Acid-base status and pH
Acid-base status (ABS), or acid-base balance, is the constancy of the ratio of acids and bases in the internal environment of the body. It is evaluated by the pH indicator, which is the negative decimal logarithm of the hydrogen ion concentration. Thus, at an H⁺ concentration of 10⁻⁷ mol/L, the pH is 7.
Normally, the pH of arterial blood is 7.40, and venous blood is 7.35. The lower value in venous blood is associated with its saturation with carbon dioxide: under the action of the enzyme carbonic anhydrase, CO₂ combines with water to form carbonic acid H₂CO₃, which partially dissociates into HCO₃⁻ and H⁺.
The rigidity of the pH constant is due to the dependence of protein function—enzymes, transporters, receptors—on a strictly defined optimal pH. A shift in pH disrupts or completely halts their functioning. The maintenance of pH is provided by effector systems: pulmonary ventilation, blood buffer systems, the excretory function of the kidneys, sweating, and GI tract activity.
Bicarbonate buffer system
It consists of carbonic acid H₂CO₃ and sodium bicarbonate NaHCO₃ (in dissociated form as the HCO₃⁻ ion) in a ratio of approximately 1 : 20.
- In case of H⁺ excess, the HCO₃⁻ ion binds the hydrogen ion to form H₂CO₃, which, under the action of carbonic anhydrase, breaks down into H₂O and CO₂; carbon dioxide is eliminated by the lungs.
- In case of an excess of bases (OH⁻), carbonic acid donates H⁺, which combines with the hydroxyl ion to form water.
This system works in close cooperation with pulmonary ventilation.
Protein buffer system
The buffer properties of proteins are due to their amphoteric nature—the ability to act as both acids and bases.
- In an alkaline environment, proteins act as H⁺ donors: the donated hydrogen ion neutralizes OH⁻ to form water.
- In an acidic environment, proteins dissociate to form OH⁻, which binds excess H⁺, also forming water.
Phosphate buffer system
Formed by dihydrogen phosphate NaH₂PO₄ and monohydrogen phosphate Na₂HPO₄ in a ratio of about 1 : 4. In case of base excess, dihydrogen phosphate donates H⁺, turning into monohydrogen phosphate and forming water. In case of H⁺ excess, monohydrogen phosphate binds a hydrogen ion, turning into dihydrogen phosphate.
The phosphate buffer predominantly functions in the renal tubules and inside cells, where the concentration of phosphates is higher than in plasma. In the tubules, the Na⁺ ion from Na₂HPO₄ is reabsorbed into the blood, and its place is taken by H⁺, formed during the dissociation of carbonic acid in tubular cells. The resulting NaH₂PO₄ is excreted in the urine, and the HCO₃⁻ ion enters the blood, replenishing the bicarbonate buffer. Thus, the phosphate system supports the bicarbonate one.
Hemoglobin buffer system
Hemoglobin is capable of binding and releasing hydrogen ions, changing its affinity depending on oxygen saturation.
- In tissues: the KHbO₂ compound releases O₂ for tissue respiration. The CO₂ resulting from metabolism enters the erythrocyte, where, under the action of carbonic anhydrase, it turns into H₂CO₃ with subsequent dissociation into HCO₃⁻ and H⁺. The hydrogen ion displaces K⁺ from hemoglobin and binds with it—forming HHb.
- In the lungs: high O₂ partial pressure promotes hemoglobin oxygenation. A change in protein conformation leads to the release of H⁺. The incoming KHCO₃ dissociates into K⁺ and HCO₃⁻; bicarbonate binds H⁺ to form H₂CO₃, which breaks down into H₂O and CO₂, which is exhaled. Potassium re-attaches to hemoglobin, restoring the KHbO₂ compound.
Cooperation of buffer systems and effector organs
All blood buffer systems work in coordination: the hemoglobin system ensures CO₂ transport and is closely linked to the bicarbonate system, the phosphate system replenishes the bicarbonate reserve through the renal tubules, and the protein system complements the action of the others in plasma and tissues.
Maintaining pH is impossible without the participation of effector systems: pulmonary ventilation (removal of CO₂), kidneys (secretion of H⁺, reabsorption of HCO₃⁻), GI tract (secretion of acids and bases by glands), sweating (elimination of a portion of acids and bases). The joint action of these mechanisms ensures the narrow physiological limits of blood pH.
Key points
- Arterial blood pH is 7.40, venous is 7.35; the difference is due to the content of CO₂ and H⁺.
- pH rigidity is necessary for the function of enzymes and other proteins at their optimal pH.
- The bicarbonate buffer (H₂CO₃ / NaHCO₃, 1:20) works in cooperation with the lungs.
- The protein buffer is realized due to the amphotericity of proteins.
- The phosphate buffer (NaH₂PO₄ / Na₂HPO₄, 1:4) predominates in the renal tubules and inside cells.
- The hemoglobin buffer binds H⁺ in tissues and releases it in the lungs during oxygenation.
- Carbonic anhydrase catalyzes the key reaction CO₂ + H₂O ⇌ H₂CO₃ in all buffer interactions.
- pH constancy is maintained by the joint work of blood buffers, lungs, kidneys, GI tract, and skin.