The pulmonary circuit receives the whole of the cardiac output, and the flow through it is governed by events in the systemic circulation. Its function is gaseous exchange, and the low resistance and high capacitance of the circuit is subservient to this function. Local flow is affected by intra-alveolar oxygen tension, and regional flow by passive hydrostatic effects due to gravity. The distribution of ventilation is also affected by gravity owing to the effect of the latter on regional compliance. When the cardiac output falls, as it does in most methods of induced hypotension, there is an accompanying increase in physiological dead space owing to an increase in the alveolar component.