English: Diagram of the cross-current respiratory gas exchanger in the lungs of birds, Air is forced through the parabronchus in only one direction. Pulmonary capillaries surround the parabronchus in the manner shown, bringing de-oxygenated blood (from below in the diagram), and oxygenated (or partially oxygenated) blood drains leaves towards the top in the diagram.The blood from all of the pulmonary capillaries mixes before being distributed round the body.