This is a study of several reflex interactions between the cardiovascular and respiratory systems that will be carried out in anesthetized dogs. My aims are: 1) to describe, and to discriminate among, the reflex and chemically-mediated effects of acid aspiration on the systemic and pulmonary circulations and on airways, 2) to display the influence of baroreflexes from systemic and pulmonary arteries and from individual cardiac chambers on breathing, and 3) to appraise the effects on systemic vascular resistance of large negative pleural pressure excursions during labored breathing that may initiate cardiac, pulmonary and aortic stretch reflexes. An important feature of this study is the isolation of the primary physiologic event of interest from secondary events, so that a single control system can be described in terms of a physiologic input (e.g. chamber transmural pressure) and a physiological consequence (e.g. respiratory activity of a muscle). Isolation will be achieved by selective cardiopulmonary bypass perfusion, servocontrol of pressures and anatomical isolation of cardiovascular compartments. Injection of acid into an isolated lung lobe, deformations of cardiopulmonary compartments with step, impulse and ramp waveforms, and enhanced respiratory variations of pleural pressure (dissociated from lung volume change) will be used as stimuli related to the three aims. Results will be expressed in terms of systemic and pulmonary vascular resistance, airway resistance, lung compliance, heart rate, and timing and drive characteristics of respiratory muscles, as appropriate to the aims.