The renal cortex and renal pyramids of the renal medulla constitute the parenchyma (functional portion) of the kidney. Within the parenchyma are the functional units of the kidney - about 1 million microscopic structures called nephrons. Urine formed by the nephrons drains into large papillary ducts, which extend through the renal papillae of the pyramids. The papillary ducts drain into cuplike structures called minor and major calyces. Each kidney has 8 to 18 minor calyces and 2 or 3 major calyces. A minor calyx receives urine from the papillary ducts of one renal papilla and delivers it to a major calyx. From the major calyces, urine drains into a single large cavity called the renal pelvis and then out through the ureter to the urinary bladder.
