Kidney

Abstract

The kidney is a paired bean-shaped organ located superiorly in the retroperitoneum, averaging 12 cm long, 6 cm wide, and 2.5 cm thick, and weighing between 115 and 170 g. It is contained within a thin fibrous capsule that ends at the renal sinus. Fat surrounds the cortical surface of the kidney and the blood vessels, nerves, and collecting structures at the hilum. The perirenal fat is enclosed within a thin but resilient translucent sheath known as Gerota’s fascia. Each kidney is composed of 8–18 lobes, each of which constitutes a conical medullary pyramid capped by cortical tissue; downward extensions of cortical tissue between the pyramids are called columns of Bertin. The cortex is normally about 1 cm thick over the pyramids. Medullary rays of Ferrein, consisting of collecting ducts, straight segments of proximal and distal tubules, and vasa recta, extend from the cortex into the medulla. The inner medulla (papilla) drains through the orifices of the terminal collecting ducts (Bellini’s ducts) into a minor calyx. Each kidney contains one to two million nephrons, which are the functional unit of the kidney. Each nephron includes a glomerulus, proximal tubule, distal tubule, connecting segment, and collecting duct, which actually is derived from the ureteric bud.