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 offiltrate 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 hydrogenion 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 mineralions.
