Vasa recta: counter-current exchange preserves the medullary gradient
The vasa recta, capillaries where blood flows in opposite directions (counter-current exchange), preserve the medullary gradient: the concentration of dissolved particles rises deeper into the medulla (the inner kidney).
Remember
The loops of Henle (hairpin-shaped tubules) build the medullary gradient and the vasa recta keep it, by passive counter-current exchange that limits solute washout (solute carried away by blood) rather than eliminating it.
The kidney makes concentrated urine by building a vertical osmotic gradient. The medulla (the inner part of the kidney) holds more dissolved particles per litre than the cortex (the outer part), and it becomes more concentrated the deeper you go. The loop of Henle (the hairpin-shaped part of the tubule that dips into the medulla) builds this gradient using energy, by counter-current multiplication.
The medulla needs a blood supply, and ordinary blood flow would wash the gradient away. The vasa recta, long, straight capillaries running alongside the loops, solve this by acting as a counter-current exchanger: blood flows in opposite directions in their descending and ascending parts.
Going down into the concentrated medulla, blood gains solutes (sodium chloride and urea) and loses water. Coming back up towards the cortex, it loses solutes to the interstitium (the tissue between the tubules) and regains water. Much of the solute is therefore kept in the medulla, and the reabsorbed water is carried away in the blood. This limits washout, the dissipation of the gradient. The exchange is passive: it follows concentration gradients and involves no pumping.
The exchange minimises solute loss rather than eliminating it, so the loops keep topping the gradient up. When a drug markedly increases medullary blood flow, the exchange becomes less efficient, so solute leaves the medulla faster and part of the gradient is washed out. The collecting ducts (the final drainage tubes) then have less pull on water, and the urine stays relatively dilute even when ADH (antidiuretic hormone, which increases water reabsorption) is high.
In short, the loop of Henle is the multiplier that makes the gradient, and the vasa recta are the exchanger that keeps it.
Key facts
- Division of labour
- the loop of Henle sets up the gradient and the vasa recta maintain it.
- Where they sit
- long, straight capillaries that run alongside the loops of Henle of the juxtamedullary nephrons (the nephrons whose long loops dip into the medulla).
- Descending blood
- gains solutes and loses water in the salty medulla.
- Ascending blood
- loses solutes and gains water.
- Energy
- the exchange is passive, so the vasa recta do not pump solutes.
- Limit
- the exchange minimises solute loss rather than eliminating it.
- Blood flow
- faster flow would carry solute out of the medulla and wash the gradient out, whereas the vasa recta cope with continuous flow through their opposing limbs, not through speed.
Exam traps
- The vasa recta are not a counter-current multiplier. That term belongs to the loop of Henle, which builds the gradient, whereas the vasa recta are an exchanger that conserves a gradient already in place.
- Rapid flow does not protect the gradient. It carries solutes out of the medulla before they can be returned, so it increases washout and flattens the gradient.
- The vasa recta do not pump ions. Active pumping of sodium chloride into the interstitium is how the loop of Henle builds the gradient, while the vasa recta exchange passively.
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