Acid-base balance: difference between acidosis and alkalosis, both metabolic and respiratory

By Cristiano Antonino On May 4, 2023

The terms “acidosis” and “alkalosis”, in physiology and pathology, mean a condition characterized by the alteration of the pH of the blood plasma, due to an excess of acidity or basicity

Under normal conditions, the pH of our body’s blood is maintained within a narrow range between 7.35 and 7.45 to ensure the proper functioning of the metabolic processes and the release of the right amount of oxygen to the tissues.

That said, when the pH value is altered, two situations can occur:

acidosis: consists of a decrease in pH below the value of 7.35 (acidemia);
alkalosis: consists of an excessive basicity of the blood which causes the pH to rise above 7.45.

Some authors prefer to indicate this condition with the term “acidemia” and with “acidosis” any condition that tends to add acids or remove bases from body fluids, even in the presence of a still normal pH.

In theory, the term “acidosis” could refer to any extracellular body fluid

By convention – however – academically this term is applied to plasma, considering it representative of body water.

The causes that can determine these two conditions are extremely varied.

In fact, many ailments and pathologies can interfere with the pH control in the body and cause the blood pH to go beyond the normal limits, such as for example some types of fever, encephalitis, neoplasms, some types of diet, traumas, pathologies lung diseases, pregnancy, drugs, neuroses and thyroid diseases.

Respiratory and metabolic acidosis and alkalosis

Both acidosis and alkalosis can be classified into “respiratory” and “metabolic” based on the plasma concentration of bicarbonate and carbon dioxide, which when evaluated together provide a rough indication of the main systems involved.

In summary we are talking about:

respiratory acidosis when the reduction (or tendency to decrease) in body plasma pH is linked to an increase in carbon dioxide (CO2) dissolved in the blood;
metabolic acidosis when the reduction (or tendency to decrease) in body plasma pH is due to an accumulation of acids or a reduction in bicarbonates;

and of:

respiratory alkalosis when the increase (or tendency to increase) in body plasma pH is caused by a decrease in carbon dioxide (CO2) dissolved in the blood;
metabolic alkalosis when the increase (or tendency to increase) in body plasma pH is caused by a loss of acids or an accumulation of bicarbonates.
anion gap

Calculation of the anion gap is often useful in the differential diagnosis of metabolic acidosis

The anion gap is calculated by subtracting the sum of the concentrations of Cl– and HCO3– from the plasma concentration of Na; an increase in the anion gap reveals the presence of an excess of acid equivalents, metabolic acidosis.

When normal anion gap metabolic acidosis is present, impaired renal H+ excretion should be suspected.