= Alveolar-capillary barrier
The \b[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.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 22.1, Alveoli and the respiratory membrane; Section 22.4, Gas Exchange]{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.<The Ultrastructure of Mouse Lung: Some Remarks Regarding the Fine Structure of the Alveolar Basement Membrane by H. E. Karrer>[Primary abstract, Journal of Biophysical and Biochemical Cytology 2(4 Suppl), 287–292]{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 $S_jp$ at gas <partial pressure> $p$. Equal molar flux through successive layers and <Fick's first law> give
$$
J=\frac{p_A-p_B}{\displaystyle\sum_j\delta_j/(D_jS_j)},\qquad
\dot n=AJ,
$$
where $A$ 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.
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