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Abstract

The lungs of the domestic fowl were prepared for scanning electron microscopy after vascular and airway latex rubber casting to demonstrate the spatial organization of the various structural components that are involved in the gas exchange that takes place in the parabronchial tissue mantle. The bulk of the intrapulmonary air flows through the parabronchial lumen and then centrifugally diffuses into the exchange tissue through the atria, the infundibula, and the air capillaries. The blood flows centripetally from the interparabronchial arteries, then into the intraparabronchial arterioles, and finally into the blood capillaries, which together with the air capillaries constitute the functional terminal gas exchange units. The relationship between the air flow in the parabronchial lumen and the incoming blood (into the exchange tissue) has been shown to be crosscurrent, where the directions of the flow of these two gas exchange media are essentially perpondicularly disposed to each other; whereas the relationship between the blood capillaries and the air capillaries is countercurrent, the blood flowing towards the parabronchial lumen and the air in the opposite direction, i.e., towards its periphery. Both these spatial structural relationships between the air and blood are significant factors that contribute to the remarkable efficiency of the avian lung in gas exchange. The crosscurrent system enhances the duration of interaction between the parabronchial air and the blood, the oxygen being extracted from a parabronchus along its whole length, whereas the overall contribution of the countercurrent relationship, which is superimposed on the crosscurrent system, is not expected to be very high, mainly because of the tortuosity and profuse interlacing of the blood and air capillaries over a relatively short distance, that of approximately the thickness of the parabronchial tissue mantle.