The kidney is an important organ of the urinary system, and it forms the urinary system with the ureter, bladder and urethra. The kidneys are located on both sides of the lumbar spine, one on each side, shaped like a fava bean close to the posterior abdominal wall. The kidneys are substantial organs, and fresh kidneys are reddish-brown in color and rich in blood. The size of the kidney varies from person to person, generally speaking, the average length is 9-12cm, width is 5-6cm, thickness is 3-4cm, and the average weight is 120-150g. The size and weight of the kidney in women are slightly smaller than those in men of the same age. The outer edge of the kidney is elevated and the inner edge is depressed in the middle, which is the site of access to the renal blood vessels, lymphatic vessels, nerves and ureters, called the renal hilum, and these structures in and out of the renal hilum are collectively called the nephron. The renal unit is the basic unit that makes up the function and structure of the kidney, including the renal tubule and the renal tubule connected to it, and the renal tubule consists of the glomerulus and the renal capsule. Each kidney has about one million kidney units.