Alveoli are the tiny, thin-walled air sacs where oxygen passes into the blood and carbon dioxide passes out. An adult has several hundred million of them, giving a total surface area of roughly 70 square metres. Each is wrapped in a dense mesh of capillaries, with a barrier less than one micrometre thick between air and blood.
| Location | Throughout both lungs, at the ends of the respiratory bronchioles, alveolar ducts and alveolar sacs. |
|---|---|
| Parts |
|
| Blood Supply | Pulmonary capillaries (from pulmonary arteries) |
| Venous drainage |
Exchange oxygen and carbon dioxide between air and blood by diffusion. Surfactant from type II cells lowers surface tension so the alveoli do not collapse when breathing out.
During exercise more alveolar capillaries open, increasing the area for gas exchange. Diseases that thicken the barrier (fibrosis) or fill the alveoli (pneumonia, fluid) reduce oxygen transfer, often first noticed as breathlessness on exertion. Premature babies lack enough surfactant and are given it directly into the lungs.
Infection that fills alveoli with inflammatory fluid and cells. It causes fever, cough, breathlessness and low oxygen levels.
Widespread injury to the alveolar wall with leaky capillaries and fluid-filled alveoli. It causes severe low oxygen and usually needs intensive care.
Surfactant deficiency in premature infants causing alveoli to collapse. It is treated with surfactant and breathing support.
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