Respiration Physiology (1987) 69, 81-100 81 Elsevier RSP 01303 Adaptive variation in the mammalian respiratory system in relation to energetic demand" VI. The pulmonary gas exchanger Ewald R. WeibeP, Loretta B. Marques l, Minerva Constantinopol 1, Fabienne Doffey 1, Peter Gehr 1 and C. Richard Taylor 2 ~Department of Anatomy, University of Berne, Berne, Switzerland and ZMuseum of Comparative Zoology, Harvard University, Cambridge, MA 02138, U.S.A. (Accepted for publication 7 March 1987) Abstract. This paper explores the question whether the morphometric diffusing capacity of athletic dogs and ponies is larger than that of goats and calves, in proportion to the 2.5-fold higher limit for 02 consumption in the athletic species. Comparing pony and calf we find the lung volumes, the gas exchange surfaces, and the capillary volume to be about 1.5-fold larger in the pony, whereas red cell size and the barrier thicknesses are about the same. Lung and capillary volumes, as well as the gas exchange surfaces do not differ significantly between dog and goat, whereas red cell size, capillary hematocrit and plasma barrier thicknesses differ greatly. The membrane diffusing capacity calculated from morphometric data is about 1.25 times higher in the athletic species. The 02 conductance of the blood is higher in the athletic species because of their higher hemoglobin content, its value also depends on the assumptions made with respect to the reaction kinetics of the blood (Oo2). The results are only consistent if the initial reaction rate ofO2 with erythrocytes is assumed to be independent of red cell size. In this case we find that DLo2 of the athletic species is 1.5 times larger than that of the less aerobic species, and that the pressure head for 02 diffusion is also 1.5 times larger. The 2.5-fold greater 0 2 transfer capacity of the lung of athletic animals is therefore achieved by increasing by equal parts the structurally determined diffusing capacity and the driving force for O 2 diffusion. Calves; Cattle; Diffusing capacity; Dogs; Goats; Horses; Lung structure; Morphometry; Ponies In the second paper of this series (Taylor et aL, 1987) it was demonstrated that 0 2 consumption can be increased by a factor of 12-29 fold above rest, as an animal augments its energy requirements with increasing exercise intensity, and that this increase is limited: when "~/o2max has been reached a further increase in energy require- ments must be covered anaerobically through the gtycolytic pathway, resulting in lactate accumulation. It was also shown that the limit of oxidative metabolism is different for different species: in 'athletic' species, such as dog and pony, Vo2max is 2.5 times higher Correspondence address." Ewald R. Weibel M.D., Department of Anatomy, University of Berne, Biihlstrasse 36, CH-3000 Berne, Switzerland. 0034-5687/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)