The loop of Henle employs a sodium counter-current mechanism to provide hypertonic conditions - approximately 1200 mmol/l, mainly sodium chloride and urea - in the deep medulla of the kidney. This permits the production of concentrated urine if required.
The active component of the LOH is the ascending limb. Here chloride, and hence sodium, are pumped out of the tubule into the interstitium.
The ascending limb of the LOH is impermeable to water. The descending limb of the LOH is permeable to sodium chloride and water.
The net effect is for sodium chloride to leave the ascending limb and to enter the descending limb, having first passed through the renal medullary interstitium. Some water is also lost from the descending limb.
As the tubular fluid passes through the ascending limb it becomes increasingly dilute as the sodium chloride is removed. As a result the fluid entering the distal convoluted tubule is hypotonic (150 mmol/l).
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