Which segment is the site of secretion and regulated reabsorption of ions and pH homeostasis?

Explanation

Tubular secretion: Substances such as `H^(+), K^(+), NH_(4)^(+) , creatinine and organic acids move into the filtrate from the peritubular capillaries into the tubular fluid. Most of the water is absorbed in the proximal convoluted tubule and `Na^(+)` is exchanged for water in the loop of Henle. Hypotonic fluid enters the distal convoluted tubule and substances such as urea and salts pass from peritubular blood into the cells of DCT. The urine excreted contains both filtered and secreted substances. Once it enters the collecting duct, water is absorbed and concentrated hypertonic urine is formed. For every `H^(+)` secreted into the tubular filtrate, a Na+ is absorbed by the tubular cell. The H+ secreted. combines with `HCO_(3)^(-), HPO_(3)^(-)` and `NH_(3_^(-)` and gets fixed as `H_2CO_(4)^(+), H_2PO_(4)^(+)` and `NH_(4)^(+)` respectively. Șince `H^(+)` gets fixed in the fluid reabsorption of `H^(+)` is prevented.