Vasa recta
Vasa recta renis
Also known as: straight vessels, arteriolae rectae, venulae rectae
Long, hairpin-shaped capillaries derived from the efferent arterioles of juxtamedullary glomeruli that supply the medulla and act as countercurrent exchangers, preserving the medullary osmotic gradient.
Overview
Efferent arterioles of juxtamedullary nephrons give rise to bundles of descending vasa recta that plunge into the medulla alongside the loops of Henle, loop back at various depths and return as ascending vasa recta to the arcuate or cortical radiate veins. Descending vasa recta have a continuous endothelium with pericytes and express UT-B urea transporters and AQP1; ascending vasa recta are highly fenestrated.
As blood descends it loses water and gains NaCl and urea from the hypertonic interstitium, becoming hypertonic; as it ascends it regains water and loses solute. This countercurrent exchange lets the vasa recta nourish the medulla and carry away the water reabsorbed from the descending limbs and collecting ducts without washing out the gradient. Medullary blood flow is low (≈10% of renal blood flow, inner medulla only ≈1–2%), which also helps preserve the gradient but leaves the medulla hypoxic.
- Location
Renal medulla, parallel to the loops of Henle and collecting ducts.
Function
- Countercurrent exchange — preserve the corticomedullary osmotic gradient
- Remove water reabsorbed in the medulla
- Supply O₂ and nutrients to the medulla
- Physiology
Medullary washout: increased medullary blood flow (e.g. osmotic diuresis, vasodilators) dissipates the gradient and reduces maximal urine concentration.
Clinical significance
Medullary hypoxia
Low medullary flow and high TAL O₂ consumption make the outer medulla the site of maximal ischaemic tubular injury (acute tubular necrosis of S3 and medullary TAL) and the papillae prone to necrosis; sickling in the hypertonic, hypoxic vasa recta causes papillary necrosis and isosthenuria in sickle cell disease.
Test yourself
Which feature distinguishes juxtamedullary nephrons from cortical nephrons?
- Their glomeruli lie in the outer cortex
- They lack a thick ascending limb
- They have long loops reaching the inner medulla and their efferent arterioles give rise to the vasa recta
- They make up about 85% of all nephrons
Answer: They have long loops reaching the inner medulla and their efferent arterioles give rise to the vasa recta
Juxtamedullary nephrons (≈15%) have glomeruli near the corticomedullary junction, long loops with thin ascending limbs extending to the papilla, and efferent arterioles forming the vasa recta — they create the medullary osmotic gradient.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.