Morphometrics of the avian lung. 3. The structural design of the passerine lung.

The lungs of 46 adult, wild passerine birds belonging to 8 species have been analysed morphometrically, both by light and electron microscope. Volumes were estimated by point counting, surface areas by intersection counting, and thicknesses by intercept length measurements. The mean values obtained for these passerine species appertaining to both lungs together were: volume of the lung per kilogram body weight 25 cm3/kg, volume density of the exchange tissue 52%, surface area of the blood-gas (tissue) barrier per gram body weight 47.48 cm2/g, surface density of the blood-gas (tissue) barrier 323.8 mm2/mm3, capillary loading 1.15 cm3/m2, harmonic mean thickness of the blood-gas (tissue) barrier 0.127 micron, arithmetic mean thickness 0.745 micron and the total morphometric pulmonary diffusion capacity 7.08 ml O2/min/mm Hg/kg. These values indicate that the passerine lung is specially well adapted for gas exchange, mainly by having a thin and extensive blood-gas (tissue) barrier, in response to the high oxygen demand by this group of bird.