Morphometries of the avian lung: The structural-functional correlations in the design of the lungs of birds
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1. Birds present remarkably variable pulmonary morphometric characteristics which closely correspond with factors such as phylogeny, body size, mode of life and habitat. These factors determine the oxygen demands, energetics and exercise capacities of the various species. 2. 2. Genetics, domestication and longstanding intense selection for production traits such as weight gain and egg production appear to have played a role in the deterioration of the morphometric parameters of the galliform species. Captive non galliform birds, however, appear to retain pulmonary structural adaptations and hence the potential for efficient gas exchange probably for occasional instances which may require explosive energy production. 3. 3. The morphometric pulmonary parameters such as lung volume, volume of the pulmonary capillary blood and surface area of the blood-gas (tissue) barrier scale linearly with body weight while the harmonic mean thickness of the blood-gas (tissue) barrier does so very weakly. This suggests that barrier thickness has been optimized in birds and is thus probably the least adaptable parameter in the lung. The weight specific surface area of the blood-gas (tissue) barrier scales negatively with body weight indicating that the small birds which generally are more energetic and have a higher O2 consumption, have adaptively superior lungs than the larger ones. 4. Bats have better pulmonary morphometric parameters than birds. This may be a compensation for having retained their functionally inferior mammalian lungs. Refinement of the structural components of the bat lung does not appear to have sufficed in effecting the exchange of the large amounts of oxygen required for flight and has called for adaptation and subsequently co-option of extrapulmonary components such as the heart and blood into the respiratory processes. 5. In comparative pulmonary morphometry, regressional and statistical analysis enables the magnitudes of various parameters in different groups of animals to be assessed and wide ranging conclusions to be drawn. Pulmonary modelling which entails integration of pulmonary parameters to formulate factors which best define the global structural capacity of a gas exchanger has been attempted on respiratory organs of a number of animals. The major limiting factors which have been the course of the current methodological variations, have been the interpretation and correlation of measurements made on tissue preparations with the functional (in life) state and the lack of the essential morphophysiological data on pulmonary tissues of many animals. There is need to harmonize the various approaches.