Acid-Base Balance and Theories

Acid-Base Balance

• Blood pH is maintained within a narrow range (7.35-7.45) for optimal enzyme function.
• Metabolic reactions produce acids (volatile and non-volatile).
• Bicarbonate is generated from organic acids, which can significantly affect pH.
• Understanding acids, bases, and pH is crucial for comprehending acid-base balance.

Bronsted-Lowry Theory

• Acids are proton donors.
• Bases are proton acceptors.
• Examples: HCl → H+ + Cl-; H₂CO₃ ↔ H⁺ + HCO₃⁻;

Strong vs. Weak Acids & Bases

• Strong acids dissociate completely in solution, producing high H+ concentrations. (e.g., HCl, HNO₃, H₂SO₄)
• Weak acids do not completely ionize, resulting in lower H+ concentrations. (e.g., H₂CO₃, CH₃COOH)
• Conjugate bases of strong acids are weak bases, while conjugate bases of weak acids are strong bases.

Dissociation of Water

• The degree of dissociation increases with temperature.
• Water's H+ concentration is 1/100,000,000 gram moles per liter at 25°C.
• This applies to all solutions.

Buffers

• Resist changes in pH upon the addition of small amounts of acid or alkali.
• Composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• Examples: CH₃COOH/CH₃COONa; H₂CO₃/HCO₃⁻.
• Effectiveness is highest at pH = pKa

Factors Affecting Buffer pH

• pKa value: Lower pKa = lower buffer pH
• Ratio of salt to acid: Ration remains the same with no change in pH.

Buffering Capacity

• Measures the ability to resist changes in pH when acids or bases are added.
• Determined by the absolute concentration of salt and acid.
• Most effective when salt:acid = 10:1 or 1:10.

Maintenance of Acid-Base Balance

• Three mechanisms: buffers, respiration, and kidneys.

Buffer Types

• Bicarbonate buffer (major extracellular buffer).
• Phosphate buffer (major intracellular buffer).
• Protein buffer (important in RBCs and plasma)
• Hemoglobin Buffer

Bicarbonate Buffer

• A critical plasma buffer system (HCO₃⁻ and H₂CO₃)
• Ratio of HCO₃⁻ to H₂CO₃ should be 20:1 to maintain pH 7.4.
• Bicarbonate concentration is higher (20 times).
• Controlled by kidneys (metabolic component).
• Controlled by lungs (respiratory component)

Phosphate Buffer

• Primary intracellular buffer
• pKa = 6.8 (close to blood pH)
• Less efficient if plasma pH is 7.4 due to low concentrations

Protein Buffers

• Important in plasma and intracellular fluids
• Plasma proteins (Pr-/HPr) act as buffers at pH 7.4
• Hemoglobin (important in RBCs).

Respiratory Mechanisms

• Controls H₂CO₃ (carbonic acid) levels in plasma.
• Hyperventilation (excretion of CO₂) or hypoventilation (accumulation of CO₂) adjusts pH.
• Rapid response mechanism (2-3 minutes)
• Not sufficient for complete pH restoration.