Trade-offs in the evolution of the respiratory apparatus of chordates

Abstract

In the evolution of the respiratory apparatus repeated trade-offs between it and other organ systems are evident. Gills of non-vertebrate chordates have both a respiratory and alimentary function, whereby in amphioxus the gills filter incoming water, extract and transport suspended particles, and ventilate the atrial epithelium, which is the main respiratory surface. In craniates a muscular pump replaces the ciliary ventilator. The branchial pump of gnathostomes encloses the heart, resulting in the biomechanical expedience of coupling branchial and cardiac beat frequency. For aquatic vertebrates an air-filled air breathing organ means a trade-off between buoyancy and respiratory functions. In terrestrial tetrapods, multi-functionality of lungs and of respiratory musculature results in complex trade-offs and synergic combinations. The shift to aspiration breathing has resulted in a mechanical constraint in some lizards, due to the dual locomotor and ventilatory role of the hypaxial musculature. The most highly derived amniotes, mammals and birds, however, evolved along different pathways to high-performance aerobes. Whereas the kinetic bronchoalveolar mammalian lung results in a trade-off between large surface area and work of inflation, the avian system combines a large surface area and thin air-blood barrier of the constant volume lung with ease of inflation of the air sacs, resulting in energy-efficient gas exchange.