For orientation, examine R-P, Figs. 20.1 & 20.6. The kidney has a thin, tough
connective tissue capsule beneath which lies the glandular tissue. The glandular tissue
surrounds a large central cavity called the renal sinus. Adjacent to the renal sinus on the
medial side of the organ is a notch called the hilus where the renal vessels and the ureter
enter and leave the organ. The glandular portion of the kidney is composed of a number
of conical pyramids, the renal lobes. Each renal lobe has its base on the capsule, and its
apex (the renal papilla) projects into a minor calyx. The renal lobe consists of two
regions, the cortex and the medulla. The cortex has a granular appearance due to the
presence of renal corpuscles and renal tubules. These are components of the nephron, the
functional unit of the kidney. The medulla has a striated appearance due to the presence
of collecting tubules. Structures called medullary rays appear as vertical striations in the
cortical substance. Renal columns are projections of cortical tissue between the bases of
medullary pyramids.