The kidney is the most important organ of the human urinary system and one of the most important organs in maintaining the stability of the internal environment. The function of the kidneys is to excrete metabolic wastes and regulate water-electrolyte balance to maintain the stability of the body’s internal environment. The human body has two kidneys, right and left, located behind the peritoneum. Their size and weight vary with age and sex. The surface of the kidney is a dense connective tissue peritoneum. The renal parenchyma consists of two parts: the external cortex and the deep medulla. They perform the same function, changing the volume of urine, the concentration of various electrolytes, and the amount of other metabolites excreted. The abnormal internal environment is regulated accordingly in order to achieve homeostasis. This powerful regulating ability is based on the huge filtration area of glomeruli in both kidneys, sufficient blood supply and flexible filtration regulation ability, plus the powerful reabsorption and secretion ability of each segment of the renal tubule, which makes the kidney excrete metabolic waste very powerful. The unique structural features of the kidney are described above. The interaction between the various renal units, glomeruli and tubules, and their ability to regulate the changes in body fluids enable them to perform their basic functions.