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Metabolic Acidosis

Q: Which mechanisms provide acute protection during acidosis?

Acute compensation includes the activation of extracellular and cellular buffers, as well as increased alveolar ventilation to eliminate carbon dioxide.

Q: What role do the kidneys play in long-term compensation?

The kidneys perform ammoniagenesis, increase the secretion of titratable acids, enhance renal acidogenesis, and promote sodium reabsorption coupled with proton excretion.

Q: What causes secondary hyperaldosteronism?

It is caused by elevated angiotensin II levels, adrenal steroidogenesis disorders, and renal juxtaglomerular apparatus hyperplasia, as seen in Bartter syndrome.

Acidosis metabolica

For medical students2 min readUpdated 2026-10-10

Metabolic acidosis triggers immediate and long-term protective mechanisms aimed at neutralizing excess acids and restoring acid-base balance. Buffer systems, as well as the respiratory and urinary systems, are involved in these compensatory reactions.

Respiratory compensationPulmonary hyperventilation to eliminate carbon dioxide.

Renal mechanismsActivation of ammoniagenesis and titration of excreted acids.

Bone buffersMobilization of bone bicarbonate and phosphate buffers.

Extracellular buffersImmediate activation of bicarbonate and protein buffers.

Acute Compensation Mechanisms

Immediate protective reactions are activated without delay, providing rapid buffering and a respiratory response:

Activation of extracellular buffers: The bicarbonate and protein systems are the first to respond.
Respiratory compensation: Increased alveolar ventilation (marked hyperventilation) facilitates the active elimination of $CO_2$.
Cellular buffers: Hydrogen ions ($H^+$) are exchanged for potassium ions ($K^+$).

Long-Term Compensation Mechanisms

Long-term reactions engage at later stages, ensuring full replenishment of buffer capacities and the excretion of non-volatile acids through the kidneys and other systems:

Activation of ammoniagenesis: Formation of ammonium ions ($NH_4^+$) and effective proton excretion.
Increased acid secretion: Elimination of ions in the form of titratable acids ($NaH_2PO_4$).
Bone buffers: Mobilization of bone tissue bicarbonate and phosphate buffers.
Gastric secretion: Enhanced formation of hydrochloric acid ($HCl$) in the stomach to clear $H^+$ from the bloodstream.
Renal acidogenesis: Additional secretion of hydrogen ions and increased sodium ion ($Na^+$) reabsorption coupled with $H^+$ and $K^+$ secretion.
Hepatic mechanisms: Stimulation of gluconeogenesis from lactate and urea synthesis.

Secondary Hyperaldosteronism

This condition develops due to the stimulation of the zona glomerulosa of the adrenal cortex by factors of extra-adrenal origin.

Main Etiological Factors:

Angiotensin II: Elevated concentrations occur in chronic arterial hypertension and hypovolemia (via activation of the renin-angiotensin-aldosterone system).
Steroidogenesis disorders: Blockade of glucocorticoid and androgen synthesis leads to a compensatory rise in aldosterone production.
Juxtaglomerular apparatus pathology: Hyperplasia of the renal juxtaglomerular apparatus, characteristic of Bartter syndrome.

Endocrine Regulatory Factors

Regulatory changes also involve other endocrine structures:

ACTH: Elevated hormone levels stimulate glucocorticoid synthesis.
Parathyroid glands: Their hypofunction is accompanied by decreased blood calcium ($Ca^{2+}$) levels (hypocalcemia) and elevated phosphate content ($Na_2HPO_4$), causing hyperphosphatemia.

Mnemonic

Exhale $CO_2$ ($CO_2$) immediately and engage your buffers; the kidneys and bones will step in for the long haul.

Frequently asked questions

What are the types of metabolic acidosis based on the anion gap?

Metabolic acidosis is classified into two types depending on the anion gap.

High anion gap — typical of ketoacidosis, lactic acidosis, renal failure, and poisonings (methanol, ethylene glycol, salicylates).
Normal anion gap — also known as hyperchloremic metabolic acidosis, occurs with diarrhea resulting in bicarbonate loss and in renal tubular acidosis.

What are the main causes of metabolic acidosis?

The main causes of metabolic acidosis are a deficit of bicarbonate or an excess of non-volatile acids in the body.

Accumulation of non-volatile acids — occurs in tissue hypoxia (lactate), diabetes mellitus (ketoacidosis), and circulatory disorders.
Impaired acid excretion — develops due to impaired renal excretory function.
Bicarbonate loss — observed during diarrhea.
Exogenous causes — salicylate and methanol poisoning, as well as excessive administration of non-volatile acid solutions.

What clinical symptoms are characteristic of metabolic acidosis?

A characteristic clinical manifestation of metabolic acidosis is compensatory pulmonary hyperventilation.

Kussmaul breathing — deep, labored breathing aimed at compensating for acid-base disorders.
Altered mental status — varying degrees of impaired consciousness can develop, up to coma (diabetic ketoacidotic or uremic).
Cardiac arrhythmias — can occur as a consequence of metabolic acidosis.

What are the blood gas values in uncompensated metabolic acidosis?

In uncompensated metabolic acidosis, the laboratory findings include:

Blood pH — pH < 7.4.
Hypoxemia — decreased $O_2$.
Hypercapnia — increased $CO_2$.

An individual clinical example of metabolic acidosis: pH 7.25, $PaCO_2$ 28 mmHg, $HCO_3^-$ 12 mEq/L; anion gap 24; adequate compensation with an expected $PaCO_2$ of 26 ± 2.

Which mechanisms provide acute protection during acidosis?