3% of the hydrostatic pressure change at the carotid sinus, chosen as an arbitary reference, so that pressure at the carotid sinus changed only 51% as much as it would have if systemic pressure had not changed. The responses to head raising and lowering were dynamically asymmetrical, in a way which permits them to be described as being dependent on the rate of head lowering as well as on vertical position. When chloralose-anesthetized dogs maintained with positive-pressure breathing were studied under water, the response was abolished, suggesting its hydrostatic basis. Chronic carotid baroreceptor deafferentation or carotid sinus excision only diminished the response to changes in head position in most animals, and a transient response to head lowering persisted in all. An apparent CNS adaptation to the disruption of baroreceptor afferents makes it impossible to judge the quantitative role of the carotid sinus reflex in the relation of head position to blood pressure in intact dogs. The effect of head motion on blood pressure are predictable in a model of blood pressure control basedon the known dynamic relations between carotid sinus and systemic pressures, from which we tentatively conclude that vertical head motion acts on blood pressure through the carotid sinus reflex. If so, the measurement of vascular responses to head motion can serve as a nondestructive, quantitative test of baroreceptor reflex dynamics in intact, unanesthetized animals.