Immediate compensatory mechanisms
Immediate protective reactions are triggered instantly, providing rapid buffer defense and a respiratory response:
- Activation of extracellular buffers: the bicarbonate and protein systems are the first to be activated.
- Respiratory compensation: an increase in alveolar ventilation volumes (marked hyperventilation) promotes active elimination of $CO_2$.
- Cellular buffers: exchange of hydrogen ions ($H^+$) for potassium ions ($K^+$) is initiated.
Long-term compensatory mechanisms
Long-term reactions are engaged 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 efficient proton excretion.
- Increased acid secretion: elimination of ions in the form of titratable acids ($NaH_2PO_4$)
- Bone buffers: recruitment of bone bicarbonate and phosphate buffers.
- Gastric secretion: enhanced formation of hydrochloric acid ($HCl$) in the stomach to remove $H^+$ from the bloodstream.
- Renal acidogenesis: additional secretion of hydrogen ions and increased reabsorption of sodium ions ($Na^+$) coupled with the secretion of $H^+$ and $K^+$.
- Hepatic mechanisms: stimulation of gluconeogenesis from lactate and urea synthesis.
Secondary hyperaldosteronism
This condition develops due to the stimulation of the adrenal cortex zona glomerulosa by factors of extra-adrenal origin.
Main etiological factors:
- Angiotensin II: elevated concentrations are observed in chronic arterial hypertension and hypovolemia (via activation of the renin-angiotensin-aldosterone system).
- Impaired steroidogenesis: blockade of glucocorticoid and androgen synthesis leads to a compensatory rise in aldosterone production.
- Juxtaglomerular apparatus pathology: hyperplasia of the renal juxtaglomerular apparatus, characteristic, for example, of Bartter syndrome.
Endocrine regulatory factors
Regulatory changes also affect other endocrine structures:
- ACTH: an increase in this hormone stimulates glucocorticoid synthesis.
- Parathyroid glands: their hypofunction is accompanied by a decrease in blood calcium ($Ca^{2+}$) levels (hypocalcemia) and an increase in phosphate content ($Na_2HPO_4$), causing hyperphosphatemia.
Briefly: answers to topical questions
What are the types of metabolic acidosis based on the anion gap?
Metabolic acidosis is classified into two types depending on the magnitude of the anion gap.
- High anion gap — characteristic of ketoacidosis, lactic acidosis, renal failure, and poisonings (methanol, ethylene glycol, salicylates).
- Normal anion gap — also termed hyperchloremic, occurs in diarrhea with bicarbonate loss and renal tubular acidosis.
What are the main causes of metabolic acidosis?
The main causes of metabolic acidosis are a deficiency of bicarbonate or an excess of non-volatile acids in the body.
- Accumulation of non-volatile acids — occurs during tissue oxygen deprivation (lactate), diabetes mellitus (ketoacidosis), and circulatory disorders.
- Impaired acid excretion — develops when renal excretory function is impaired.
- Loss of bicarbonates — observed in diarrhea.
- Exogenous causes — poisonings with salicylates, methanol, as well as excessive intake 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 — labored, acidotic breathing aimed at compensating for acid-base disorders.
- Consciousness disturbances — varying degrees of altered consciousness can develop, up to coma (diabetic ketoacidotic or uremic).
- Cardiac arrhythmias — may occur as a consequence of metabolic acidosis.
What are the acid-base parameters in uncompensated metabolic acidosis?
In uncompensated metabolic acidosis, sources indicate the following parameters:
- Blood pH — pH < 7.4.
- Hypoxemia — decreased O2.
- Hypercapnia — increased CO2.
A separate clinical example of metabolic acidosis: pH 7.25, PaCO2 28, HCO3− 12; anion gap 24; adequate compensation, expected PaCO2 26 ± 2.
Exhale ($CO_2$) urgently and activate buffers, while the kidneys and bones will step in for the long term.
Class and exam questions
What mechanisms provide immediate protection during acidosis?
Immediate compensation includes the activation of extracellular and cellular buffers, as well as increased alveolar ventilation of the lungs to eliminate carbon dioxide.
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.
What causes secondary hyperaldosteronism?
It is caused by elevated angiotensin II levels, impaired adrenal steroidogenesis, and hyperplasia of the renal juxtaglomerular apparatus, as in Bartter syndrome.
What else to study on this topic
- Cellular buffer mechanisms and potassium ion exchange
- The role of ammoniagenesis and ammonium ion excretion in the kidneys
- The role of bone tissue in maintaining acid-base balance
- Biochemical disorders in Bartter syndrome
- The effect of parathyroid hypofunction on mineral metabolism
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