Which substance serves as a major buffer in urine?

The major buffer in urine is phosphate. Phosphate ions, primarily in the form of dihydrogen phosphate (H2PO4-) and hydrogen phosphate (HPO4^2-), play a critical role in maintaining the pH of urine. When excess hydrogen ions (H+) are present in urine, the phosphate buffer system helps to neutralize these ions, thereby preventing significant fluctuations in urine pH. This buffering capacity is essential for maintaining acid-base balance in the body, especially as the kidneys filter blood and excrete waste products.

While bicarbonate is an important buffer in blood and helps regulate pH throughout the body, its primary role occurs in the plasma rather than in the urine itself. Ammonia also contributes to urinary buffering, but phosphate remains the most significant component. Urea plays a role in the nitrogen balance and is more a metabolic product rather than a buffering agent. In contrast, phosphate's ability to exist in various ionization states and its effective interaction with hydrogen ions make it the predominant buffer in urine.