[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/) › Pleural effusion and pneumothorax

# Pleural effusion and pneumothorax

- Lesion

Fluid, pus, blood, chyle or air can collect between the pleural layers. Each one has its own causes and compresses the lung.

Updated Nov 12, 2026

## Key points

- Serous or serofibrinous pleuritis: pneumonia, TB, infarction, abscess, lupus, rheumatoid arthritis, uremia, radiation.
- Empyema: loculated yellow-green pus, usually from adjacent pneumonia; organises into thick adhesions that trap the lung.
- Hemorrhagic pleuritis: tumour, bleeding disorders, rickettsiae; search for tumour cells. Hemothorax: frank blood, usually trauma or ruptured aorta.
- Chylothorax: milky lymph from thoracic duct injury or obstruction by tumour.
- Hydrothorax (transudate): heart failure (most common), cirrhosis, nephrotic syndrome.
- Spontaneous pneumothorax: young, tall men, rupture of apical subpleural blebs; tension pneumothorax shifts the mediastinum and is an emergency.

## What it is

The pleural space normally holds a thin film of fluid. Effusions are inflammatory (exudates: pleuritis, empyema, hemorrhagic pleuritis) or noninflammatory (hydrothorax transudates, hemothorax, chylothorax). Air in the space is pneumothorax, spontaneous or traumatic. Most pleural disease is secondary to disease of the lung or elsewhere; malignant effusions are common.

## Structure

- Fibrinous pleuritis: shaggy fibrin on the pleura (friction rub), later organised into fibrous adhesions.
- Empyema: creamy pus with neutrophils, walled off by granulation tissue and fibrosis.
- Malignant effusion: tumour cells in cytology of the fluid (lung, breast, ovary, lymphoma most often).
- Spontaneous pneumothorax: small thin-walled apical blebs with pleural inflammation and eosinophils at the rupture site.

## Step by step

- Transudates form when hydrostatic pressure rises or oncotic pressure falls with intact pleura.
- Exudates form when inflammation or tumour makes pleural capillaries leaky or blocks lymphatic drainage.
- Bacteria reaching the space convert a parapneumonic effusion into empyema.
- Air enters through a ruptured bleb or a chest wound and the lung recoils away from the chest wall.
- If a flap valve lets air in but not out, pressure rises with each breath: tension pneumothorax compresses the heart and great veins.
- Any large collection compresses the lung (compression atelectasis).

## What is in the pleural space

| Collection | Content | Main causes |
| --- | --- | --- |
| [Hydrothorax](https://durgical.com/en/conditions/pleural-effusion/) | Transudate | Heart failure, cirrhosis, nephrotic syndrome |
| Serofibrinous pleuritis | Exudate with fibrin | Pneumonia, TB, infarct, lupus, RA |
| [Empyema](https://durgical.com/en/conditions/parapneumonic-effusion-and-empyema/) | Pus | Spread from bacterial pneumonia or abscess |
| [Hemothorax](https://durgical.com/en/conditions/haemothorax/) | Blood | Trauma, ruptured aortic aneurysm |
| Chylothorax | Milky lymph | Thoracic duct trauma, tumour |
| [Pneumothorax](https://durgical.com/en/conditions/pneumothorax/) | Air | Ruptured blebs, emphysema, trauma |

## Clinical notes

- Pleuritic pain and dyspnoea; dullness for fluid, hyperresonance and absent breath sounds for air.
- Light's criteria on the fluid separate transudate from exudate.
- Tension pneumothorax: hypotension, tracheal deviation away from the affected side; needs immediate decompression.

## Memory aid

Water, wine-red, white milk, pus or puff: name what fills the pleura and the cause follows.

## Test yourself

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

## What usually causes spontaneous pneumothorax in a young adult?

Rupture of small apical subpleural blebs, often in tall young men.

## What is chylothorax and what causes it?

Milky lymph in the pleural space from thoracic duct injury or obstruction of lymphatics, often by tumour.

## Why is tension pneumothorax life-threatening?

A flap-valve leak lets air in but not out; rising pressure collapses the lung and shifts the mediastinum, impairing venous return to the heart.

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## Elsewhere in DurgiPedia

- [Pleural fluid: formation and drainage](https://durgical.com/en/physiology/pleural-fluid-formation-and-drainage/)
- [Serous membranes: pleura and peritoneum](https://durgical.com/en/histology/serous-membranes-pleura-and-peritoneum/)

## Connections

## Related 5

- [Edema and effusions: the four mechanisms](https://durgical.com/en/general-pathology/edema-and-effusions-the-four-mechanisms/)
- [Atelectasis: resorption, compression and contraction](https://durgical.com/en/general-pathology/atelectasis-resorption-compression-and-contraction/)
- [Emphysema](https://durgical.com/en/general-pathology/emphysema/)
- [Pleural tumours: mesothelioma and solitary fibrous tumour](https://durgical.com/en/general-pathology/pleural-tumours-mesothelioma-and-solitary-fibrous-tumour/)
- [Bacterial pneumonia: lobar vs bronchopneumonia](https://durgical.com/en/general-pathology/bacterial-pneumonia-lobar-vs-bronchopneumonia/)

## Related conditions 4

- [Pleural effusion](https://durgical.com/en/conditions/pleural-effusion/)
- [Parapneumonic effusion and empyema](https://durgical.com/en/conditions/parapneumonic-effusion-and-empyema/)
- [Haemothorax](https://durgical.com/en/conditions/haemothorax/)
- [Pneumothorax](https://durgical.com/en/conditions/pneumothorax/)

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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: [Pleural Disorders](https://medlineplus.gov/pleuraldisorders.html) (MedlinePlus, US National Library of Medicine, 2025); [Collapsed Lung](https://medlineplus.gov/collapsedlung.html) (MedlinePlus, US National Library of Medicine, 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/pleural-effusion-and-pneumothorax/