Distribution of alveolar wall per unit volume in the human lung.

Firstly, to assess the reproducibility of a computerised image analysis method of measuring alveolar size in terms of the alveolar wall surface area per unit volume (AWUV) in normal non-smoking adult human lungs fixed in inflation, one lung was fixed by simple manual prebronchial perfusion with formaldehyde (wet-fix), another by prolonged perfusion with formaldehyde followed by air until semi-dried (semi-dry). Blocks from both lungs were taken to assess the relative value of plastic and paraffin wax embedding. There was no statistical difference in AWUV between plastic and paraffin sections from each lung, but the wet-fix method gave statistically significantly (P < 0.05) higher AWUV readings than the semi-dry method. Secondly, when a whole semi-dry lung slice was divided into 66 blocks, the AWUV distribution was not random but largest in the centre and smallest in the periphery anteriorly and superiorly. This is obviously important in sampling of lung tissue for morphometry and other purposes.