pulmonary interstitium

If the alveoli are the rooms where air meets blood, the interstitium is the scaffolding and plaster holding the building together. It is the thin layer of connective tissue, cells, and fibers that lies between the air sacs and the surrounding capillaries, giving the lung its springy structure without getting in the way of gas exchange.

Anatomically, the interstitium is a continuous network running from the central airways out to the surface lining the lung. The most delicate part sits within the alveolar walls, where it is squeezed almost to nothing so that oxygen and carbon dioxide can cross the alveolar–capillary membrane quickly. It also contains the collagen and elastin fibers responsible for the lung's elastic recoil.

Because this framework is normally so thin, even small amounts of fluid, inflammatory cells, or scar tissue within it can stiffen the lung and widen the distance gases must travel. This is exactly what happens in interstitial lung disease, which is named for this tissue compartment. Understanding the interstitium therefore helps explain why these diseases reduce both lung volumes and gas transfer.