Renal medulla

The renal medulla is the inner region of the kidney, situated between the renal cortex and the renal pelvis. It is composed of multiple cone-shaped structures called renal pyramids, which contain the loops of Henle, medullary collecting ducts, vasa recta, and the medullary interstitium. The renal medulla plays a critical role in the regulation of water and electrolyte balance, as well as in the concentration of urine, primarily through the countercurrent multiplication and exchange mechanisms established by its unique tubular and vascular architecture.

The medulla is characterized by steep gradients of osmolality and low oxygen tension, which are essential for its function but also make it particularly susceptible to ischemic injury and metabolic stress.

The organization of nephrons and blood vessels in the medulla supports the generation and maintenance of these gradients, enabling the kidney to produce urine that is hyperosmotic to plasma and thus conserve water under conditions of dehydration or increased antidiuretic hormone activity.