Surface tension arises wherever an air–water interface exists, because cohesive attraction between water molecules at the surface pulls inward and resists expansion of that interface; in the lung, the fluid lining each alveolus creates such an interface and therefore an inward collapsing force that must be overcome to inflate the air sac. Type II pneumocytes, which cover only a small fraction of the alveolar surface, secrete surfactant — a predominantly phospholipid molecule with both polar and non-polar regions — that intercalates between surface water molecules and lowers surface tension in proportion to its surface density. This matters because the law of Laplace makes the pressure required to inflate a sphere depend on its radius, so smaller alveoli would otherwise carry greater surface tension and resist inflation relative to larger ones; surfactant neutralizes that size dependence so alveoli of differing radii inflate comparably. The concept belongs to pulmonary and respiratory physiology within human physiology, and together with compliance it constitutes the elastic component of the work of breathing.
Surface tension arises wherever an air–water interface exists, because cohesive attraction between water molecules at the surface pulls inward and resists expansion of that interface; in the lung, th…