As noted earlier in this chapter, the primary function of the lungs is to facilitate gas exchange between the atmosphere and the blood. Toward this end, the capillary system in the lungs is very different than that found elsewhere. Conforming to the alveolar geometry (Fig. 10.3), capillary blood flow in the lungs is better described as a sheet flow rather than a tube flow; that is, the blood flows within the thin planar walls of the alveoli, which appear as parallel membranes separated by hexagonally positioned posts. Fung and his colleagues sought to quantify the pressure–flow relation in this sheet flow and began with a nondimensionalization. Here, let us perform a similar procedure and compare to that reported by Fung (1993).