SPECIAL COMMUNICATIONS t Theory of sheet flow in lung alveoli

lary blood vessels in the pulmonary alveoli are so short and SO closely knit that a new term-“sheet flow”-is desirable to avoid the usual notion of a blood vessel as a tube. From the viewpoint of fluid mechanics the new terminology is particu-larly pertinent. In this article we consider the flow pattern of the blood in such a sheet. A theoretical approach as well as a large-scale model study has been undertaken to determine the streamlines, the velocity distribution, and the pressure gradient in the pulmonary alveolar septa. The role of the elasticity of the system is considered. It is shown that in the range of linear elasticity, the fourth power of the thickness of the sheet satisfies the Laplace equation. The thickness distribution as a function of pulmonary arterial pressure, venous pressure, and alveolar air pressure is illustrated by several examples. pulmonary physiology; blood flow; capillary blood vessels