What Is Acidosis?

Acidosis is a disturbance of acid-base balance in which acid accumulates in the body or bicarbonate is lost, driving blood pH downward. Arterial pH is normally held between 7.35 and 7.45, an unusually tight range for a physiological variable, and a measured value below 7.35 is termed acidemia. The distinction matters clinically: acidosis names the underlying process, while acidemia names the resulting low pH, and a patient can have two opposing processes at once whose net effect on pH is small. Enzyme activity, cardiac contractility, oxygen affinity of hemoglobin, and the distribution of potassium across cell membranes all shift with pH, which is why acid-base status is among the first measurements taken in critical care.

Acid-base regulation depends on three systems working on different timescales: chemical buffers in blood and tissue act within seconds, the lungs adjust carbon dioxide clearance within minutes, and the kidneys adjust bicarbonate reabsorption and acid excretion over hours to days. The bicarbonate buffer system links these, and the Henderson-Hasselbalch relation among pH, bicarbonate concentration, and the partial pressure of carbon dioxide gives the framework used to classify every acid-base disorder.

Metabolic and Respiratory Forms

Metabolic acidosis begins with a fall in serum bicarbonate, either because acid has consumed it or because it has been lost through the kidneys or gastrointestinal tract. Respiratory acidosis begins with retention of carbon dioxide from inadequate alveolar ventilation, as in chronic obstructive pulmonary disease, neuromuscular weakness, oversedation, or airway obstruction. Each triggers compensation by the other system: metabolic acidosis stimulates hyperventilation that lowers arterial carbon dioxide, and respiratory acidosis prompts renal bicarbonate retention that develops over several days. Compensation moves pH toward normal but does not restore it, so a fully normal pH alongside abnormal bicarbonate and carbon dioxide values suggests a mixed disorder rather than a compensated one.

The Anion Gap

Metabolic acidosis is subdivided using the serum anion gap, calculated as sodium concentration minus the sum of chloride and bicarbonate, with a reference range of roughly 8 to 12 milliequivalents per liter. A widened gap indicates that unmeasured anions have accumulated, pointing to lactate, ketoacids, or toxic metabolites from methanol or ethylene glycol ingestion, or to retained organic acids in kidney failure. A normal gap points instead to bicarbonate loss with chloride retention, as in diarrhea or renal tubular acidosis. The clinical reference on anion gap and non-anion gap metabolic acidosis sets out this classification along with the corrections applied when serum albumin, the main unmeasured anion in health, is low.

Measurement and Monitoring

Acid-base status is established by arterial blood gas analysis, which reports pH together with the partial pressures of carbon dioxide and oxygen, with bicarbonate calculated from those values. Blood gas analyzers use potentiometric electrodes for pH and carbon dioxide and an amperometric electrode for oxygen, and modern cartridges add lactate, electrolytes, and hemoglobin on the same sample. Because lactate rises early in tissue hypoperfusion, point-of-care devices are used to shorten turnaround time in emergency departments, and comparisons of handheld lactate analyzers against routine laboratory results report agreement adequate for triage decisions from samples of well under a milliliter. Research instrumentation is moving toward continuous measurement, including transcutaneous flexible sensors for photonic detection of pH and lactate that track both markers without repeated blood sampling.

Applications

Understanding and detecting acidosis is central to work in several fields, including:

• Critical care monitoring and early sepsis recognition
• Blood gas analyzer and biosensor design
• Dialysis and renal replacement therapy management
• Anesthesia and mechanical ventilation control
• Neonatal monitoring, including fetal scalp and cord blood sampling
• Sports physiology and exercise testing