Which of the following is NOT a function of the kidneys in maintaining acid-base balance?

The function of the kidneys in maintaining acid-base balance is multifaceted, and each of the options provided plays a significant role, except for the correct choice.

The kidneys are primarily involved in the regulation of bicarbonate levels, which acts as a buffer for hydrogen ions, thereby helping to maintain the pH of the blood. Reabsorption of bicarbonate is essential for increasing bicarbonate levels in the blood, contributing to a more alkaline environment when necessary.

Excretion of hydrogen ions is another vital function as it allows the body to eliminate excess acidity. By removing hydrogen ions through urine, the kidneys help to maintain a stable pH level in the extracellular fluid.

The conversion of ammonia to ammonium is also crucial. Ammonia can bind with hydrogen ions, thus assisting in the excretion of acid in the form of ammonium. This process is particularly important when the body is in a state of acidosis, as it helps to remove excess acid from the body's systems.

However, the production of carbonic acid is not a direct function of the kidneys. Carbonic acid is usually formed in the body through the combination of carbon dioxide and water and primarily regulated through respiratory function. The kidneys do participate in maintaining acid-base balance through other means, but