Renal medulla

Medulla renalis

The inner part of the kidney parenchyma, made of 8–18 striated, cone-shaped renal pyramids containing the loops of Henle, collecting ducts and vasa recta, which build the hypertonic gradient for concentrating urine.

Overview

Interstitial osmolality≈300 (outer) → ≈1200 mOsm/kg (papilla)

Pyramids per kidney≈8–18

The medulla consists of the renal pyramids, whose bases face the cortex at the corticomedullary junction and whose apices — the renal papillae — project into the minor calyces. Between the pyramids lie the cortical renal columns. The striated appearance of the pyramids comes from parallel straight tubules (loops of Henle, collecting ducts) and the vasa recta.

The medulla is divided into an outer medulla (outer stripe and inner stripe, containing the thick ascending limbs) and an inner medulla (thin limbs of long loops and the inner medullary collecting ducts) reaching to the papilla. Its interstitial osmolality rises from ≈300 mOsm/kg at the corticomedullary junction to ≈1200 mOsm/kg at the papillary tip, generated by countercurrent multiplication and urea recycling and preserved by the low, sluggish blood flow in the vasa recta (≈10% of renal blood flow).

Because of that low flow and the high O₂ consumption of the thick ascending limbs, the medulla is relatively hypoxic (PO₂ ≈10–20 mmHg in the inner medulla) and especially vulnerable to ischaemic and toxic injury.

Location

Deep to the cortex, surrounding the renal sinus.

Parts & subdivisions

Renal pyramids

8–18 cone-shaped masses; each with its overlying cortex forms a renal lobe.

Renal papillae

Apices of the pyramids; perforated by papillary ducts (area cribrosa).

Renal columns

Cortical tissue between pyramids carrying interlobar vessels.

Outer and inner medulla

Outer medulla contains thick ascending limbs and outer medullary collecting ducts; inner medulla contains thin limbs and the urea-permeable inner medullary collecting ducts.

Function

Generate and maintain the corticomedullary osmotic gradient (≈300 → 1200 mOsm/kg)

Concentrate urine under ADH control

Convey urine from collecting ducts to the papillae

Physiology

Countercurrent multiplier (loops of Henle) + urea recycling (inner medullary collecting duct, ADH-dependent) + countercurrent exchange (vasa recta).

Clinical significance

Renal papillary necrosis

Ischaemic necrosis of the papillae, which lie at the end of the vasa recta supply — causes include analgesic (NSAID) nephropathy, diabetes, sickle cell disease, obstruction and pyelonephritis (POSTCARDS). Sloughed papillae can obstruct the ureter, causing colic and haematuria.

Medullary sponge kidney and nephrocalcinosis

Cystic dilatation of medullary collecting ducts predisposes to calcium stones; medullary nephrocalcinosis occurs in distal RTA and hyperparathyroidism.