[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Lung](https://durgical.com/en/general-pathology/areas/lung/) › Diffuse alveolar damage (ARDS)

# Diffuse alveolar damage (ARDS)

- Lesion

In ARDS the alveolar lining and capillaries are injured. Protein-rich fluid leaks, dead cells and fibrin form hyaline membranes, and the lung may scar.

Updated Nov 12, 2026

## Key points

- Hallmark: waxy pink hyaline membranes lining alveolar ducts and alveoli, made of fibrin-rich edema fluid and remnants of necrotic epithelium.
- Phases: exudative (first week: edema, membranes) then proliferative/organising (type II pneumocyte hyperplasia, interstitial fibroblasts).
- Neutrophils drive the injury: proteases, oxidants, cytokines and NETs.
- Four triggers cause more than half of cases: sepsis, diffuse pulmonary infection (viral including SARS-CoV-2, Pneumocystis), gastric aspiration and trauma.
- Same pattern as hyaline membrane disease of the preterm newborn, but a different cause.
- Acute interstitial pneumonia = DAD with no identifiable trigger.

On this page

- The basics Key points
- What it is
- Structure
- Step by step
- The phases of diffuse alveolar damage
- Markers
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Clinical notes

- Study Test yourself

- Connections Connections

## What it is

Acute lung injury is the sudden onset of hypoxaemia with bilateral edema that is not due to heart failure; ARDS is its severe clinical form. Its tissue pattern is diffuse alveolar damage (DAD): leaky capillaries, dead pneumocytes and hyaline membranes, followed by an organising phase. Sepsis, diffuse lung infection, gastric aspiration and severe trauma cause most cases.

## Structure

- Gross (acute): heavy, firm, red, boggy lungs; patchy ground-glass and consolidation on imaging.
- H&E, exudative phase: congested capillaries, interstitial and alveolar edema, fibrin, scattered neutrophils, and eosinophilic hyaline membranes on alveolar walls.
- H&E, organising phase: plump hyperplastic type II pneumocytes, loose fibroblastic tissue in the interstitium and airspaces.
- Late: interstitial fibrosis in a minority of survivors.

## Step by step

- A direct (aspiration, pneumonia) or indirect (sepsis, trauma, pancreatitis) insult injures pneumocytes and endothelium.
- Alveolar macrophages release TNF, IL-1 and chemokines; endothelium expresses adhesion molecules and procoagulants.
- Neutrophils stick, migrate into the alveoli and release proteases, oxidants and NETs, injuring more cells.
- Leaky capillaries flood the interstitium and alveoli with protein-rich fluid.
- Type II cells die, surfactant is lost; fibrin and cell debris settle as hyaline membranes.
- Patchy injury leaves perfused but unventilated areas: shunt and hypoxaemia that resists oxygen.
- If the trigger ends, macrophages clear debris and TGF-β drives repair; type II cells regrow the lining, or fibrosis follows.

## The phases of diffuse alveolar damage

| Phase | Time | What the pathologist sees | Consequence |
| --- | --- | --- | --- |
| Exudative | First days to a week | Edema, fibrin, neutrophils, hyaline membranes | Shunt, stiff lungs |
| Proliferative (organising) | From the second week | Type II pneumocyte hyperplasia, fibroblasts in septa and spaces | Slow improvement or progression |
| Fibrotic or resolved | Weeks to months | Interstitial fibrosis, or restored lining | Recovery or chronic restriction |

## Markers

- Hyaline membranes
- Type II pneumocyte hyperplasia

## Clinical notes

- Profound dyspnoea and tachypnoea, then hypoxaemia refractory to oxygen, diffuse bilateral infiltrates, stiff lungs needing high ventilator pressures.
- Mortality is high and rises with severity; deaths come mostly from sepsis and multiorgan failure.
- Most survivors recover function; some keep fibrosis. Risk is higher with alcohol use disorder and smoking.

## Memory aid

Leak, line, lay down: capillaries leak, membranes line the alveoli, fibroblasts lay down scar.

## Test yourself

Think of the answer, then tap the question to see it.

## What are hyaline membranes made of?

Fibrin-rich, protein-rich edema fluid mixed with the debris of necrotic alveolar epithelial cells.

## Which four conditions cause more than half of ARDS cases?

Sepsis, diffuse pulmonary infection, gastric aspiration and mechanical trauma including head injury.

## Why is the hypoxaemia of ARDS refractory to oxygen?

Many alveoli are flooded or collapsed but still perfused, so blood shunts past them; extra inspired oxygen cannot reach that blood.

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## Elsewhere in DurgiPedia

- [The alveolus: type I and II pneumocytes, surfactant, the blood–air barrier](https://durgical.com/en/histology/the-alveolus-type-i-and-ii-pneumocytes-surfactant-the-blood-air-barrier/)

## Connections

## Related 7

- [When leukocytes injure tissue: proteases, oxidants and NETs](https://durgical.com/en/general-pathology/when-leukocytes-injure-tissue-proteases-oxidants-and-nets/)
- [Neonatal respiratory distress syndrome: surfactant and hyaline membranes](https://durgical.com/en/general-pathology/neonatal-respiratory-distress-syndrome-surfactant-and-hyaline-membranes/)
- [Pulmonary edema: hemodynamic vs alveolar-capillary injury](https://durgical.com/en/general-pathology/pulmonary-edema-hemodynamic-vs-alveolar-capillary-injury/)
- [Viral pneumonia](https://durgical.com/en/general-pathology/viral-pneumonia/)
- [Nonspecific interstitial and cryptogenic organizing pneumonia](https://durgical.com/en/general-pathology/nonspecific-interstitial-and-cryptogenic-organizing-pneumonia/)
- [Shock: types, stages and the organs it damages](https://durgical.com/en/general-pathology/shock-types-stages-and-the-organs-it-damages/)
- [How septic shock develops](https://durgical.com/en/general-pathology/how-septic-shock-develops/)

## Related conditions 7

- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [COVID-19](https://durgical.com/en/conditions/covid-19/)
- [Influenza (flu)](https://durgical.com/en/conditions/influenza/)
- [Pulmonary contusion](https://durgical.com/en/conditions/pulmonary-contusion/)
- [Fat embolism syndrome](https://durgical.com/en/conditions/fat-embolism-syndrome/)
- [Aspiration pneumonia and pneumonitis](https://durgical.com/en/conditions/aspiration-pneumonia-and-pneumonitis/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Acute Respiratory Distress Syndrome](https://www.ncbi.nlm.nih.gov/books/NBK436002/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/diffuse-alveolar-damage/