The alveolus: thin on purpose
An alveolus is a microscopic air sac, and you have hundreds of millions of them. Each is wrapped in a mesh of capillaries so tight that the air and the blood are separated by an astonishingly thin barrier — the alveolar–capillary membrane, thinner than a wisp of tissue paper. Gas exchange happens here purely by diffusion: oxygen drifts from the higher concentration in air toward blood, and carbon dioxide drifts the other way. No pumping, no work — just molecules moving down a gradient across a barrier kept deliberately thin.
Surfactant: the anti-collapse coating
A wet bubble wants to collapse — surface tension pulls its walls inward. Millions of tiny wet alveoli would do the same, so the lung coats them with surfactant, a soapy film made by type II pneumocytes that lowers surface tension and keeps the sacs open between breaths. Surfactant is also why a newborn's first cries matter so much, and why premature babies, who haven't made enough yet, can struggle to breathe. It is one of the most elegant tricks in the whole body.
Lobes and segments: the lung's address system
Zoom back out and the lungs are neatly partitioned. The right lung has three lobes (upper, middle, lower); the left has only two, because the heart takes up room on that side. Each lobe is further carved into bronchopulmonary segments — about ten per lung — each fed by its own segmental bronchus and artery. Because a segment is a self-contained unit, a surgeon can sometimes remove just one diseased segment and leave the rest of the lung working.
- Right lung: upper, middle, and lower lobes (three).
- Left lung: upper and lower lobes (two), with a tongue-shaped lingula on the upper.
- Each lobe divides into bronchopulmonary segments, each with its own airway and artery.
- This address system lets doctors say exactly where a nodule or pneumonia sits.