There are 3 processes involved in urine formation which are glomerular filtration, tubular reabsorption and tubular secretion. Tubular secretions occurs from the epithelial cells lining the renal tubules and collecting ducts. The importance of this process is the removal of excessive quantities of dissolved substances in the body. This also maintains the pH of the blood at the normal level. Substances secreted into the tubular fluid include potassium, hydrogen, ammonium, creatinine, urea, some hormones and drugs. The balance of H+ and NH4+ in the blood keeps the normal pH. Typically pH of urine is about six.
