Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
Something—infection, breathing in stomach contents, major injury, bad blood transfusion, or other insult—damages the paper-thin wall between air sacs and blood vessels, so protein-rich fluid floods the air sacs and oxygen cannot cross into the blood.
A Flooded, Collapsed Sponge
Imagine a kitchen sponge (the alveolus) designed to soak up water from one side (air) and release it to the other side (blood). Normally, the sponge's walls are coated with soap (surfactant) that keeps it fluffy and open. Now imagine a fire (the insult: infection, aspiration, trauma) damages the sponge's walls, turning them from a tight weave into a mesh full of holes. Water (protein-rich fluid) floods in from the blood side, soaking the entire sponge and washing away the soap coating. The sponge collapses into a wet, dense clump (atelectasis). When you try to squeeze fresh water through it (oxygen), almost none reaches the other side because the sponge is waterlogged and flat—the water just gets trapped inside (intrapulmonary shunt). You squeeze harder and harder (higher oxygen, faster breathing) but the collapsed, flooded sponge cannot absorb anymore. Your arm gets tired (respiratory muscle fatigue), and eventually you cannot squeeze at all (respiratory failure).
ARDS Definition Task Force (Berlin Definition), JAMA 2012 · American Thoracic Society / European Society of Intensive Care Medicine / Society of Critical Care Medicine Clinical Practice Guideline: Mechanical Ventilation in ARDS, 2017 · Surviving Sepsis Campaign International Guidelines 2021 · National Heart, Lung, and Blood Institute ARDS Clinical Network (ARDSNet) · reviewed 2026-07-07