The 'distal tubule' although anatomically defined is not a distinct section of the nephron in terms of physiological function. Functionally, the distal tubule is a segment of the nephron in which the tubule cells undergo transition from 'ascending limb of Henle type' cells to 'collecting tubule type' cells. The walls of the former cell type have only a very low and essentially constant permeability to water, whereas the 'collecting tubule type' cells have a variable water permeability, which is regulated by the hormone ADH (anti-diuretic hormone, vasopressin).
The potential difference across the distal tubular wall, varies with distance along the tubule. In the early part, the lumen is positive with respect to the interstitium (as in the ascending limb of Henle), but in the later parts the luminal potential becomes negative and may reach -45 mV. This negative potential is caused by active sodium re-absorption.
The collecting tubules have cortical and medullary sections, each section having somewhat different properties. Both are relatively impermeable to water, urea and NaCl, but the water permeability is increased by ADH. Thus, ADH leads to urine concentration by permitting the osmotic abstraction of water into the interstitium. Theoretically the urine in the collecting tubule can achieve the same osmolality as the medullary interstitium but in reality it is usually rather less than this.
ADH also increases the urea permeability of the medullary collecting tubules, but has no effect on the urea permeability of the cortical collecting tubules. This impermeability of the cortical part of the collecting tubule to urea, is one of the factors that make urea so important in the urine concentration mechanism.
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