COMPARATIVE ASPECTS OF THE STRENGTH OF PULMONARY CAPILLARIES IN RABBIT, DOG, AND HORSE

In previous studies of rabbit and dog lung, we demonstrated stress fai lure of pulmonary capillaries at high transmural pressures (P-tm). The P-tm necessary to elicit stress failure was 40 cmH(2)O higher in dog than rabbit, and the total blood-gas barrier (BGB) thickness was great er in dog than rabbit. This suggests that stress failure may be relate d to BGB thickness, and is consistent with the Laplace relationship wh ich states that wall stress is proportional to capillary radius but in versely proportional to wall thickness. In the present studies, we com pared BGB thickness and an index of capillary radius in lungs from 3 r abbits, 3 dogs, and 2 horses perfusion fixed at a P-tm of similar to 3 0 cmH(2)O. Thicknesses of the BGB were measured at right angles to the barrier at random points on the capillary wall determined by test lin e intersections. Capillary radius was determined from the mean of majo r and minor axes measured on electron micrographs. Capillary pressure for failure in the horse was taken to be the mean of pulmonary arteria l and left atrial pressures observed in galloping thoroughbreds known to develop exercise-induced pulmonary hemorrhage, although the actual pressure required for failure may be less than this. Average capillary radii were 3.6, 3.4, and 3.2 mu m for rabbits, dogs, and horses, resp ectively. We found that the BGB was thinnest in the rabbit, intermedia te in the dog, and thickest in the horse. Calculated capillary wall st ress values for the median total BGB thickness at a nominal P-tm of 30 cmH(2)O were 2.5 x 10(4), 1.7 x 10(4), and 1.5 x 10(4) N.m(-2) for ra bbits, dogs, and horses, respectively. This species ranking fits with the pressures required to cause stress failure which are approximately 50, 90, and 130 cmH(2)O in rabbit, dog, and horse, respectively. We c onclude that the differences in capillary radius of curvature and BGB thickness account for some of the observed differences in P-tm necessa ry to cause stress failure. However, other factors may also be importa nt in determining the strength of the BGB.