Stability of alveoloi is maintained by:

Reduce surface tension by surfactant Ganong writes -"The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. Surfactant is a mixture of dipahnitoylphosphaticlylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. In spherical structures like the alveoli, the distending pressure equals two times the tension divided by the radius (P = 2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure." "Pulmonary surfactant therefore helps stabilize the sizes of the alveoli and helps keep them open and available to paicipate in gas exchange." -Human Physiology: From Cells to Systems By Lauralee Sherwood p477 There are two factors which prevent the alveolar collapse: Reduction of surface tension due to surfactant The surface tension of the alveolar lining fluid varies with surface area. This is because surfactant reduces surface tension in propoion to its surface concentration. Surfactant is insoluble in water and floats on the surface of the alveolar lining fluid. In larger alveoli the surfactant is more spread out (dilute) and the surface tension is higher. As the alveoli reduce in size during expiration the concentration of the surfactant increases and the more it reduces the surface tension. 2. Alveolar interdependence There is interaction between adjacent groups of alveoli. Therefore, collapsing alveoli pull on adjacent alveoli preventing fuher collapse. This is termed alveolar interdependence.