Kidney/ˈkɪdni/ Definition 1 of 3Pro Tip 1/3Body Cleansing The kidneys filter blood, removing toxins and regulating water balance. Her kidney malfunction led to a buildup of toxins. Pro Tip 2/3Kidney Pair Humans have two kidneys located in the lower back, one on each side of the spine. The doctor checked the health of both his kidneys. Pro Tip 3/3Health Context Discussions about kidneys are often related to diseases, transplants, diets, or injuries. After the accident, they assessed his kidney health.