Quantitative analysis of renal medullary anatomy in rats and rabbits.

The mean renal tubular diameters, number of tubules per unit of cross-sectional area, and fraction of the total volume occupied by each medullary structure were determined at various levels of the renal medulla of the rat and rabbit. Statistical estimates of anatomic variables were made using spatial sampling techniques on histologic sections. Osmotic diuresis and renal venous occlusion were used to allow fixation of renal tubules and blood vessels in an open state. The distribution of volume fractions of medullary structures are similar in rats and rabbits. Diameters of outer medullary tubular segments and inner medullary thin limbs of Henle are also similar in rats and rabbits. Marked differences between rats and rabbits, however, are seen in the size and number of collecting ducts in the inner medulla. Rabbit inner medullary collecting ducts increase in diameter and decrease in number in the papillary direction relatively closer to the cortex than do those of the rat. Luminal diameters of papillary collecting ducts are more than twice as great in the rabbit as in the rat. An additional finding was that short loops of Henle in the rabbit have their bends relatively closer to the cortex than those of the rat. The quantitative anatomic data derived in this study, when combined through mathematical modeling with knowledge of transport properties of renal tubular membranes, should lead to a clearer understanding of renal function.