Tubular secretion is a process in the kidneys where materials are transferred from peritubular capillaries to the renal tubular lumen, which is the opposite process of reabsorption/24%3A__Urinary_System/24.3%3A_Physiology_of_the_Kidneys/24.3E%3A_Tubular_Secretion). This process occurs mainly by active transport and passive diffusion/24%3A__Urinary_System/24.3%3A_Physiology_of_the_Kidneys/24.3E%3A_Tubular_Secretion). The substances that are secreted largely include hydrogen, creatinine, ions, and other types of waste products, such as drugs/24%3A__Urinary_System/24.3%3A_Physiology_of_the_Kidneys/24.3E%3A_Tubular_Secretion). The purpose of tubular secretion in kidney physiology is to remove waste products, such as excess potassium or hydrogen ions, from the bloodstream into the tubular fluid. This process occurs primarily in the distal and collecting tubules of the nephron. The secreted substances come from the blood in peritubular capillaries and pass through the interstitial fluid before going through the wall of the tubule into the inside of the tubule. Tubular secretion is an essential kidney function for rapidly clearing endogenous substances and administered medications from the circulation.