vasa recta

**Anatomy** - Arise from efferent arterioles of juxtamedullary nephrons (long loops of Henle). Cortical nephrons' efferent arterioles feed the peritubular capillary network in the cortex instead. - Hairpin-shaped: descending vasa recta (arterial) and ascending vasa recta (venous) run alongside the loop of Henle and collecting duct. - Ascending vasa recta drain into the arcuate and interlobular veins, then the renal vein. - The descending limb is lined by continuous endothelium with UT-B urea transporters and aquaporin-1. The ascending limb is fenestrated, which favors uptake of water and solutes.

**Physiology: countercurrent exchange** - The medullary interstitium is hyperosmolar, reaching ~1200 mOsm/L at the papillary tip. This gradient is built by the countercurrent multiplier (loop of Henle: NKCC2 in the thick ascending limb, plus urea recycling in the inner medullary collecting duct via UT-A1/A3 under ADH). - Descending vasa recta: blood becomes progressively hyperosmolar as water leaves and solutes (NaCl, urea) enter. - Ascending vasa recta: the process reverses. Water enters and solutes leave, so blood exits only slightly more concentrated than it entered. - Net effect: the vasa recta remove the water and solutes reabsorbed from the loop and collecting duct *without washing out the gradient*. They are a passive countercurrent exchanger, not a multiplier.

**Clinical significance** - **Medullary washout:** Increased medullary blood flow (e.g., with vasodilators, osmotic diuresis, or high-volume states) dissipates the gradient and impairs urine concentration. Loop diuretics also reduce the gradient by inhibiting NKCC2. - **Papillary necrosis:** The medulla has low PO2 because of countercurrent oxygen exchange and a high metabolic demand from the thick ascending limb. This makes it prone to ischemic injury. Risk factors are sickle cell disease or trait, analgesic (NSAID) nephropathy, diabetes, pyelonephritis, and obstruction (mnemonic: "SAAD" – Sickle cell, Analgesics, Alcohol, Diabetes). Presents with hematuria, flank pain, and sloughed papillae. - **ATN:** The outer medulla is vulnerable to hypoxic injury (straight S3 segment of the proximal tubule and medullary thick ascending limb) because of this low oxygen tension. - **ADH and the vasa recta:** ADH (via V2) increases urea permeability and reduces medullary blood flow in some contexts, helping maintain the gradient. Prostaglandins increase medullary blood flow, so NSAIDs can enhance concentrating ability but risk ischemia.

**High-yield pearls** - Vasa recta = peritubular capillaries of juxtamedullary nephrons. - Function: preserve the medullary osmotic gradient (countercurrent exchange). - The "multiplier" is the loop of Henle. The "exchanger" is the vasa recta. - Slow, low blood flow (~5% of renal blood flow to the medulla) helps maintain the gradient.