Functions in the acid-base balance

Functions in the acid-base balance b. Ammonia excretion  

Along with the lungs, the kidneys are particularly involved in keeping the pH value of the extracellular fluid constant (see p. 288). The contribution made by the kidneys particularly involves resorbing HCOs and actively excreting protons. 

An important function of the secreted H ions is to promote HCOs resorption (top right). Hydrogen carbonate, the most important buffering base in the blood, passes into the primary urine quantitatively, like all ions. In the primary urine, HCOa reacts with H ions to form water and CO2, which returns by free diffusion to the tubule cells and from there into the blood. In this way, the kidneys also influence the C02/HC03 buffering balance in the plasma. 

Approximately 60 mmol of protons are excreted with the urine every day. Buffering systems in the urine catch a large proportion of the H ions, so that the urine only becomes weakly acidic (down to about pH 4.8). 

An important buffer in the urine is the hydrogen phosphate/dihydrogen phosphate system (HP04 /H2P04 ). In addition, ammonia also makes a vital contribution to buffering the secreted protons. 

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