Page 9 - LN- EXCRETION

i. Proximal Convoluted Tubule (PCT)
The epithelial cells of the PCT have numerous microvilli (simple cuboidal brush-border
epithelium) which increase the surface area available for reabsorption.
The process of reabsorption mostly (65%) takes place within PCT (i.e., nearly all of the
essential nutrients, 70-80% of electrolytes and water). PCT also helps in the absorption of
–
HCO from the filtrate.
Selective secretion of hydrogen ions, ammonia and potassium ions takes place here to
maintain the pH and ionic balance of the body fluids. The filtrate is considered isotonic to
blood plasma.
ii. Henle’s Loop
Reabsorption in Henle’s loop is minimum, besides it, this plays an important role in
maintaining the high osmolarity of medullary interstitial fluid. Two portions of Henle’s loop,
play different role in osmoregulation such as
a. Descending Limb of Loop of Menu?
Water is reabsorbed here due to increasing osmolarity of interstitial fluid but, sodium and
other electrolytes are not reabsorbed here. This concentrates the filtrate as it moves down.
b. Ascending Limb of Loop of Menu?
–
+
This segment is impermeable to water but permeable to K ,Cl and Na and partially
+
+
permeable to urea. Thus, in the thick ascending limb of the loop of Henle Na , K , Mg and
2+
+
–
Cl are reabsorbed.
Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of
electrolytes to the medullary fluid.
iii. Distal Convoluted Tubule (DCT)
Active reabsorption of sodium ions from the filtrate (under the influence of aldosterone)
takes place. Water is also reabsorbed here under the influence of Antidiuretic Hormone
(ADH).
With associated secretion of potassium (K ), hydrogen (H ) ions, NH , some Cl (chloride) ions
–
–
+
+
and HCO are also reabsorbed here. It is necessary to maintain the pH and sodium-
–
potassium balance in blood. This makes the filtrate isotonic to blood plasma.