alveolus

An alveolus is one of the lung’s hundreds of millions of microscopic air sacs — the tiny bubbles where breathing actually pays off. Picture a cluster of soap-bubble-thin pouches at the very end of the airways, each wrapped in a fine mesh of blood vessels.

Across the wafer-thin barrier between the air inside and the blood outside, oxygen diffuses into the bloodstream and carbon dioxide passes the other way to be breathed out. The walls are kept moist and coated with surfactant, a soap-like film that stops these delicate sacs from collapsing. Pooled together, the alveoli of both lungs offer a gas-exchange surface roughly the size of a tennis court.

When alveoli fill with fluid (pneumonia, pulmonary edema), are destroyed (emphysema), or stiffen (fibrosis), gas exchange falls and breathlessness follows. Their health is the ultimate measure of whether the lungs can do their job.