Physiology for MRCEM Primary

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Respiratory physiology

Alveolar interfaces

[box type=”download”] — Alveolar surface tension as a contributor to lung compliance via Laplace’s law
— Surfactant as a tension-reducing phospholipid mixture secreted by type II pneumocytes
— Clinical consequences of reduced surfactant [/box]

Surface tension of the fluid lining the alveoli tends to collapse the alveoli.
This is a manifestation of Laplace’s law, which shows that the pressure in a bubble (or alveolus) is proportional to the surface tension (T) and radius (P
A small bubble will therefore have a higher pressure than a larger one
This high pressure can draw fluid into alveoli (transudation).
These pressures are minimized in the lung by surfactant, a mixture of phospholipids secreted by type II pneumocytes.
Surfactant floats on the alveolar fluid surface, and reduces surface tension. As alveoli shrink and the fluid surface area is reduced, surfactant is thus concentrated, further reducing surface tension, so alveolar pressure actually falls (P
Surfactant thus prevents alveolar collapse and helps
maintain similar alveolar sizes, and reduces lung stiffness and transudation.
Premature babies with insufficient surfactant develop neonatal respiratory distress syndrome, with stiff lungs, lung collapse and transudation.