Describe the structure of surfactant (25% of marks). Explain the effects of surfactant upon surface tension and lung mechanics (75% of marks).
The answer required a description of surfactant composition (phospholipids 85%, neutral
lipids 5%, and proteins 10%). Phospholipid dipalmitoylphosphatidylcholine is the main
surface active component. It was expected candidates would provide a description of the
arrangement of the phospholipids with the hydrophilic head in the aqueous phase and the
hydrophobic tail in the airspace of the alveolus. The effects of surfactant required an
explanation of surface tension and how this affects alveoli. One good way to explain this was describing how La Place’s law would affect alveoli with, and without,surfactant. As the
alveoli decrease in size, the surfactant molecules are pushed together and exert a greater
surface tension lowering effect.
Surfactant is also important in the lung elastic recoil and hysteresis and for alveolar stability
preventing collapse and thereby improving lung compliance and decreasing work of
breathing. Surfactant also helps oppose the starling forces in the lung and keep fluid from
being drawn into the alveoli. Candidates often misunderstood La Place’s law and did not
explain how surfactant decreases surface tension.