Countercurrent Exchange in the Vasa Recta Preserves Hyperosmolarity of the Renal Medulla
Blood flow must be provided to the renal medulla to supply the metabolic needs of the cells in this part of the kidney. Without a special medullary blood flow system, the solutes pumped into the renal medulla by the countercurrent multiplier system would be rapidly dissipated.
Special features of the renal medullary blood flow that contribute to the preservation of the high solute concentrations:
1. The sluggish blood flow (accounting for less than 5 per cent of the total renal blood flow) is sufficient to supply the metabolic needs of the tissues but helps to minimize solute loss from the medullary interstitium.
2. The vasa recta serve as countercurrent exchangers, minimizing washout of solutes from the medullary interstitium.
The countercurrent exchange mechanism
As blood descends into the medulla toward the papillae, it becomes progressively more concentrated, partly by solute entry from the interstitium and partly by loss of water into the interstitium.
By the time the blood reaches the tips of the vasa recta, it has a concentration of about 1200 mOsm/L, the same as that of the medullary interstitium.
As blood ascends back toward the cortex, it becomes progressively less concentrated as solutes diffuse back out into the medullary interstitium and as water moves into the vasa recta.
* Thus, although there is a large amount of fluid and solute exchange across the vasa recta, there is little net dilution of the concentration of the interstitial fluid at each level of the renal medulla because of the U shape of the vasa recta capillaries, which act as countercurrent exchangers. Thus, the vasa recta do not create the medullary hyperosmolarity, but they do prevent it from being dissipated.
