Form and Function of Lungs: The Evolution of Air Breathing Mechanisms

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Abstract
SYNOPSIS. Structural evolution of the vertebrate lung illustrates the principle that the emergence of seemingly new structures such as the mammalian lung is due to intensification of one of the functions of the original piscine lung. The configuration of the mechanical support of the lung in which elastic and collagen fibers form a continuous framework is well matched with the functional demands. The design of the mammalian gas exchange cells is an ingenious solution to meet the functional demands of optimizing maintenance pathways from nucleus to the cytoplasm while simultaneously providing minimal barrier thickness. Surfactant is found in the most primitive lungs providing a protective continuous film of fluid over the delicate epithelium. As the lung became profusely partitioned, surfactant became a functionally new surface-tension reduction device to prevent the collapse of the super-thin foam-like respiratory surface. Experimental analyses have established that in lower vertebrates lungs are ventilated with a buccal pulse pump, which is driven by identical sets of muscles acting in identical patterns in fishes and frogs. In the aquatic habitats suction is the dominant mode of feeding generating buccal pressure changes far exceeding those recorded during air ventilation. From the perspective of air ventilation the buccal pulse pump is overdesigned. However in terrestrial habitats vertebrates must operate with higher metabolic demands and the lung became subdivided into long narrow airways and progressively smaller air spaces, rendering the pulse pump inefficient. With the placement of the lungs inside a pump, the aspiration pump was established. In mammals, the muscular diaphragm represents a key evolutionary innovation since it led to an energetically most efficient aspiration pump. Apparently the potential energy created by contraction of the diaphragm during inhalation is stored in the elastic tissues of the thoracic unit and lung. This energy is released when lung and thorax recoil to bring about exhalation. It is further determined experimentally that respiratory and locomotory patterns are coupled, further maximizing the efficiency of mammalian respiration. Symmorphosis is exhibited in the avian breathing apparatus, which is endowed with a key evolutionary innovation by having the highly specialized lung continuously ventilated by multiple air sacs that function as bellows. Functional morphologists directly deal with these kinds of functional and structural complexities that provide an enormous potential upon simple changes in underlying mechanisms.

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