thin membranous sheath that covers the outer surface of each kidney. The capsule is composed of tough fibres, chiefly collagen and elastin (fibrous proteins), that help to support the kidney mass and protect the vital tissue from injury. The number of elastic and smooth muscle fibres found in the capsule tends to increase with the individual's age. The capsule receives its blood supply ultimately from the interlobar arteries, small vessels that branch off from the main renal arteries; these vessels travel through the cortex of the kidney and terminate in the capsule. The maximum thickness of the membrane is usually 2 to 3 millimetres (0.08–0.12 inch). The capsule surrounds the outer walls and enters into a hollow region of the kidney known as the sinus. The sinus contains the major ducts that transport urine and the arteries and veins that supply the tissue with nutrients and oxygen. The capsule connects to these structures within the sinus and lines the sinus wall.

In a normal person, the capsule is light reddish-purple in colour, translucent, smooth, and glistening; it can usually be easily stripped from the rest of the kidney's tissue. A diseased kidney frequently sends fibrous connections from the main body of tissue to the capsule, which makes the capsule adhere more strongly. Difficulty in removing a capsule is noted at autopsy as an indication that the kidney was diseased.