Exhalation Mechanics and the Evolution of Aspiration Breathing in Tetrapods

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Brainerd 9419892 The mechanism by which complex biological systems with many interacting parts evolve has engaged biologists since Charles Darwin. The goal of this research is to determine how one such complex system, the respiratory pump used for breathing, has changed during vertebrate evolution. Reptiles, birds, and mammals ventilate their lungs with an aspiration pump in which ribs, and in some cases a diaphragm muscle, expand the chest cavity and suck air into the lungs. Air-breathing fish lack the rib and diaphragm structures that are necessary for aspiration breathing. They take air into the mouth cavity and compress it, forcing air into the lungs. Mouth pumps use only muscles of the head region while aspiration pumps use only muscles of the chest and abdominal regions. The transition between the two is a significant change in a complex system. Some amphibians, however, appear to be intermediate, using head muscles for inhalation and body muscles for exhalation. This research will determine whether the use of body muscles for exhalation is common to most species of amphibians, and thus is a probable evolutionary step between mouth pump and aspiration breathing. Pressure transducers, muscle activity transducers, air flow transducers, and x-ray video will be used to determine ventilation patterns in representatives of all three orders of extant amphibians. The results will test the hypothesis that use of body muscles for exhalation is an ancestral character of the amphibians, and hence a probable intermediate stage in the evolution of the aspiration pump. In the long term, this research will aid in identifying the sequence of behavioral, morphological and neural changes that resulted in a reorganization of a complex physiological system for function in a new environment.