Internal Structure of the Kidney

Each kidney is wrapped in three protective tissue layers. The outermost, the renal fascia, is a thin sheet of fibrous connective tissue that anchors the kidney to the abdominal wall and to nearby tissues. The middle layer, the adipose capsule, is a cushion of fatty tissue that protects the kidney by absorbing physical shock. The innermost layer, the renal capsule, is a smooth, transparent, fibrous membrane continuous with the outer covering of the ureter; it acts as a barrier that helps stop infection from spreading into the kidney.

Inside this capsule, a longitudinal section reveals two histologically distinct regions. The renal cortex is the outer, peripheral zone — reddish and granular in appearance because it is packed with renal (Malpighian) corpuscles and convoluted tubules along with a dense supply of blood vessels. The renal medulla is the inner zone — paler and striated in appearance, because it consists mainly of the loops of Henle and collecting ducts, all bundled together in a conical arrangement to form the renal pyramids. Tongues of cortical tissue dip down between adjacent pyramids as the renal columns (columns of Bertini). The narrow, innermost tip of each pyramid is the renal papilla. …