Changes in alveolar capillary configuration at different levels of lung inflation in the rat. An ultrastructural and morphometric study.

Changes in alveolar capillary configuration during positive pressure lung inflation were investigated by transmission and scanning electron microscopy as well as by morphometric methods. As the airway pressure was raised, an increasing number of alveoli expanded. However, in fully inflated or overinflated lungs, a few collapsed areas persisted. In such areas, the air-blood tissue barrier was folded into the capillary space, dividing it into intercommunicating chambers. In expanded alveoli, the capillaries appeared as a continuous space delimited at each side by a tissue leaflet. This space was crossed by tissue bridges consisting of the thick portion of air-blood barrier. In overinflated lungs, the capillary space was compressed between the alveoli and thus collapsed. Our findings suggest that the thin porne. The variations of the capillary configuration seem to be determined either by "passive" deformations or by compression of the alveolocapillary membrane, or as a result of contraction of interstitial cells which are located in the thick portion of the air-blood barrier and are attached to the basement membranes of two adjacent alveoli. The relationship between alveolar dynamics and capillary configuration is considered to be a basic factor in ventilation/perfusion auto-regulation.