Physiology of the distal convoluted tubule and collecting duct

Each human kidney contains approximately 1.3 million nephrons that are made up of a glomerulus and a renal tubule. The components of the renal tubule include the proximal convoluted tubule (PCT), the loop of Henle (LOH), the distal convoluted tubule (DCT) and the collecting duct (CD). Each component is made up of different cell subtypes that correlate with differences in function.(1) The distal tubule follows the macula densa, and consists of two segments, the DCT and the connecting segment. The connecting segment extends to the CD which has a cortical and a medullary component. These segments have an important role in the absorption of many ions, and water reabsorption.(2) The tubular fluid entering the distal tubule is hypotonic (100-200 mOsm). It contains about 20% of the water filtered by the glomerulus and about 10% of the sodium filtered. The early distal tubule reabsorbs a further 5% of the sodium, and the late distal tubule and CD fine tune reabsorption of the last little bit (around 5%), determining exactly how much sodium will be excreted.(3) The absolute level of transport of ions and water in these segments is much less than in most upstream segments, but the variability of the rate of transport is much higher.(4) Sodium reabsorption in the late distal tubule and CD is regulated by hormones.(1) In the context of renal tubule physiology, the term basolateral or serosal membrane refers to the cell membrane oriented away from the lumen of the tubule, whereas the term luminal or apical membrane refers to the cell membrane oriented towards the lumen.