COMPARATIVE PULMONARY STRUCTURE AND FUNCTION IN TWO LARGE MAMMALS, THE HORSE AND THE COW

Abstract

<p>Pulmonary structure and function were compared in healthy adult horses and cows to examine the hypothesis that differences in pulmonary function between two species of similar size were related to differences in pulmonary structure. The functional residual capacity and tidal volume were higher in the horses than in the cows while the respiratory rate was higher in the cows. The pattern of air flow differed between the two species, and flow rates were higher in the cows. The dynamic compliance was higher in the horses while the work of breathing was higher in the cows. Separation of the lower pulmonary system (trachea to pleural cavity) from the total pulmonary system (nares to pleural cavity) indicated that most of the resistance and work of breathing were in the upper airways (nares to trachea), and there were no differences in the mechanical indices of pulmonary function in the lower pulmonary system between the two species. To more adequately characterize the lungs, horses and cows were anaesthetized and pressure-volume manoeuvres were performed to determine the lung capacities and quasistatic compliance. The lung capacities were significantly greater in the horses, but the lung compliance did not differ significantly between the two species. Quantitative and qualitative observations revealed few significant differences between the structure of the lung parenchyma in horses and cows. The obvious structural differences were in the size and shape of the lungs, the branching pattern of the airways, and the amount and distribution of the interlobular septa. During the measurements of pulmonary mechanics in the standing animals there were several anomalous observations and examination of the pressure recordings revealed phase lags between the total and lower pulmonary systems. It would appear that it is invalid to assume that inertia is a negligible factor in pulmonary function in large mammals such as the horse and the cow, due to the large abdomen in these species. The size and shape of the abdomen has probably been a significant factor in the evolution of lung structure in horses and cows, and abdominal movements are important in determining the breathing patterns in these two species.</p>