Hydrogen Ion Metabolism

Sources of Hydrogen Ions

Hydrogen ions (H+) are central to acid-base balance in biological systems. Their sources are diverse and include both metabolic and environmental origins.

• Phosphoric and sulfuric acids: Produced from metabolism of proteins and nucleic acids.

• Hydrochloric acid: Secreted in the stomach for digestion.

• Carbon dioxide metabolism: CO2 dissolves in water to form carbonic acid (H2CO3).

• Lactic acid production: Occurs during anaerobic metabolism.

• Ketone body formation: Results from fatty acid metabolism, especially in diabetes or starvation.

Example: Hydrogenase-containing microorganisms in the large intestine contribute to hydrogen ion production, which is a natural component of human flatus.

Hydrogen Ion Metabolism and Physiological pH

The body maintains a narrow physiological pH range (7.35–7.4) through continuous generation, buffering, and elimination of hydrogen ions. Buffering systems and compensatory mechanisms prevent drastic changes in pH.

• Buffering systems: Regulate free H+ to prevent rapid pH drops.

• Renal and respiratory compensation: Work together to maintain acid-base balance.

Acid-Base Control Systems

Chemical Control: Buffer Systems

Buffer systems are the first line of defense against pH changes. They act rapidly to neutralize excess acids or bases.

• Bicarbonate buffer system

• Phosphate buffer system

• Protein buffer system

Bicarbonate Buffer System

The bicarbonate buffer system is the most important extracellular buffer. It involves the equilibrium between bicarbonate (HCO3-) and carbonic acid (H2CO3).

• When a strong acid is added, H+ reacts with HCO3- to form H2CO3, which then dissociates to CO2 and H2O.

• The lungs can readily dispose of or retain CO2, making this buffer highly effective.

Equation:

Phosphate Buffer System

The phosphate buffer system is more important in intracellular fluid and renal tubules.

• Accounts for about 5% of nonbicarbonate buffer value in plasma and 16% in erythrocytes.

• Reacts with acids and bases to stabilize pH.

Equation:

Protein Buffer System

Proteins, especially albumin and hemoglobin, are major contributors to non-bicarbonate buffering.

• Imidazole groups of histidine residues in proteins provide buffering capacity.

• Hemoglobin is the main buffer in erythrocytes.

Physiological Control: Respiratory and Renal Mechanisms

Physiological mechanisms provide longer-term regulation of acid-base balance.

• Respiratory mechanisms: Control CO2 elimination or retention.

• Renal mechanisms: Control excretion or conservation of acids and bases.

Respiratory Mechanisms in Acid-Base Balance

Role of Respiration

The respiratory system regulates acid-base balance by adjusting the rate of CO2 elimination.

• O2 and CO2 exchange occurs in the lungs.

• Fall in pH stimulates chemoreceptors in arteries, aorta, and CNS.

• The body responds by increasing or decreasing respiration rate.

• Pathological conditions may require assisted ventilation.

Renal Mechanisms in Acid-Base Balance

Renal Regulation

The kidneys are the final defense against pH changes, adjusting acid and base excretion as needed.

• In acidosis: Increased acid excretion, conservation of base.

• In alkalosis: Increased base excretion, conservation of acid.

Mechanisms of Renal Regulation

• Na+-H+ exchange: H+ ions are extruded into tubular fluid in exchange for Na+.

• Production and excretion of ammonia (NH4+): Ammonia is produced from amino acids and combines with H+ to form ammonium ions, which are excreted.

• Reclamation of bicarbonate (HCO3-): Bicarbonate is reabsorbed to maintain pH.

Potassium (K+) Competition: K+ competes with H+ in the Na+-H+ exchanger. High intracellular K+ leads to less acidic urine and increased body fluid acidity; K+ depletion leads to more acidic urine and more alkaline body fluids.

Ammonia Production: Renal tubules produce ammonia from glutamine and other amino acids. At normal pH, the NH4+ to NH3 ratio is about 100:1. Ammonia diffuses into the tubular lumen and combines with H+ to form ammonium ions, which are trapped and excreted.

Acid-Base Imbalances

Classification of Acid-Base Disturbances

Acid-base disturbances are classified based on their origin and effect on blood pH and electrolytes.

• Metabolic acidosis

• Metabolic alkalosis

• Respiratory acidosis

• Respiratory alkalosis

Metabolic Acidosis

Metabolic acidosis is characterized by decreased plasma bicarbonate and a drop in pH.

• Caused by excess production of organic acids (e.g., ketoacidosis), reduced acid excretion (renal failure), or excessive loss of bicarbonate (diarrhea, renal tubular acidosis).

• Respiratory compensation lowers PCO2 to raise pH.

Anion Gap Formula:

Normal AG = 8–10 mEq/L (due to unmeasured plasma anions).

• Increased anion gap acidosis: Caused by organic acids (methanol, uremia, ketoacidosis, lactic acidosis, salicylate intoxication).

• Normal anion gap acidosis: Caused by loss of bicarbonate-rich fluid; can be hypokalemic, normokalemic, or hyperkalemic.

Metabolic Alkalosis

Metabolic alkalosis occurs when excess base is added, base elimination is decreased, or acid-rich fluids are lost, leading to primary bicarbonate excess.

• Respiratory compensation raises PCO2 to lower pH toward normal.

Respiratory Acidosis

Respiratory acidosis results from decreased elimination of CO2 through the lungs, increasing PCO2.

• Causes include direct depression of the respiratory center (drugs, CNS trauma, infections, comatose states) and conditions affecting the respiratory apparatus (COPD, pulmonary fibrosis, airway obstruction, chest wall disease, neuromuscular disorders).

Respiratory Alkalosis

Respiratory alkalosis is caused by increased rate/depth of respiration, leading to excessive elimination of CO2 and increased pH.

• Results in decreased PCO2 and increased HCO3-/CO2 ratio.

• Causes include direct stimulation of the respiratory center (anxiety, fever, CNS infection, drugs, pregnancy, hyperthyroidism) and pulmonary disorders (pneumonia, pulmonary emboli, interstitial lung disease, congestive heart failure).

Summary Table: Acid-Base Disturbances

Additional info: The notes expand on the original content by providing definitions, equations, and a summary table for clarity and completeness. The context of hydrogen ion metabolism is relevant to microbiology, especially regarding microbial contributions to acid-base balance and physiological mechanisms.