Each kidney is comprised of an outer structure, or renal cortex, and an inner structure, or renal medulla. Near the center of the kidney is the hilum. The hilum is the entrance to the renal sinus. The renal sinus is the main cavity of the kidney that contains the renal pelvis, the renal calyces, blood vessels, and nerves.
The functional unit of the kidney is the nephron. The nephron is the primary structure in the kidney that produces urine as part of the process of filtering the blood. A nephron is made up of a renal corpuscle and renal tubules: the proximal and distal convoluted tubules, the nephron loop (loop of Henle), and the collecting duct. The renal corpuscle surrounds a collection of capillaries known as a glomerulus. A glomerulus, which is granular in structure, is a cluster of capillaries located at the end of a kidney tubule where material is filtered from the blood. It is surrounded by a capsule commonly known as Bowman's capsule, which is located at the end of a renal tubule. The filtration occurs from the glomerulus to the Bowman's capsule. Blood to be filtered travels through the renal artery into the kidney at the level of the hilum. It flows into the afferent arteriole. Urea (a by-product of protein metabolism), salts, water, ammonia, bile by-products, and other wastes are left behind and eventually may become part of the urine. Filtered blood then leaves the kidney through outgoing blood vessels called the efferent arterioles, peritubular capillaries, and eventually through the renal vein back to the heart. While the blood remains in the kidney, some water and nutrients are reabsorbed back into the bloodstream. The renal cortex is the outer layer of the kidney and is granular in structure. It is composed of glomeruli and tubules and connects the pyramids of the renal medulla.
The renal medulla is the middle, or inner, region of the kidney that functions as a collecting area for the waste and water that has been filtered by the renal cortex. The renal medulla is divided into sections, known as the renal pyramids. The pyramid of the renal medulla, also called Malpighian pyramid, is one of several sections of tissue in the medulla that contains tubes that transport urine from the cortex to the calyces. The urine produced in the renal cortex is transported through the pyramids to the renal calyces. A renal calyx is a cup-shaped cavity where urine accumulates before passing through the ureter to the bladder. Each pyramidal "point" (called a papilla) projects into a calyx. Droplets of urine from the pyramids first collect in a minor calyx, a cavity that collects droplets of urine from the renal pyramids. Two to three minor calyces converge to deposit collected urine into a major calyx. A major calyx is a junction of two to three minor calyces in the kidney that collects urine flowing from the kidney and passes it to the ureter. The major calyx, in turn, joins with other major calyces to form the renal pelvis. The renal pelvis is a large, funnel-shaped, upper portion of the ureter that serves to collect the urine before it flows into the bladder. It is also the point where two or three major calyces converge. The renal column, also called Bertin's column, is an extension of the renal cortex into the medulla between the renal pyramids providing support and connection for the cortex.