Internal structure of kindey

Internally, the parenchyma of each kidney consists of an outer renal cortex and an inner medulla.

The renal cortex is approximately 5 mm thick. Extensions of the renal cortex project internally and are called renal columns

The renal columns divide the renal medulla into triangular-shaped tissue collections called the pyramids. Each kidney has more or less 10 to 16 pyramids. The pyramids contain nephrons, which are the functional units of the kidneys.

The bases of the renal pyramids are directed toward the renal cortex (outward). In contrast, the apex of each pyramid projects inward toward the renal sinus, which is a cavity within the kidney. It contains the hilum, renal pelvis, renal calyces, fat tissue, blood vessels, and nerves. 

The apical projections of renal pyramids are known as the renal papillae, which contain the openings of the papillary ducts that drain renal tubules and are surrounded by a minor calyx.

The minor calyces represent the proximal parts of the tubes that will eventually form the two ureters. They receive urine from the papillary ducts. Several minor calyces fuse and form a major calyx. Two or three major calyces combine to form the renal pelvis, a funnel-shaped superior end of each ureter.