Acid–Base Balance and pH Regulation in the Human Body

Hydrogen Ions (H+) and pH

Definition and Importance of H+ in Biological Systems

The concentration of hydrogen ions (H+) in aqueous solutions is a fundamental aspect of cellular and systemic physiology. All water-based solutions, including those in the human body (intracellular fluid, plasma, interstitial fluid), contain some H+ ions. The regulation of H+ is critical because these ions are highly reactive and can disrupt the structure and function of proteins and other biomolecules.

• Acid: A substance that increases the concentration of H+ ions when added to a solution.

• Base (Alkali): A substance that decreases the concentration of H+ ions when added to a solution.

• Even pure water contains H+ due to the dissociation reaction: $\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$

• In pure water, [H+] ≈ $1 \times 10^{-7}$ M.

• Excess H+ can denature proteins, especially enzymes and channel proteins, making them non-functional.

pH: Measurement of Hydrogen Ion Concentration

pH is the standard method for expressing the concentration of H+ in a solution. It is defined as the negative logarithm (base 10) of the H+ concentration:

$\mathrm{pH = -\log[H^+]}$

• Lower pH values indicate higher H+ concentrations (more acidic).

• Higher pH values indicate lower H+ concentrations (more basic or alkaline).

• Examples:

  • pH 2 = 0.01 M [H+]

  • pH 5 = 0.00001 M [H+]

  • pH 10 = 0.0000000001 M [H+]

Acid–Base Balance in the Human Body

pH Scale and Physiological Ranges

The pH scale ranges from 0 (most acidic) to 14 (most basic). In the human body, the homeostatic value for plasma pH is approximately 7.4 (typically 7.38–7.42). Deviations from this range can have significant physiological consequences.

• Acidosis: Plasma pH < 7.35 (excess H+); can cause decreased CNS activity, muscle weakness, arrhythmias, and coma.

• Alkalosis: Plasma pH > 7.45 (deficient H+); can cause increased CNS activity, tingling, muscle cramps, and respiratory paralysis.

• Neutral: pH = 7 (equal [H+] as pure water).

Sources of Acids and Bases in the Body

Acids and bases in the body originate from various sources:

• Dietary acids: Amino acids, fatty acids, etc.

• Metabolic acids: Lactic acid (anaerobic metabolism), ketoacids (lipid breakdown).

• CO2: The largest source; CO2 dissolves in blood and forms H+ and HCO3- ($\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$).

• Bases are rare in diet/metabolism; the body primarily eliminates excess acids.

Mechanisms of pH Homeostasis

Buffers, Lungs, and Kidneys

The body maintains pH homeostasis through three main mechanisms:

1. Buffers: Substances that minimize pH changes by absorbing or releasing H+ as needed.

   • Bicarbonate (HCO3-): The most important buffer in plasma. Reaction: $\mathrm{H^+ + HCO_3^- \rightleftharpoons CO_2 + H_2O}$

   • Other buffers: Proteins, hemoglobin, phosphates, ammonia.

   • Buffers do not eliminate H+, but reduce the impact of acid/base addition.

2. Lungs (Ventilation): Regulate CO2 levels, thus affecting H+ concentration.

   • Increased ventilation (hyperventilation): Decreases CO2, decreases H+, raises pH.

   • Decreased ventilation (hypoventilation): Increases CO2, increases H+, lowers pH.

   • Allows rapid response to pH changes, but limited by respiratory needs.

3. Kidneys: Regulate excretion of H+ and HCO3- in urine.

   • Low pH: Kidneys excrete H+, reabsorb HCO3-.

   • High pH: Kidneys retain H+, excrete HCO3-.

   • Slower but essential for long-term acid-base balance.

Acid–Base Disturbances: Classification and Diagnosis

Types of Acid–Base Disturbances

Acid–base disturbances are classified by their origin:

• Respiratory disturbances: Caused by changes in CO2 levels.

  • Respiratory acidosis: Too much CO2 (e.g., emphysema, hypoventilation).

  • Respiratory alkalosis: Too little CO2 (e.g., hyperventilation).

• Metabolic disturbances: Caused by non-respiratory factors.

  • Metabolic acidosis: Excess acid from metabolism (e.g., diabetic ketoacidosis).

  • Metabolic alkalosis: Excess base or loss of acid (e.g., vomiting, loss of gastric acid).

Diagnosis Using Blood Values

Measuring the partial pressure of CO2 (PCO2) and the concentration of HCO3- in the blood helps distinguish between respiratory and metabolic causes:

Disturbance	pH	CO2	HCO3-
Respiratory Acidosis	Low	High	High
Metabolic Acidosis	Low	Low	Low
Respiratory Alkalosis	High	Low	Low
Metabolic Alkalosis	High	High	High

Summary Table: Plasma PCO2, Ions, and pH in Acid–Base Disturbances

The following table summarizes the typical changes in plasma PCO2, HCO3-, and pH during different acid–base disturbances:

Condition	pH	PCO2	HCO3-
Respiratory Acidosis	Low	High	High
Metabolic Acidosis	Low	Low	Low
Respiratory Alkalosis	High	Low	Low
Metabolic Alkalosis	High	High	High

Key Equations and Concepts

• pH calculation: $\mathrm{pH = -\log[H^+]}$

• Water dissociation: $\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$

• Bicarbonate buffer system: $\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}$

Additional info: The above notes integrate textbook-level explanations, relevant equations, and visual aids to provide a comprehensive overview of acid–base balance and regulation in the human body, suitable for ANP college students.