The human kidney filters blood and balances the body’s fluid composition. This bean-shaped organ is structurally divided into two main regions: the outer renal cortex and the inner renal medulla. The renal medulla is the darker, reddish-brown tissue situated directly beneath the cortex. It is the deep, inner region where the final adjustments to the filtrate are made before it is excreted as urine. The medulla’s unique architecture creates the concentration gradient necessary for water reabsorption and waste removal.
Macroscopic Organization
The most visible feature of the renal medulla is its organization into distinct, cone-shaped segments known as renal pyramids. Each kidney typically contains between 10 to 18 of these triangular structures, which form the bulk of the medullary tissue. The broad base of each pyramid rests adjacent to the renal cortex, establishing the corticomedullary border.
The pyramids possess a striated appearance because they are composed of millions of parallel, straight tubular and vascular segments. These segments are parts of the nephrons that dip down from the cortex. These pyramids converge inward toward the center of the kidney.
The tip of each renal pyramid, which points toward the renal pelvis, is called the renal papilla. The papilla contains the openings of the largest collecting ducts. It is the site where the processed fluid, now called urine, is released into the minor calyces, which lead to the ureter.
The Tubular Network
The renal medulla is predominantly composed of the straight segments of the nephron and the collecting duct system. These microscopic tubes are extensions of nephrons that originate in the cortex, looping deeply into the medulla before returning. The most prominent structure is the Loop of Henle, which consists of descending and ascending limbs.
The descending limb of the Loop of Henle enters the medulla and has a thin segment characterized by simple squamous epithelium. The ascending limb forms a hairpin turn, transitioning into a thick segment composed of simple cuboidal epithelium. The thick ascending limb is histologically distinguishable by its larger cells and wider lumen.
Running parallel are the Medullary Collecting Ducts, which receive processed fluid from multiple nephrons. These ducts are lined with simple cuboidal or columnar epithelial cells that become progressively taller toward the papilla. The epithelium contains two specialized cell types that regulate the final composition of the urine.
The principal cells are the most numerous, responsible for sodium reabsorption and water movement. Intercalated cells regulate the body’s acid-base balance. These ducts merge to form the larger papillary ducts, which ultimately empty the final urine product at the renal papilla.
Specialized Blood Supply
The renal medulla contains a unique, slow-flowing vascular network distinct from the peritubular capillaries found in the cortex. This specialized blood supply is composed of the Vasa Recta, or “straight vessels.” These straight capillaries descend into the medulla, running parallel to the Loops of Henle and the collecting ducts.
The Vasa Recta originate from the efferent arterioles of the juxtamedullary nephrons, located closest to the corticomedullary border. This arrangement forms a countercurrent exchange system that supplies nutrients without washing away the high concentration of solutes in the surrounding interstitial fluid.
The medullary interstitium, the tissue surrounding the tubules and vessels, is another component of the medulla. This space contains specialized cells and a concentrated extracellular matrix rich in solutes. This highly concentrated environment allows the collecting ducts to efficiently reabsorb water and produce concentrated urine.