Physiological mechanisms of pH regulation

Respiratory mechanism (Pulmonary ventilation)

The respiratory system provides rapid compensation for ABB disorders. This process is controlled via the respiratory center, which responds to the concentration of hydrogen ions ($H^+$) and the partial pressure of carbon dioxide ($pCO_2$).

In acidosis (acidification of the environment):

1. The concentration of $H^+$ in the blood and brain extracellular fluid increases.
2. This stimulates central and peripheral chemoreceptors.
3. Neurons of the inspiratory center are activated.
4. Hyperventilation develops — breathing becomes more frequent and deeper.
5. The body actively rids itself of $CO_2$, which leads to a drop in carbonic acid ($H_2CO_3$) levels and a decrease in the amount of free hydrogen ions.

In alkalosis (alkalinization of the environment), the process goes in the opposite direction:

• 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 increasing their concentration back to normal.

Quantitative dependence here is extremely strict: if the pH drops by 0.1, the ventilation volume increases by exactly twofold. Conversely, when the pH rises 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 slower than the lungs, but perform fundamental work in altering ionic composition. All key events unfold in the renal tubules.

The mechanism consists of two opposing processes:

• Reabsorption — the return of bicarbonate ($HCO_3^-$) and other beneficial metabolites from primary urine into the bloodstream.
• Secretion — active excretion of hydrogen ions ($H^+$) into the tubular lumen. This process is coupled with the reabsorption of sodium ions ($Na^+$).

The intensity of these processes is not constant. It flexibly adapts to the current needs of the body: during ABB 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.

The following reactions affecting ABB occur continuously in hepatocytes:

• Oxidation of various organic acids.
• Deamination of amino acids (cleavage of the amino group).
• Synthesis of urea, which utilizes ammonia and ammonium chloride.

Auxiliary regulation mechanisms

In addition to the lungs, kidneys, and liver, additional systems participate in maintaining pH, and their role is considered auxiliary:

1. Sweating. Through sweat, the skin can excrete a certain volume of both acidic and basic metabolic products.
2. Digestive tract. The pancreas secretes pancreatic juice rich in bicarbonates ($HCO_3^-$) into the intestinal lumen. In addition, some acids and bases can be excreted directly via the gastrointestinal tract.
3. External link of self-regulation. These are human behavioral reactions: dietary, drinking, and salt behavior. Through them, the body receives water, salts, and nutrients from the outside (exogenously), which also affects the final acid-base balance.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of sensitivity of peripheral and central chemoreceptors
• Biochemistry of urea synthesis in the liver as a neutralization method
• Ion pumps of the renal tubules: hydrogen-sodium exchange
• The role of pancreatic juice in chyme buffering

Other topics in the "Blood" section

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