Mechanisms of gas exchange in bird lungs

The avian embryo exchanges the oxygen and carbon dioxide with the ambient air by diffusion. The respiratory organ is the chorioallantois, endowed with a rich circulation. Between ambient air and chorioallantoic capillary blood are interposed the porous shell fibrous shell membranes, and the chorioendothelium which compose the diffusion barrier. The air cell is formed between the two shell membranes in the blunt end of the egg. The diffusion barrier is divided into an outer barrier (shell plus outer membrane) and an inner barrier (inner membrane plus chorioendothelium and capillary blood). The resistance to gas diffusion (the reciprocal of the diffusive conductance) in the outer barrier is almost fixed throughout incubation while that in the inner barrier decreases as the embryo develops. Because of the fixed outer barrier conductance, the embryo is obliged to take up oxygen under hypoxic conditions against increasing metabolism with development and encounters a relative respiratory acidosis. In connection with the diffusive hypoventilation caused by the fixed outer barrier conductance the respiratory factors of the allantoic circulation change progressively with development to moderate the restraint of gas exchange through the shell. Blood oxygen capacity and hemoglobin increase with development in association with an increase in erythrocyte count and hematocrit value. In addition, a progressive leftward shift of the oxygen dissociation curve occurs. The increases in the allantoic blood flow and chorioallantoic capillary volume contribute to the increasing conductance of the inner barrier. Furthermore regulation of acid base balance is inferred in the developing embryo.

G. Hiller

Papers

Oxygen affinity of chicken blood before and after hatching.