A comprehensive account of the functional morphology of the vertebrate respiratory organs must delve into how different organs and organ-systems have evolved, developed, been refined, and integrated for the purpose of gas exchange. Oxygen, a product of photosynthesis by plants and cyanobacteria, has been the most singularly pervasive molecular factor in setting the form, patterning, and geographic distribution of extant animal life. In biology, the evolution of sound designs has been attained at enormous cost. Respiration, the process in which molecular oxygen is appropriated from an external fluid medium and channeled to the tissue cells for energy production, incorporates a complex, highly integrated arsenal of structural components and functional processes. Respiratory organs afford the first interface between oxygen in the ambient environment and metabolic machinery of the body. In respiratory organs, the geometric association of the structural components determines the movements and presentations of the gas-exchange fluid media within and across the organs.