22nd June 2023

Collecting Duct

The permeability of the late distal tubule, cortical and medullary collecting ducts to water is controlled by antidiuretic hormone (ADH), produced by the hypothalamus, and stored in the posterior pituitary gland. In the presence of high levels of ADH (e.g. secondary to dehydration), the number of aquaporin-2 channels in the distal convoluted tubule and collecting duct increases, increasing water reabsorption, and thereby reducing the volume and increasing the osmolality of urine. Thus, ADH has an important role in controlling the degree of concentration of the urine.

ADH is clinically relevant in conditions such as: SIADH (syndrome of inappropriate ADH secretion), resulting in excessive blood dilution leading to hyponatraemia; Diabetes insipidus (DI), an insufficient release of ADH from the posterior pituitary (central DI) or reduced response of the collecting ducts to ADH (nephrogenic DI), resulting in polyuria and subsequent polydipsia.

The late DCT and cortical collecting duct are almost completely impermeable to urea, therefore, urea in these segments is excreted in the urine. Conversely to the cortical collecting duct, the medullary collecting duct is permeable to urea. In the interstitium, urea is an effective osmole (i.e. unable to cross from extracellular fluid to intracellular fluid, raising its osmolality) which allows increased reabsorption of water from the renal tubule.