 Alveolar-capillary barrier

The alveolar-capillary barrier is the thin tissue interface separating alveolar gas from blood in pulmonary capillaries. Gas transfer crosses the alveolar epithelium, intervening basement membrane material and the capillary endothelium. This arrangement places a short tissue diffusion path between a large ventilated surface and circulating blood; it does not mean that air and blood occupy the same compartment.^[ref]

Electron microscopy made the layers experimentally distinguishable. In a 1956 study, H. E. Karrer examined three electron micrographs of mouse lung and reported a basement membrane of variable apparent width between endothelium and alveolar epithelium. The paper explicitly recognized oblique sectioning as a possible cause of some apparent thickening.^[ref] A micrograph establishes spatial structure in a prepared specimen; apparent width in one section is not automatically the shortest physiological diffusion distance.

A planar steady-state diffusion model illustrates the functional consequence. Let layer $j$ have thickness ${\delta }_j$, diffusion coefficient $D_j$ and gas solubility coefficient $S_j$, with concentration $S_{j}p$ at gas partial pressure $p$. Equal molar flux through successive layers and Fick's first law givewhere $A$ is exchange area. These are diffusion resistances of serial alveolar barrier layers: the same flux crosses each layer, while its partial-pressure drops add.

In an original normalized example, suppose one layer contributes one quarter of the initial total diffusion resistance. Quadrupling only its thickness makes total resistance $3/4+4/4=7/4$ of its original value, reducing transfer to $4/7$ at the same partial pressure difference and area. If exchange area also halves, total transfer becomes $2/7$ of the original. The layer carrying most of the added resistance can matter more than a simple average thickness.

The model assumes steady transport, uniform parallel layers and specified gas solubilities. Actual gas exchange also depends on ventilation, blood delivery and reactions within blood. A structural image alone does not measure all of those processes or establish the cause of a measured gas-transfer deficit.