During urine formation, the tubular cells secrete substances like $\mathrm{H}^{+}, \mathrm{K}^{+}$and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.

PCT $\rightarrow$ Selective secretion of $\mathrm{H}^{+}$, ammonia and $\mathrm{K}^{+}$into the filtrate.

DCT $\rightarrow$ Capable of reabsorption of $\mathrm{HCO}_3^{-}$and selective secretion of $\mathrm{H}^{+}, \mathrm{K}^{+}$and $\mathrm{NH}_3$.