A hydrostatic Michell framework supports frog lungs

A braced framework of tubular struts, in the walls and air spaces of frog l ungs, suspends the respiratory surface and holds the lung open at zero tran smural pressure withstanding imploding forces created by abdominal viscera, much as would the supports of a bell tent. The struts are tubes, having a larger second moment of area than do solid struts of the same cross-section al area, and so are stronger, and contain pulmonary vessels within a flexib le wall. The orthogonal arrangement of the struts in the framework, explain ed in part by Maxwell's Lemma and Michell's Theorem, strengthens the framew ork and minimizes its weight; orthogonality is maintained as the lungs chan ge size. A model is presented, in which a frog might control pre- and post- pulmonary vascular resistances and, hence, blood volume in the struts, with out compromising pulmonary perfusion. Such adjustments could vary the area of lung and the extent of perfused capillaries exposed to pulmonary gas, he lping match the lung's surface area, weight and metabolic load to activity. (C) 1999 Society for Mathematical Biology.