Human urine is usually acidic because:

1. excreted plasma proteins are acidic
2. potassium and sodium exchange generates acidity
3. hydrogen ions are actively secreted into the filtrate.
4. the sodium transporter exchanges one hydrogen ion for each sodium ion, in peritubular capillaries.

Urine has acidic nature because hydrogen ions(H⁺) are components of an acid which are secreted into the filtrate.