The kidney is the organ that filters blood and produces urine. Humans normally have two kidneys, located behind the abdominal cavity on either side of the spine. Blood enters each kidney through a renal artery, and the kidneys continuously adjust its water, salt, and waste content.
Inside each kidney are roughly one million nephrons, its microscopic filtering units. Filtration begins in a glomerulus, where water and small dissolved substances leave the blood. The nephron then reabsorbs useful materials such as glucose, amino acids, sodium, and much of the filtered water. Remaining wastes, including urea, become urine, which travels through the ureters to the bladder.
A common mix-up is thinking the kidney simply strains out all waste. It also helps regulate blood pressure, acid-base balance, electrolytes, and red-blood-cell production through hormones such as renin and erythropoietin. The liver detoxifies many chemicals, while the spleen filters aging blood cells; neither produces urine.