compensation mechanisms for normal acid-base disturbances.

Acids produced by normal metabolism are added to the pool and disrupt homeostasis.

‘These are called metabolic acids because they do not arise from CO2’ – does this mean CO2 produced as a result of cellular respiration does not contribute to metabolic acidosis? Does it count as respiratory?

Cells possess buffers that protect them from their own metabolism.The most common intracellular buffer is phosphate. Two reasons: cells have lots of phosphate anyway, as it is primarily used for energy metabolism (ATP, ADP, creatine phosphate). Also, phosphate has a pKa of 6.8, which is closer to the cell’s ideal then bicarb.

Cells also possess membrane transport proteins that regulate the export of protons / base according to pH. A decrease in extracellular pH caused by one cell group will cause others to retain their H+ and increase buffering instead, until normal extracellular pH is restored. In this manner, the H+ ions in a body are dispersed evenly.


Respiration compensates for metabolic acids by expiring CO2.

As H+ is increasingly produced, it collides more frequently with bicarb to form carbonic acid. The metabolic acid that is was previously bound to is no longer a threat and is excreted by the kidney, eventually. As CO2 is expelled by the lungs, carbonic acid (catalyzed by carbonic anhydrase) tends to form CO2 (rather than bicarb; where would it put the H+?)  to compensate. In the process, a H+ is removed from the pool.

As a result, the bicarbonate (and indeed total CO2) supply dwindles.

–          Bicarbonate is a counted anion. When it accepts the H+ from a metabolic acid, it becomes neutral and the charge is transferred to the conjugate base of the metabolic acid, an uncounted anion.

–          The excessive loss of Bicarb through this route (buffering), resulting in increased uncounted anions, is reflected in plasma samples as an increased anion gap.

–          Bicarbonate that leaves the body with its charge still attached will be accompanied by the other counted ions, and will not result in an increase in the normal anion gap.

Kidneys compensate for metabolic acids by producing HCO3.

In the process, H+ is removed from the pool.


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