Vasa recta: countercurrent exchanger
Loop of Henle · detail · UT-B Slow flow Low PO2
Hairpin capillaries from the efferent arterioles of juxtamedullary glomeruli that supply the medulla. By exchanging solute and water passively between descending and ascending limbs they carry away reabsorbed water without dissipating the gradient the loop built.
Traced from the start
- Efferent arteriole
- Vasa recta: countercurrent exchanger
Detail
- Origin
- Efferent arterioles of juxtamedullary nephrons to descending vasa recta, then ascending vasa recta, then arcuate veins
- Exchange
- Descending limb loses water and gains NaCl and urea (UT-B on endothelium and red cells); ascending limb reverses this, so blood leaves only slightly hypertonic to plasma
- Flow
- Medullary blood flow is only ~5-10% of total renal blood flow; the slow flow is essential because high flow washes the gradient out
- Oxygen
- Inner medullary PO2 is only ~10-20 mmHg because countercurrent exchange shunts oxygen too
- Hormone
- ADH constricts descending vasa recta (V1), further lowering medullary flow and protecting the gradient
When it goes wrong
Hypotension, NSAIDs, radiocontrast or red-cell sickling within the hypoxic, hypertonic vasa recta
Medullary hypoxia causing outer-medullary acute tubular necrosis or papillary necrosis (loin pain, haematuria, sloughed papillae obstructing the ureter)
Mannitol, saline loading or loop diuretics raising medullary blood flow
Partial washout of the gradient and reduced concentrating capacity
Practise this structure
1 question in the bank tagged Medullary.