8/17/2019 The Collecting Tubules.docx

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The Collecting Tubules

Histoanatomy

Urine passes from the distal convoluted tubules to collecting tubules, the last part of each nephron,

which join each other to form larger, straight collecting ducts that run to the tips of the medullary

pyramids and empty into the minor calyces. The collecting tubules are lined with cuboidal epithelium

and have a diameter of approximately 40 µm. ells of the converging collecting ducts are more

columnar and the duct diameters reach !00 µm near the tips of the medullary pyramids.

"long their entire extent, collecting tubules and ducts are composed mainly of wea#ly staining principalcells with few organelles and scanty microvilli. The intercellular limits of the cells are clearly visible in

the light microscope. Ultrastructurally the principal cells can be seen to have basal membrane

infoldings, consistent with their role in ion transport. $cattered among the principal cells are variably

dar#er intercalated cells with more abundant mitochondria which help regulate the acid%base balance

by secreting &' and absorbing &()*.

Physiology

 The principal cells reabsorb sodium and water from the

lumen and secrete potassium ions into the lumen. The

intercalated cells reabsorb potassium ions and secrete

hydrogen ions into the tubular lumen. $odium

reabsorption and potassium secretion by the principalcells depend on the activity of a sodium%potassium

"T+ase pump in each cells basolateral membrane.

 This pump maintains a low sodium concentration inside

the cell and, therefore, favors sodium di-usion into the

cell through special channels. The secretion of 

potassium by these cells from the blood into the tubular

lumen involves two steps /1 Potassium enters the

cell because of the sodium-potassium ATPase pump, which maintains a high intracellular

potassium concentration, and then /!1 once in the cell, potassium diuses down its concentration

gradient across the luminal membrane into the tubular uid . The principal cells are the primary

sites of action of the potassium%sparing diuretics, including spironolactone, eplerenone, amiloride, andtriamterene.

2edullary collecting ducts reabsorb less than 0 percent of the 3ltered water and sodium, they are the

3nal site for processing the urine. $pecial characteristics of this tubular segment are as follows

. The permeability of the medullary collecting duct to water is controlled by the level of "&. 5ith

high levels of "&, water is avidly reabsorbed into the medullary interstitium, thereby reducing the

urine volume and concentrating most of the solutes in the urine.!. Unli#e the cortical collecting tubule, the medullary collecting duct is permeable to urea and there

are special urea transporters that facilitate urea di-usion across the luminal and basolateral

membranes. Therefore, some of the tubular urea is reabsorbed into the medullary interstitium,

helping to raise the osmolality in this region of the #idneys and contributing to the #idneys overall

ability to form concentrated urine.).  The medullary collecting duct is capable of secreting hydrogen ions against a large concentration

gradient, as also occurs in the cortical collecting tubule. Thus, the medullary collecting duct also

plays a #ey role in regulating acid%base balance.