Buffer Systems in Human Physiology

Buffer Systems

• The human body maintains a stable pH (7.35-7.45 in arterial blood) for cellular functions, enzymes, and overall physiological processes.
• Buffer systems neutralize excess acids or bases, preventing sudden pH changes.
• A buffer solution resists pH changes when an acid or base is added.
• It comprises a weak acid and its conjugate base, or a weak base and its conjugate acid.
• These neutralize added acids/bases to maintain a relatively steady pH.

Bicarbonate Buffer System

• Components: bicarbonate ion (HCO3-) and carbonic acid (H2CO3).
• Location: extracellular fluid (blood plasma).
• Mechanism: When excess H+ ions are present, bicarbonate ions combine to form carbonic acid. Carbonic acid then breaks down into CO2 and H2O, releasing CO2 through respiration. If pH rises, carbonic acid releases H+ ions, lowering pH.
• Importance: This is the primary buffer in the blood, working closely with respiratory and renal systems to maintain pH.

Phosphate Buffer System

• Components: dihydrogen phosphate (H2PO4-) and hydrogen phosphate (HPO42-).
• Location: primarily in intracellular fluid and kidney tubules.
• Mechanism:
  • When pH decreases, HPO42- binds with H+ ions to form H2PO4-.
  • When pH increases, H2PO4- donates H+ ions, stabilizing pH.
  • These two ions are in equilibrium.
• The overall plasma concentration is lower than other major buffer systems (only around 5% of the non-bicarbonate buffer value of plasma).

Protein Buffer System

• Nearly all proteins act as buffers.
• Proteins are made of amino acids, with positively charged amino groups and negatively charged carboxyl groups.
• Components: amino acids and proteins (e.g., hemoglobin).
• Location: both intracellular and extracellular fluids.
• Mechanism: Amino groups accept H+ ions; carboxyl groups release H+ ions, depending on pH.

Low pH (Acidic) Conditions

• High H+ ion concentration.
• The amino group (-NH2) accepts an additional H+ ion, becoming -NH3+, reducing excess H+ and preventing further pH decrease.
• The carboxyl group (-COOH) already protonated, doesn't release H+ in acidic environments.
• Overall, amino groups accepting H+ helps stabilize pH.

High pH (Basic) Conditions

• Low H+ ion concentration.
• The carboxyl group (-COOH) releases an H+ ion, becoming -COO-, adding protons to counter the pH increase.
• The amino group (-NH2), deprotonated at high pH, remains uncharged and doesn't readily accept H+ ions.
• Overall, carboxyl groups releasing H+ in countering the basic environment help stabilize pH.

Hemoglobin Buffer System

• Location: red blood cells.
• Mechanism: Hemoglobin binds H+ ions, especially when oxygen is released from tissues, buffering changes in pH.
  • Also helps transport CO2 from tissues to lungs for exhalation.
• Importance: Key role in maintaining blood pH, especially in venous blood (high CO2).

Buffer Systems and Acid-Base Disorders

• Buffer systems work with respiratory and renal systems to compensate for acid-base imbalances.
• Respiratory System Compensation (Lungs): Rapidly regulates blood pH by adjusting CO2 levels through breathing.
• Renal System Compensation (Kidneys): Adjusts blood pH more slowly through H+ excretion/HCO3- reabsorption.

Isohydric and Chloride Shift

• Isohydric shift: Movement of H+ ions by hemoglobin, without changing overall blood pH.
• Chloride shift: Exchange of bicarbonate (HCO3-) and chloride (Cl-) ions across red blood cell membranes to maintain balance, facilitating CO2 transport.

Compensatory Mechanisms

• Compensatory mechanisms are physiological processes to restore homeostasis when encountering stressors, diseases, or other disruptions.
• They help sustain essential functions, especially under acute or chronic stress, but prolonged compensation may lead to pathologic conditions if underlying causes aren't addressed (diabetic example).

Types of Compensatory Mechanisms

• Renal (Kidneys): Regulation of acid-base balance through bicarbonate reabsorption or hydrogen ion excretion, compensation for respiratory issues.
• Respiratory (Lungs): Adjusting breathing rate and depth to maintain blood gas levels (CO2, O2).
• Hormonal (e.g., RAAS): Activates in response to low blood pressure or sodium depletion, leading to water and sodium retention to increase blood volume.