Function of the pleura

The pleura, the covering around the lung, does two things, and both of them matter. The first is to make pleural fluid. The second is to create cavities that keep the large organs of the chest apart.

That fluid is released by mesothelial cells, the cells that line both layers of the pleura, and its job is lubrication: the surfaces glide over each other instead of scraping. But it has a surprising property. On top of being slippery, it is adhesive. Put a drop of water between two panes of glass and you will see that they slide easily, yet pulling them apart is very hard. That is exactly what happens in the chest: when the chest wall expands as you breathe in, the lung does not lag behind but expands with the wall and fills with air. And the second job, keeping things separate, has an advantage of its own: it keeps one organ's movement from getting in the way of its neighbor's, and it slows the spread of an infection from one compartment to the next.