The kidneys are covered by a thin, fibrous membrane called the renal capsule. However, the outermost functional region of the kidney is the renal cortex.
The cortex contains a large quantity of nephrons, the major filtration units of the kidneys. Blood is transported via the renal artery to the glomerulus, a large cluster of smaller capillaries that surround a structure called Bowman's capsule. All the waste is removed from the blood and enters the tubules, where it is processed before exiting the kidney as urine. The renal cortex is believed to contain over 1 million blood-filtering nephrons and renal tubules.