Answer:
In addition to the role
of Proximal Convoluted Tublues (PCT) in selective reabsorption of materials
from the glomerular filtrate back into the blood of peritubular capillaries via
the renal interstitium, they also alter the composition of filtrate by the
process of secretion.
In its distal part,
epithelial cells extract certain excretory substances from the blood of peritubular
capillaries and secrete these into the filtrate.
Creatinine,
hippuric acid, pigments, drugs including penicillin are actively secreted-into
the filtrate in the proximal convolued tubule from the interstitial fluid.
Hydrogen ions and ammonia are also secreted into the proximal convoluted
tubules.
Urea enters
the filtrate via diffusion in the thin segment of ascending limb of Henle's
loop.
Maximum
hydrogen secretion occurs in the proximal convoluted tubules. Removal of hydrogen
ion and NH3 from the blood in the PCT and Distal Convoluted Tubule (DCT) helps in
maintaining pH of the blood, i.e., between 6 to 8.
Tubular secretion
although possess a minor role in functioning of the human kidney, but plays an
essential role in animals like marine fishes and desert amphibians, because
these animal do not possess well developed glomeruli hence their urine is
mainly formed by the tubular secretion of urea, creatinine and mineral ions.
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