For memory the dependent lung is the one where the blood is more likely to be, i.e. the one where gravity sends the most blood. So in sitting this is the bases, in lying this is the posteior aspects of the lobes, when standing on the head it would be the apical segments. The relevance of this relates to ventilation/perfusion. In the normal subject ventilation tends to go the the dependent lung, i think due to less resistance of the lung tissue with relatively less blood flow. This may not be the case in the person with pulmonary disease? It has been a while since I was involved directly in cardiopulmonary physio so please correct me if i am wrong.

The concept of dependent and non-dependent should be considered when performing postural drainage. Read below for my additional notes on breathing at low lung volumes and the ventilated patient. 8o