The shape of renal vasa recta capillaries and its effect on calculation of single capillary blood flow

Previous studies of the renal papilla of the rat have suggested that the vasa recta capillaries can be well approximated by elliptical cylinders (C. Holliger, K. V. Lemley, S. L. Schmitt, F. C. Thomas, C. R. Robertson, and R. L. Jamison, 1983, Circ. Res., 53, 401-413). This hypothesis was validated in a morphological study employing several methods of specimen fixation and preparation. Papillas of young (body wt = 90 g) Wistar rats were fixed and subsequently examined by light and electron microscopy. Cross-sectional shapes and orientations were determined for 300 superficial vasa recta. The ratio, beta, of vessel cross-sectional major-axis-to-minor-axis lengths was 1.39 +/- 0.24 (SD). Values of beta greater than 1.0 (the value expected for circular vessels) could not be accounted for by either fixation artifact or the angle of histologic sectioning of the papillas. A quantitative estimate of the relationship between the apparent capillary diameter measured in vivo and the capillary cross-sectional area was made using a mathematical model which accounts for cross-sectional shapes and orientations of the vasa recta. This estimate implies that current methods of calculating single vas rectum blood flow using apparent diameters and blood velocities determined in vivo probably overestimate actual blood flow by about 25%.