A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the alveolar type I epithelial cells (or pneumocytes).
<div style="display:inline-block;padding:0 3px;background-color:#BDC"><a href="//commons.wikimedia.org/wiki/File:Alveolar_wall.jpg" title="File:Alveolar wall.jpg"><b>Original: </b></a></div> <div style="display:inline-block;padding:3px 0;"><a href="//commons.wikimedia.org/wiki/User:Cruithne9" title="User:Cruithne9">Cruithne9</a></div> <div style="display:inline-block;padding:0 3px;background-color:#CCF">Vector: </div> <div style="display:inline-block;"><a href="//commons.wikimedia.org/wiki/User:Pixelsquid" class="mw-redirect" title="User:Pixelsquid">Pixelsquid</a></div>🎱
CC BY-SA 4.0
Own work based on: <a href="//commons.wikimedia.org/wiki/File:Alveolar_wall.jpg" title="File:Alveolar wall.jpg">Alveolar wall.jpg</a>: <a href="//commons.wikimedia.org/wiki/File:Alveolar_wall.jpg" class="image"><img alt="Alveolar wall.jpg" src="https://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/Alveolar_wall.jpg/48px-Alveolar_wall.jpg" decoding="async" width="48" height="38" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/Alveolar_wall.jpg/72px-Alveolar_wall.jpg 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/Alveolar_wall.jpg/96px-Alveolar_wall.jpg 2x" data-file-width="1000" data-file-height="800"></a>