Kidney of a juvenile okapi, Okapia johnstoni.

The kidney of the okapi is of the medullary crest type and is divided transversely into six lobes by encroachments of cortex into medulla. These lobes are demarcated externally by furrows. The collecting ducts open at the apex of the truncated medullary crest. The pelvis part of the kidney, or pars pelvina renis, is the entire inner medulla and is the only part exposed to the pelvic urine. The pelvis has 10 interlobar vascular eminences and extends peripherally along-side the interlobar vessels as 10 fornices. The vascular eminences are buttressed by 12 free-edged semilunar eminences, which form pouches across 12 intervascular eminences. At each pole a vascular eminence supplies blood vessels to a dorsal and a ventral intervascular eminence. The extrahilar portion of the renal vein consists mainly of a thick coat of longitudinal muscle. In the renal sinus this heavy muscular coat is absent. The tributaries of this vein open via conspicuous nonmuscular valves. The cortex is about 85% of the renal mass and has about 2.89 X 10(6) glomeruli, which form 4.85% of the cortical mass. The filtering surface of a glomerulus is 0.088 mm2, making the total filtering surface of one kidney 0.219 m2. The number of glomeruli per kidney falls into the line which relates number of glomeruli to adult body weight from mouse to elephant and is expressed by log N = 0.59 log W + 3.2 or by N - 1.585W0.59 where N is the number of glomeruli in one kidney and W the adult body weight.