Kidney: One of a pair of organs located in the right and left side of the abdomen. The kidneys remove waste products from the blood and produce urine. As blood flows through the kidneys, the kidneys filter waste products, chemicals, and unneeded water from the blood. Urine collects in the middle of each kidney, in an area called the renal pelvis. It then drains from the kidney through a long tube, the ureter, to the bladder, where it is stored until elimination. The kidneys also make substances that help control blood pressure and regulate the formation of red blood cells.