An analysis of the reflex systemic vasodilator response elicited by lung inflation in the dog.

1. A maintained inflation of the lungs caused a reflex reduction in total systemic vascular resistance in anaesthetized dogs under conditions in which the systemic circulation was perfused at constant blood flow and the arterial blood P(O2) and P(CO2) were maintained constant.2. The fall in systemic arterial perfusion pressure evoked by inflation of the lungs was accompanied by an increase in blood flow to the lower limbs and a reduction in their calculated vascular resistance. Since the fall in resistance occurred when the limb was perfused either at constant pressure or at constant blood flow, it must be due to vasodilatation.3. Lung inflation caused vasodilatation in skin, muscle, and in the splanchnic vascular bed. The responses in vertebral circulation were, however, small and variable.4. The vasodilator responses in the vascular territories studied were reflex in nature, being abolished by cutting the cervical vagosympathetic nerves, in which run the afferent fibres, or by interrupting the sympathetic pathways to the blood vessels.5. In the intact limb, muscle, skin and splanchnic vascular bed, the vasodilator responses to lung inflation were unaffected by atropine or propranolol, but were abolished by hexamethonium, dibenyline and bretylium tosylate, indicating that they were due predominantly to a reduction in the activity in sympathetic adrenergic vasoconstrictor fibres.