With metabolic acidosis, “acidosis” refers to a process that lowers blood pH below 7.35, and “metabolic” refers to the fact that it’s a problem caused by a decrease in the bicarbonate HCO3− concentration in the blood.
Normally, blood pH depends on the balance or ratio between the concentration of bases, mainly bicarbonate HCO3−, which increases the pH, and acids, mainly carbon dioxide CO2, which decrease the pH.
The blood pH needs to be constantly between 7.35 and 7.45, and in addition the blood needs to remain electrically neutral, which means that the total cations, or positively charged particles, equals the total anions, or negatively charged particles.
Now, not all of the ions are easy or convenient to measure, so typically the dominant cation, sodium Na+, which is typically around 137 mEq/L and the two dominant anions, chloride Cl−, which is about 104 mEq/L, and bicarbonate HCO3−, which is around 24 mEq/L, are measured.
The rest are unmeasured. So just counting up these three ions, there’s usually a difference, or “gap” between the sodium Na+ concentration and the sum of bicarbonate HCO3− and chloride Cl− concentrations in the plasma, which is 137 minus 128 (104 plus 24) or 9 mEq/L.
This is known as the anion gap, and normally it ranges between 3 and 11 mEq/L. The anion gap largely represents unmeasured anions like organic acids and negatively charged plasma proteins, like albumin.
So, basically, metabolic acidosis arises either from the buildup of acid in our blood, which could be because it’s produced or ingested in increased amounts, or because the body can’t get rid of it, or from excessive bicarbonate HCO3− loss from the kidneys or gastrointestinal tract.
The main problem with all of this is that they lead to a primary decrease in the concentration of bicarbonate HCO3− in the blood.
They can be broken down to two categories, based on whether the anion gap is high or normal. So, the first category of metabolic acidosis is a high anion gap metabolic acidosis.
In this case, the bicarbonate HCO3− ion concentration decreases by binding of bicarbonate HCO3− ions and protons H+, which results in the formation of H2CO3 carbonic acid, which subsequently breaks down into carbon dioxide CO2 and water H2O.