Respiratory Mechanism (Pulmonary Ventilation)
The respiratory system provides rapid compensation for acid-base disturbances. This process is controlled via the respiratory center, which responds to hydrogen ion concentration ($H^+$) and the partial pressure of carbon dioxide ($pCO_2$).
During acidosis (medium acidification):
- The $H^+$ concentration in the blood and brain extracellular fluid increases.
- This stimulates central and peripheral chemoreceptors.
- Neurons of the inspiratory respiratory center are activated.
- Hyperventilation develops — breathing becomes more frequent and deeper.
- The body actively eliminates $CO_2$, which leads to a drop in carbonic acid ($H_2CO_3$) levels and a decrease in free hydrogen ions.
During alkalosis (medium alkalinization), the process goes in reverse:
- The $H^+$ level drops, which reduces the activity of inspiratory neurons.
- Hypoventilation occurs (decreased alveolar ventilation).
- Carbon dioxide is retained in the blood, and the amount of $H_2CO_3$ increases.
- Carbonic acid dissociates, releasing hydrogen ions and raising their concentration back to normal.
Quantitative dependence here is extremely strict: if the pH drops by 0.1, the ventilation volume doubles precisely. Conversely, if the pH increases by 0.1, ventilation drops by half. The main advantage of the pulmonary mechanism is its speed: normalization of $H^+$ levels occurs within minutes.
Renal Mechanism (Excretory Function)
The kidneys regulate acid-base balance more slowly than the lungs, but perform fundamental work by altering ionic composition. All key events unfold in the renal tubules.
This mechanism consists of two opposing processes:
- Reabsorption — returning bicarbonate ($HCO_3^-$) and other useful metabolites from primary urine back into the bloodstream.
- Secretion — active elimination of hydrogen ions ($H^+$) into the tubular lumen. This process is coupled with the reabsorption of sodium ions ($Na^+$).
Lumen intensity of these processes is not constant. It flexibly adapts to the body's current needs: during acid-base shifts, the kidneys can enhance or weaken the reabsorption of bases and the secretion of acids.
Metabolic Function of the Liver
The liver takes on the crucial task of metabolically neutralizing acids, preventing them from accumulating in the body.
In hepatocytes, the following reactions affecting acid-base balance occur continuously:
- Oxidation of various organic acids.
- Deamination of amino acids (cleavage of the amino group).
- Urea synthesis, which utilizes ammonia and ammonium chloride.
Auxiliary Regulatory Mechanisms
In addition to the lungs, kidneys, and liver, additional systems participate in pH maintenance, playing an auxiliary role:
- Sweating. Through sweat, the skin can excrete a certain volume of both acidic and basic metabolic products.
- Gastrointestinal tract. The pancreas secretes pancreatic juice rich in bicarbonates ($HCO_3^-$) into the intestinal lumen. Additionally, some acids and bases can be excreted directly via the GI tract.
- External self-regulation link. These are human behavioral responses: dietary, drinking, and salt behavior. Through these, the body receives water, salts, and nutrients exogenously, which also affects the final acid-base balance.