Tubular secretion transfers molecules from peritubular capillaries to renal tubular lumen. Secretory mechanisms in the tubules are not drug specific, but depend on non-selective carrier systems. Two main systems are responsible for the active secretion of drugs which are either weak acids (anions) or weak bases (cations).
Tubular secretion is an active secretion process and it requires a carrier and a supply of energy. Because of this, the carriers can transport drug
The carriers can also be saturated, although this usually only occurs in the situations involving drug-drug interactions where one drug competitively inhibits the secretion of another.