Hairpin capillaries preserve the medullary gradient while still supplying blood
Expansion
The vasa recta are long hairpin capillaries running alongside the loops of Henle.
- On the way down, solute diffuses in and water diffuses out, so blood becomes progressively concentrated
- On the way up, the reverse happens
Because the two limbs run in opposite directions, solute is passively trapped in the medulla rather than being carried away. This is passive exchange, requiring no energy, in contrast to the active multiplier.
Two further features protect the gradient: medullary blood flow is low, only about 5 per cent of renal blood flow, and it is slow.
The cost is that the medulla operates at a very low oxygen tension, which makes the thick ascending limb and the outer medulla the region most vulnerable to ischaemia. This is why acute tubular necrosis characteristically affects that segment, and why the kidney tolerates hypotension poorly despite receiving a quarter of the cardiac output.