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The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis.The collecting duct participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin (antidiuretic hormone).
The collecting duct system begins in the renal cortex and extends deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of Henle's countercurrent multiplier system. [1]: 67
By means of a countercurrent multiplier system, which uses electrolyte pumps, the loop of Henle creates an area of high urea concentration deep in the medulla, near the papillary duct in the collecting duct system. Water present in the filtrate in the papillary duct flows through aquaporin channels out of the duct, moving passively down its ...
Water is readily reabsorbed from the descending limb by osmosis. [ 3 ] Also, the medullary interstitium is highly concentrated (because of the activity of the ascending limb), leading to a strong osmotic gradient from the descending limb to the medulla.
This allows for a countercurrent exchange system whereby the medulla becomes increasingly concentrated, but at the same time setting up an osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH.
The medulla is divided into outer and inner regions. The outer region consists of short loops of Henle and collecting ducts, and the inner region consists of long loops of Henle and collecting ducts. [28] After passing through the loop of Henle, the fluid becomes hypertonic relative to the blood plasma. [78] The renal portal system is absent in ...
The last step in the processing of the ultrafiltrate is excretion: the ultrafiltrate passes out of the nephron and travels through a tube called the collecting duct, which is part of the collecting duct system, and then to the ureters where it is renamed urine. In addition to transporting the ultrafiltrate, the collecting duct also takes part ...
The medullary ascending limb is largely impermeable to water. Sodium (Na +), potassium (K +) and chloride (Cl −) ions are reabsorbed by active transport. The predominant mechanism of active transport in this segment is through the Na + /K + /Cl − co-transporter NKCC2 as well as the sodium/hydrogen exchanger NHE3. [3]