How does pulmonary surfactant prevent alveoli from collapsing?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

Pulmonary surfactant is a complex mixture of proteins and lipids secreted by type II pneumocytes (also known as granular pneumocytes) located within the alveolar walls.12

This substance functions by spreading as a thin film across the internal surface of the alveoli, where it acts to lower alveolar surface tension. By reducing this tension, surfactant prevents the tiny air sacs from collapsing during exhalation. Without this protective coating, the surface tension would be high enough to cause the alveoli to collapse, requiring significant force to re-expand them during subsequent breaths.123