Acid+Base+Balance

Title: Acid-Base Balance Notes

Introduction

  • Source: Marieb EN, Koehn K. Anatomy and Physiology. Acid-Base Balance. 4th ed. Pearson Education Inc; 2011

  • Slides by Janice Meeking, Mount Royal College: Acid-Base Balance

pH and Its Importance

  • pH impacts functional proteins and biochemical reactions.

  • Normal pH levels:

    • Arterial blood: pH 7.4

    • Venous blood: pH 7.35

    • Intracellular fluid (ICF): pH 7.0

  • Conditions:

    • Alkalosis (alkalemia): arterial blood pH > 7.45

    • Acidosis (acidemia): arterial pH < 7.35

  • pH levels below 6.8 or above 8.0 are lethal.

Production of Hydrogen Ions (H+)

  • Major sources of H+ in the body include:

    • Metabolism of phosphorus-containing proteins leading to phosphoric acid in extracellular fluid (ECF).

    • Lactic acid produced during anaerobic respiration of glucose.

    • Fatty acids and ketone bodies from fat metabolism.

    • H+ released when CO2 is converted to bicarbonate (HCO3-) in blood.

Regulation of H+ Concentration

  • Sequential regulation mechanisms include:

    • Chemical buffer systems: Rapid response, first line of defense.

    • Brain stem respiratory centers: Act within 1-3 minutes.

    • Renal mechanisms: Most potent but take hours to days to alter pH.

Acid-Base Properties

  • Strong Acids:

    • Fully dissociate in water, greatly affect pH.

  • Weak Acids:

    • Partially dissociate, effective at stabilizing pH.

  • Strong Bases:

    • Dissociate easily to tie up H+ quickly.

  • Weak Bases:

    • Accept H+ more slowly.

Chemical Buffer Systems

  • Chemical buffer: A system to resist pH changes in the presence of strong acids or bases.

    1. Bicarbonate buffer system

    2. Phosphate buffer system

    3. Protein buffer system

Bicarbonate Buffer System

  • Mixture of bicarbonate (H2CO3) and salts.

  • Buffers both ICF and ECF; vital for ECF buffering.

  • If a strong acid is added:

    • HCO3- combines with H+ to form H2CO3.

    • pH decreases slightly until HCO3- is exhausted.

  • If a strong base is added:

    • H2CO3 dissociates to donate H+, neutralizing the base.

    • pH increases minimally.

Phosphate Buffer System

  • Similar functioning as the bicarbonate buffer system:

    • Uses dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO4^2-) as buffers.

    • Effective in urine and ICF due to high phosphate concentration.

Protein Buffer System

  • Intracellular proteins are the most abundant buffers; plasma proteins are also significant.

  • Protein molecules can act as both weak acids and bases.

    • When pH increases, carboxyl (COOH) groups release H+.

    • When pH decreases, amine (NH2) groups bind H+.

Respiratory System Regulation

  • The respiratory system has a slower response but larger capacity for altering acid-base balance:

    • Eliminates CO2.

    • Blood equilibrium: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-.

    • CO2 unloading shifts equilibrium left; CO2 loading shifts it right.

Effects of pH Imbalance

  • Hypercapnia activates medullary chemoreceptors, resulting in less CO2 and reduced H+.

  • Alkalosis depresses the respiratory center, causing decreased respiratory rate.

  • Impairments lead to:

    • Hypoventilation: Respiratory acidosis

    • Hyperventilation: Respiratory alkalosis

Renal Regulation of Acid-Base Balance

  • The kidneys control acid-base balance by:

    • Conserving or generating HCO3-.

    • Excreting HCO3-.

    • Secretion of H+ occurs primarily in the proximal convoluted tubule (PCT) and alpha intercalated cells.

    • H+ comes from H2CO3 formed by carbonic anhydrase reactions.

Mechanisms of HCO3- Regulation

  • Tubule cells are impermeable to HCO3-; CO2 forms H2CO3 in PCT cells, leading to H+ secretion and HCO3- reabsorption.

  • Intercalated cells actively secrete H+ into urine, buffered by phosphates, and generate new HCO3-.

Body Responses During Alkalosis

  • Type B intercalated cells secrete HCO3- and reclaim H+ to acidify blood.

  • The process differs from bicarbonate ion reabsorption in type A cells.

Acid-Base Imbalance Conditions

  • Respiratory and Metabolic Imbalances:

    • Respiratory acidosis and alkalosis.

    • Metabolic acidosis and alkalosis.

Evaluating Respiratory Function

  • PCO2 level is the key indicator of respiratory adequacy (normal range: 35-45 mm Hg).

    • Elevated PCO2 (> 45 mm Hg) → respiratory acidosis due to poor ventilation.

    • Low PCO2 (< 35 mm Hg) → respiratory alkalosis typically caused by hyperventilation.

Causes of Metabolic Acidosis

  • Results from:

    • Excess alcohol ingestion (producing acetic acid).

    • Loss of HCO3- (e.g., persistent diarrhea).

    • Lactic acid accumulation from shock, diabetic crisis, starvation, kidney failure.

Metabolic Alkalosis

  • Less common than acidosis:

    • Indicated by rising blood pH and HCO3-.

    • Caused by vomiting or excess base intake (e.g., antacids).

Consequences of Extreme pH Levels

  • Blood pH below 7: CNS depression, leading to coma and death.

  • Blood pH above 7.8: Nervous system excitation, muscle tetany, convulsions, respiratory arrest.

Compensation Mechanisms

  • If a physiological buffer system fails, compensation occurs:

    • The respiratory system attempts to correct metabolic imbalances.

    • The kidneys work to correct respiratory imbalances.

Respiratory Compensation for Metabolic Imbalances

  • In metabolic acidosis, high H+ levels stimulate increased respiratory rate and depth, leading to lower CO2.

  • In metabolic alkalosis, respiration slows to allow CO2 accumulation, maintaining high pH and HCO3- levels.

Summary of Responses to Acid-Base Disturbances

  • Hypoventilation indicates elevated PCO2 (respiratory acidosis) and can show renal compensation via high HCO3- levels.

  • Respiratory alkalosis is marked by low PCO2 and high pH, with renal compensation indicated by decreasing HCO3- levels.