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The kidneys are located just under the rib cage in the back, one on each side. Each adult kidney is about the size of a fist. Each has an outer layer called the cortex, which contains filtering units. The center part of the kidney (medulla) has 10 to 15 fan-shaped structures called pyramids. These drain urine into cup-shaped tubes called calyxes.

Blood travels to each kidney through the renal artery, which enters the kidney at the hilus, the indentation in the kidney that gives it its bean shape. As it enters the cortex, the artery branches to envelope the nephrons - 1 million tiny filtering units in each kidney that remove the harmful substances from the blood. Each of the nephrons contains a filter called the glomerulus, which contain a network of capillaries. As blood travels through the kidneys, water and some of the other blood components (such as acids, glucose, and other nutrients) are reabsorbed back into the bloodstream. What remains is urine, which is a concentrated solution of waste material containing water, urea (a waste product that forms when proteins are broken down), salts, amino acids, by-products of bile from the liver, ammonia, and any substances that cannot be reabsorbed into the blood. Filtered blood leaves the kidney through the renal vein and flows back to the heart.

The renal pelvis, located near the hilus, collects the urine flowing from the calyxes. From the renal pelvis, urine is transported out of the kidneys through the ureters to be stored, and eventually excreted, from the urinary bladder. The bladder expands as it fills and can hold about half a liter (2 cups) of urine at any given time. An average adult produces about 1.5 liters, or 6 cups, of urine per day. An adult needs to produce and excrete at least one third of this amount of urine in order to adequately clear waste products from the body. Producing too much or not enough urine may indicate illness.