Although the borders between pyramids and renal columns are sharply defined, the pyramids project striations into the cortical arches, known as medullary rays. These striations largely represent collecting ducts (see Plate 1-26), which extend from the cortex to the renal papillae, merging along the way into papillary ducts. The papillary ducts drain urine to 20 or more small pores at each papilla’s cribriform area (area cribrosa). One to three papillae drain into each minor calyx; two to four minor calices join to form a major calyx; and two or three major calices join to form the funnelshaped renal pelvis, which becomes the ureter after leaving the hilum. The ureter, in turn, conveys urine to the bladder for storage.

The parenchyma served by a single papilla is known as a renal lobe, and in the fetus and infant these lobes are evident as grossly visible convexities separated by deep grooves on the kidney surface. Such lobulation persists in some mammalian species throughout life, and vestigial demarcations of lobulation are occasionally present in the human adult.