Morphometric estimation of pulmonary diffusion capacity in two horse lungs

The lungs of two half-bred geldings were fixed in situ by instillation of a glutaraldehyde solution into the airways during deep anaesthesia. The body weight of both animals was 510 kg and their average lung volume about 38 l. Stratified random samples from 22 regions were morphometrically analysed from electron microscopic films. Eighty-six per cent of the total lung volume was gas exchange parenchyma. It contained 26.9 l air space and 5.6 l interalveolar septa which were composed of equal parts of capillaries and tissue. The average alveolar and capillary surface areas were 2457 m2 and 1663 m2, respectively. The arithmetic mean thickness of the air-blood tissue barrier was estimated at 1.37 micron; the harmonic mean thicknesses were 0.60 micron for the tissue barrier and 0.21 micron for the plasma barrier. From these morphometric data, total pulmonary diffusion capacity for oxygen was calculated. We obtained a maximal value of 3.55 l/min x mm Hg and a minimal value of 1.75 l/min x mm Hg. The above values are averages for the whole lung. However, there are considerable regional differences between the apical and diaphragmatic lobes and almost none between dorsal and ventral areas. The volume density of alveoli was 9% larger in the apical than in the diaphragmatic lobes. Conversely, the volume density of the capillaries was 54%, and the surface density of alveoli 16% smaller in the apical than in the diaphragmatic lobe.