Pleural fluid: formation and drainage !
- Mechanism
- Exam priority: medium
A tiny layer of fluid between lung and chest wall lets them glide. It is made and drained all the time; if the balance fails, an effusion builds up.
Updated 2026-11-01
Also called: Pleural effusion: transudate and exudate, Light's criteria
See it in 3D: Pleura · Thoracic duct
What it is
The pleural space holds only a few milliliters of fluid, which lets the lung slide and stay coupled to the chest wall. Fluid filters mainly from systemic capillaries of the parietal pleura and drains through parietal pleural lymphatics, whose flow can rise greatly. An effusion appears when filtration increases or drainage fails; telling transudate from exudate points to the cause.
How it works, step by step
- Parietal pleural capillaries, at systemic pressure, filter fluid into the pleural space.
- Parietal pleural lymphatics absorb that fluid through stomata and carry it to mediastinal nodes.
- Lymphatic drainage can increase many-fold, so a moderate rise in filtration does not cause an effusion.
- If venous pressure rises (heart failure) or oncotic pressure falls, more filters than drains: transudate.
- If the pleura is inflamed or a tumor blocks the lymphatics, protein and cells leak and drainage fails: exudate.
- Accumulated fluid separates lung from wall, the lung collapses beneath it (compressive atelectasis) and dyspnea appears.
Starling forces along a capillary
Arteriolar NFP +19 mmHgVenular NFP +1 mmHgFiltration 30.0Reabsorption 0.0Net filtration 30.0 a.u. (lymph capacity 7.5)
Inflammation: histamine and other mediators raise permeability (↑ Kf), proteins leak out (↑ πif) and vasodilation raises Pc: local edema.
Edema: net filtration (30.0 a.u.) is more than the lymph can return (7.5 a.u.), so fluid collects in the interstitium.
Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.
- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema
Transudate and exudate
| Feature | Transudate | Exudate |
|---|---|---|
| Mechanism | Altered hydrostatic or oncotic pressure | Inflammation, infection, tumor, lymphatic block |
| Pleural/serum protein | ≤0.5 | >0.5 |
| Pleural/serum LDH | ≤0.6 | >0.6 |
| Examples | Heart failure, cirrhosis | Pneumonia, tuberculosis, lung cancer |
Clinical notes
- Exam: dullness, reduced breath sounds and absent fremitus at the base; compressed lung above may give bronchial breathing.
- Hemothorax: blood in the pleura after trauma; pneumothorax: air that abolishes the negative pressure and collapses the lung.
- A complicated parapneumonic effusion (low pH, low glucose) suggests empyema and usually needs drainage.
Test yourself
Think of the answer, then tap the question to see it.
Why does heart failure give a transudate and tuberculosis an exudate?
In heart failure the pleura is healthy and only venous pressure rises, filtering water; in tuberculosis the inflamed pleura lets protein and cells through.
What keeps the lung against the chest wall if they are not attached?
Negative pleural pressure, created because the lung tends to recoil inward and the wall outward, plus the thin fluid layer that couples them.
Why can a tumor invading mediastinal nodes cause a pleural effusion?
It blocks parietal pleural lymph drainage: normally filtered fluid can no longer leave and accumulates as an exudate.
What happens to pleural pressure in a pneumothorax and why does the lung collapse?
Air entry brings pleural pressure up to atmospheric; without negative pressure, the lung's elastic recoil collapses it.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Starling forces at the capillary
- Mechanics of breathing: pressures and muscles
- How the lymphatic system works
- Physiology of pulmonary edema
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 22.2 The Lungs (OpenStax (Rice University), 2022); Anatomy, Thorax, Pleurae (StatPearls (NCBI Bookshelf), 2026); Pleural Effusion (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 39). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.