1. Hypoventilation of one lobe of lung was studied in open-chest anaesthetized dogs. Lobar blood flow, pulmonary-artery pressure and gas exchange were measured, the latter from breath-by-breath analysis with a mass spectrometer.

2. Hypoventilation of the lobe by reducing the respiratory pump stroke led, at each step, to a reduction in blood flow to that lobe. The flow () reduction was variable, but always less than the ventilation () reduction, so that the ratio to the lobe was reduced. O2 tension and pH fell and CO2 tension rose in effluent blood. Thus regulation achieved by flow reduction varied between individuals and was of low gain.

3. Anatomical or series dead space (VD series) was reduced in proportion to ventilation. When VD series was less than the apparatus dead space, some gas exchange still took place.

4. Oxygen uptake () and CO2 output () were reduced during hypoventilation. fell more than , so that the respiratory exchange ratio (R) was reduced.

5. Whether the deterioration in gas tensions in effluent blood during hypoventilation of the lobe was due to shunt of blood past unventilated alveoli, or to mismatching, was not resolved.

6. The plateau phase of the CO2-output curves at low tidal volumes was usually regular; thus either hypoventilation was uniform, or some ventilation units were totally closed.

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