Surface tension, surfactant and Laplace's law !
- Concept
- Exam priority: medium
A thin film of fluid lines the air sacs and pulls them shut. Surfactant is a natural detergent that weakens that pull and keeps them open.
Updated 2026-11-01
Also called: Pulmonary surfactant, Law of Laplace
See it in 3D: Right lower lobe
What it is
At the alveolar air–liquid interface, water molecules attract each other and create surface tension, which pulls alveoli toward collapse and accounts for much of the lung's elastic recoil. Type II pneumocytes secrete surfactant (mainly dipalmitoylphosphatidylcholine plus proteins SP-A to SP-D), which lowers that tension, stabilizes alveoli of different sizes and helps keep them dry.
Key facts
- Removing the air–liquid interface (filling the lung with saline) lets it inflate with far less pressure: surface tension explains much of elastic recoil.
- Surfactant also reduces the suction of fluid from capillary into alveolus, so it protects against edema.
- An amniotic lecithin/sphingomyelin ratio ≥2 was used as a sign of lung maturity.
Equations
- P
- pressure tending to collapse the alveolus
- T
- surface tension
- r
- alveolar radius
Smaller radius means higher collapsing pressure, unless tension falls in proportion, which is what surfactant does.
Surface tension (order of magnitude)
| Parameter | Value | Note |
|---|---|---|
| Pure water at 37 °C | ≈70 dynes/cm | |
| Alveolar fluid with surfactant | ≈5–30 dynes/cm | lowest at end-expiration |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Preterm infant: expiratory grunting (closing the glottis to hold pressure), retractions and cyanosis in the first hours.
- Adult ARDS: protein-rich fluid inactivates surfactant; ventilation with PEEP holds unstable alveoli open.
- Postoperative atelectasis: shallow breaths without sighs let alveoli with little surfactant close.
Test yourself
Think of the answer, then tap the question to see it.
Without surfactant, why does a small alveolus empty into a larger one?
By Laplace, with equal tension the collapsing pressure is higher at the small radius; air flows from higher to lower pressure, into the large alveolus.
Why does surfactant deficiency lower lung compliance?
High surface tension increases elastic recoil, so more transpulmonary pressure is needed for the same volume and alveoli tend to collapse.
Why is the infant of a mother with poorly controlled diabetes at higher risk of neonatal respiratory distress?
Fetal hyperinsulinemia slows type II pneumocyte maturation and delays surfactant synthesis.
How does surfactant help keep alveoli dry?
By lowering surface tension it reduces the negative pressure in the alveolar fluid lining that would otherwise pull water from the capillaries into the air space.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Lung and chest wall compliance
- Mechanics of breathing: pressures and muscles
- The work of breathing
- Physiology of pulmonary edema
- Newborn physiology and fetal maturation
Related conditions
In Biochemistry
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: 22.3 The Process of Breathing (OpenStax (Rice University), 2022); 22.7 Embryonic Development of the Respiratory System (OpenStax (Rice University), 2022); Acute Respiratory Distress Syndrome (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). 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.