The thick ascending limb is the epithelial segment of the Loop of Henle that returns fluid toward the renal cortex while reabsorbing sodium chloride. Its relatively low membrane permeability to water lets it remove salt without equivalent water loss, helping generate dilute tubular fluid and the medullary gradient used in countercurrent multiplication.
One molecular entry route is NKCC2, a sodium–potassium–chloride cotransporter on the apical membrane of its epithelial cells. The transporter is a membrane protein; the DNA sequence encoding it and the anatomical segment expressing it are different levels of description.[ref]
In a 1973 experiment, isolated rabbit segments were perfused internally while their outside solution was controlled separately. Raising bath osmolality did not produce appreciable net water movement, but the collected fluid contained less sodium and chloride than the perfusate. These two measurements distinguish salt removal from simple dilution by added water.[ref]
As an idealized example, removing 20 units of solute from 100 units carried in one volume unit lowers concentration from 100 to 80 while that volume remains fixed. This illustrates a diluting segment; it does not mean that a living tubule must have exactly zero water permeability in every condition.
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