The first portion of the distal convoluted tubule is similar to the thick ascending limb of the loop of Henle. Na⁺ and Cl^– are transported out of the tubule fluid, and water cannot follow. As a result, the tubule fluid becomes even more dilute. The later sections of the distal convoluted tubule, however, can be permeable to water, and therefore water can be osmotically drawn from the hypotonic tubule fluid into the interstitial fluid. As the tubule fluid flows through the distal tubule to the collecting duct, it equilibrates with the osmolarity of the blood plasma.

An important function of the distal tubule is the fine-tuning of the ionic composition of the urine. Even though bulk reabsorption of substances such as calcium, phosphate, bicarbonate, and potassium occurs in the proximal convoluted tubule, changes in the concentrations of these substances occur in the distal convoluted tubule. In the case of potassium, for example, if a person is potassium depleted, this ion is reabsorbed in the distal convoluted tubule, but if a person has an abundance of potassium, this ion is secreted in the distal convoluted tubule. As you will see in Key Concept 51.6, this exchange of K⁺ is controlled by the hormone aldosterone. Another example is reabsorption of Ca²⁺ in the distal convoluted tubule, which is controlled by the actions of vitamin D. The fine-tuning of urine composition continues in the collecting duct. As you can imagine, the list of ion transporters in the distal convoluted tubule is large.