The kidneys function is to filter wastes, such as urea, creatinine, and salts, from the blood and excrete them, along with water, as urine. The kidney has a bean-shaped structure, with concave and convex surfaces. The concave surface, the renal hilum, is the point at which the renal artery enters the organ, and the renal vein and ureter leave. The kidney is surrounded by tough fibrous tissue, the renal capsule, which is itself surrounded by perinephric fat, renal fascia and paranephric fat. The anterior (front) border of these tissues is the peritoneum, while the posterior (rear) border is the transversalis fascia.
The kidney is divided into two major structures: the renal cortex, which is superficial, and the renal medulla, which lies deep below the surface. Grossly, these structures take the shape of 8 to 18 cone-shaped renal lobes, each containing renal cortex surrounding a portion of medulla called a renal pyramid (of Malphigi). Between the renal pyramids are projections of cortex called renal columns of Bertin. Spanning the cortex and medulla are the nephrons, which are the basic functional structures of the kidney; they are microscopic tubules that filter the toxic wastes, salts and get rid of excess water; there about one million nephrons per kidney. The initial filtering portion of a nephron is the renal corpuscle, located in the cortex, which is followed by a renal tubule that passes from the cortex deep into the medullary pyramids. Part of the renal cortex, a medullary ray is a collection of renal tubules that drain into a single collecting duct.
The tip, or papilla, of each pyramid empties urine into a minor calyx, minor calyces empty into major calyces, and major calyces empty into the renal pelvis, which becomes the ureter.
The essential tissue composition of kidney is that of a gland with highly modified secretory units and highly specialized ducts (nephrons). Kidneys excrete urine, produced by modifying a filtrate of blood plasma. The fundamental unit of the kidney is the nephron, and the fundamental unit of the nephron is the glomerulus.