The kidney is divided into an outer cortex and inner medulla. The medulla is composed of renal pyramids, separated by renal columns. The renal pyramids empty urine into the calyces that drain into the renal pelvis. From there urine flows into the ureter and is transported to the bladder to be stored. Each kidney contains more than a million microscopic functional units called nephrons.
Nephrons consist of vascular and tubular components.
The loop of Henle is located in the medulla. Filtrate from the distal convoluted tubule is drained into collecting ducts, which plunge through the medulla to empty urine into the calyces. The glomerular ultrafiltrate, formed under the force of blood pressure, has a low protein concentration. The GFR can be regulated by constriction or dilation of the afferent arterioles. The reabsorption of most of the remaining water occurs as a result of the action of the countercurrent multiplier system.