Function of medullary collecting duct
Ans. d. (All of the above) (Ref. Guyton physiology 11th /pg. 337-338)MEDULLARY COLLECTING DUCT# Although the medullary collecting ducts reabsorb less than 10% of the filtered water and sodium, they are the final site for processing the urine and, therefore, play an extremely important role in determining the final urine output of water and solutes. The epithelial cells of collecting ducts are cuboidal in shape with smooth surfaces and few mitochondria.# Special characteristics of this tubular segment are as follows:(1) The permeability of the medullary collecting duct to water is controlled by the level of ADH. With high levels of ADH, water is avidly reabsorbed into the medullary interstitium. thereby reducing the urine volume and concentrating most of the solutes in the urine.(2) Unlike the cortical collecting tubule, the medullary collecting duct is permeable to urea. Therefore, some of the tubular urea is reabsorbed into the medullary interstitium, helping to raise the osmolality in this region of the kidneys and contributing to the kidneys' overall ability to form a concentrated urine.(3) 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 key role in regulating acidbase balance.Educational Points:The functional characteristics of the late distal tubule and cortical collecting tubule can be summarized as follows:# The tubular membranes of both segments are almost completely impermeable to urea, similar to the diluting segment of the early distal tubule; thus, almost all the urea that enters these segments passes on through and into the collecting duct to be excreted in the urine, although some reabsorption of urea occurs in the medullary collecting ducts.# Both the late distal tubule and the cortical collecting tubule segments reabsorb sodium ions, and the rate of reabsorption is controlled by hormones, especially aldosterone.# The intercalated cells of these nephron segments avidly secrete hydrogen ions by an active hydrogen-ATPase mechanism. This process is different from the secondary active secretion of hydrogen ions by the proximal tubule because it is capable of secreting hydrogen ions against a large concentration gradient, as much as 1000 to 1. Thus, the intercalated cells play a key role in acid-base regulation of the body fluids.# The permeability of the late distal tubule and cortical collecting duct to water is controlled by the concentration of ADH, which is also called vasopressin. With high levels of ADH, these tubular segments are permeable to water, but in the absence of ADH, they are virtually impermeable to water. This special characteristic provides an important mechanism for controlling the degree of dilution or concentration of the urine.
Avid water reabsorption under the influence of high ADH levels
Resorption of some urea into interstitium
Secretion of H+ ion even against large concentration gradient
All of the above
Kidneys and Body Fluids