Once in the intravascular fluid it combines with water to form carbonic acid under the influence of carbonic anhydrase within the red blood cell, then the majority of it attaches to haemoglobin as carbaminohaemaglobin. Having travelled through the heart to the lungs it will be converted to carbonic acid then dissociate back into water and CO2. Much of the CO2 is excreted at this time by normal respiratory processes. Any remaining CO2 is converted back into carbonic acid and again, undergoing the same chemical processes, is transported via the circulatory system to the kidneys. That is to say, in acidosis the kidneys will excrete hydrogen and retain bicarbonate, making it available for further buffering.
Protein is the most powerful buffer.
Protein predominates in the intracellular fluid compartment and is therefore, more important as an intracellular buffer than as a plasma buffer. Haemoglobin is part of the protein buffering system and when the body has a large acid load and Hb is acting as a buffer O2 carrying capacity may well be reduced. Phosphate acts predominantly as an intracellular and urinary buffer. This is because phosphate levels are higher intracellularly and in the renal tubular fluid than in blood plasma. The respiratory response to an increase in acid production is rapid and initially very effective in altering pH. It has already been established that respiratory regulation of pH occurs via the carbonic acid buffering pathway and is dependent on adequate stores of bicarbonate and adequate gas exchange.