Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen. This secretion is caused mainly by active transport. Usually only a few substances are secreted. These substances are present in great excess, or are natural poisons.
The third process by which the kidneys clean or filter blood (regulating its composition and volume) is called tubular secretion and involves substances being added to the tubular fluid. This removes excessive quantities of certain dissolved substances from the body and also maintains the blood at a normal healthy pH level; typically in the range of pH 7.35 to pH 7.45. The substances that are secreted into the tubular fluid for removal from the body include: potassium ions (K+), hydrogen ions (H+), ammonium ions (NH4+), creatinine, urea, some hormones, and some drugs (e.g., penicillin). Tubular secretion occurs from the epithelial cells that line the renal tubules and collecting ducts .
It is the tubular secretion of H+ and NH4+ from the blood into the tubular fluid (i.e., urine - which is then excreted from the body via the ureter, bladder, and urethra) that helps to keep blood pH at its normal level. The movement of these ions also helps to conserve sodium bicarbonate (NaHCO3). The typical pH of urine is about 6.0.
Urine that is formed via the three processes of filtration, reabsorption, and secretion, trickles into the kidney pelvis. At this final stage it is only approximately one percent of the originally filtered volume, but it includes high concentrations of urea, creatinine, and variable concentrations of ions.