Acid-Base Balance

Fundamentals of Acid-Base Chemistry and Physiology

• Definition of pH:

  • Refers to the potential or power of Hydrogen.

  • A chemical scale that identifies the relative acidity or basicity (alkalinity) of a solution.

  • Directly evaluates the concentration of hydrogen ions, denoted as $[H^+]$.

• Inverse Logarithmic Relationship:

  • pH is mathematically defined as the negative logarithm of hydrogen ion concentration.

  • Operates on an inverse proportion:

  • An increase in $[H^+]$ causes a decrease in pH, resulting in a more acidic state.

  • A decrease in $[H^+]$ causes an increase in pH, resulting in a more alkaline (basic) state.

• Chemical Properties of Acids and Bases:

  • Acids:

  • Contain higher relative concentrations of $H^+$ ions.

  • Function as chemical donors of $H^+$ ions.

  • Strong acids dissociate completely when placed in solution.

  • Weak acids dissociate only partially in solution.

  • Bases (Alkaline Substances):

  • Contain lower relative concentrations of $H^+$ ions.

  • Function as chemical acceptors of $H^+$ ions.

  • Give up hydroxy ions ($OH^-$) in solution.

• Physiological Neutrality and Normal Systemic Values:

  • Neutrality on the logarithmic scale is represented by $7.0$, where $[H^+]$ precisely equals $[OH^-]$.

  • Systemic normal ranges are tightly regulated within serum (blood):

  • Heart Rate (HR): $60 - 100\,\text{bpm}$

  • Serum Sodium ($Na^+$): $135 - 145\,\text{mEq/L}$

  • Serum pH: $7.35 - 7.45$

  • Optimal biological neutral pH baseline in serum: $7.40$

  • Physiological Limits:

  • Normal physiological functioning occurs optimal within $7.35 - 7.45$.

  • Cellular systems can survive short periods within an expanded range of $7.20 - 7.50$ or $7.60$

  • Absolute limits incompatible with life: pH $< 6.8$ or pH $> 7.8$

• Systemic Tendencies toward Acid Production:

  • Biological processes naturally produce significantly more acids than bases.

  • Origins of systemic acid load:

  • Ingestion of acidic food and drink.

  • Metabolic degradation of lipids and proteins.

  • Cellular metabolic waste production generating carbon dioxide ($CO_2$), which acts as a volatile acid.

  • Clinical Implications of Severe Alkalosis:

  • Alkalosis causes hemoglobin to increase its affinity for oxygen (shifting the oxyhemoglobin dissociation curve), hindering oxygen unloading at the tissue level.

  • Alkaline pH imbalances are clinically significantly more difficult to correct than acidotic imbalances.

Anatomical and Tissue-Specific pH Variations

• Localized Differences Across Biological Tissues:

  • pH is not uniform throughout the body; variation supports localized function and digestion.

  • Stomach:

  • The most acidic environment in the human body.

  • Specially adapted to contain Hydrochloric Acid ($HCl$).

  • $HCl$ outside the stomach causes extensive tissue destruction.

  • Lower Esophageal Sphincter (LES):

  • Maintains a mechanical barrier to prevent gastric acid reflux into the esophagus.

  • Duodenum:

  • Possesses intrinsic neutralizing defenses to counteract stomach acid entering the intestinal tract.

  • Specific Tissue pH Ranges:

  • Skeletal Muscle: $6.9 - 7.2$

  • Liver: $7.2$

  • Bone: $7.4$

  • Pleural Fluid: $7.6$

Physiological Roles and Dynamics of Hydrogen

• Essential Biological Functions of Hydrogen:

  • Cellular Membranes: Critical for structural stability and transport properties of cell membranes.

  • Enzyme Activity: Maintains optimal tertiary structure and reaction kinetics for enzymes.

  • Hydration: Core elemental component of water ($H_2O$) maintaining organism hydration.

  • Energy Production: Direct constituent in biochemical energy generation pathways.

• Biomolecular Component:

  • Essential constituent in fundamental macronutrient structures:

  • Sugars

  • Proteins

  • Starches

  • Fats

Types of Body Acids

• Volatile Acids:

  • Defined by their capability to undergo chemical conversion into gas.

  • Cleared and excreted from the body via pulmonary exhalation by the lungs.

  • The sole volatile acid produced in the body is Carbonic Acid ($H_2CO_3$).

  • Conversion Equation:

  • $H_2CO_3 \rightleftharpoons H_2O + CO_2$

• Non-Volatile (Fixed) Acids:

  • Acids that cannot be converted into a gaseous state.

  • Must be cleared and eliminated through renal excretion by the kidneys.

  • Primary non-volatile acids include:

  • Lactic acid

  • Phosphoric acid

  • Sulfuric acid

  • Acetoacetic acid

  • Beta-hydroxybutyric acid

  • Exception for Lactic Acid:

  • Unlike other fixed acids, lactic acid can also be metabolically processed by metabolic organs, primarily the liver and kidneys.

Chemical Buffer Systems

• Buffer Dynamics:

  • Chemical substances in body fluids that immediately combine with acids or bases to resist shifts in pH.

  • Work by accepting or releasing $H^+$ ions.

  • Onset: Almost instantaneous.

  • Duration: Short-lived homeostatic line of defense.

• Primary Chemical Buffer Systems:

  • Bicarbonate - Carbonic Acid Buffer System:

  • The primary buffer system operating in the Extracellular Fluid (ECF).

  • Cellular waste $CO_2$ combines with $H_2O$ in serum to form carbonic acid ($H_2CO_3$).

  • Carbonic Anhydrase enzyme (highly concentrated in lungs and kidneys) catalyzes breakdown:

    • Pulmonary site breakdown: $H_2CO_3 \rightleftharpoons H_2O + CO_2$

    • Renal site breakdown: $H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

  • Universal Equation:

    • $HCO_3^- + H^+ \rightleftharpoons H_2CO_3 \rightleftharpoons CO_2 + H_2O$

  • Critical Stoichiometric Ratio:

    • Normal systemic ratio is $20:1$ of Bicarbonate ($HCO_3^-$) to Carbonic Acid ($H_2CO_3$).

    • $H_2CO_3$ is 20 times more potent in altering blood pH than $HCO_3^-$; for every single $H_2CO_3$ lost, $20$ ions of $HCO_3^-$ must be eliminated to preserve baseline ratio.

    • The primary reservoir of $HCO_3^-$ provides immediate neutralizing capacity against ongoing metabolic acid production.

  • Phosphate Buffer System:

  • The primary buffer system operating in the Intracellular Fluid (ICF).

  • Chemical equation:

    • $H^+ + HPO_4^{2-} \rightleftharpoons H_2PO_4^-$

    • Hydrogen ion + Hydrogen Phosphate ion $\rightleftharpoons$ Dihydrogen Phosphate ion.

  • Protein Buffer Systems:

  • Functional across almost all structural and circulating body proteins.

  • Carboxyl groups ($-COOH$) act as weak acids to donate $H^+$ (found in amino acids, acetic acid).

  • Amino groups ($-NH_2$) act as basic sites to accept $H^+$ (found in amino acids, ammonia $NH_3$).

  • Hemoglobin Buffer:

    • Grouped under protein buffers.

    • Binds and transports cellular $CO_2$ and $H^+$ at the capillary bed level.

Cellular Compensation and Electrolyte Shifts

• Intracellular Cation Redistribution:

  • Both Hydrogen ($H^+$) and Potassium ($K^+$) are positively charged cations.

  • Potassium ($K^+$) is concentrated primarily inside the cell (main intracellular cation).

• Shift Mechanism in Acidosis:

  • In states of acid accumulation (increased $[H^+]$, decreased pH):

  • Excess $H^+$ shifts from extracellular space into cells.

  • To maintain cellular electrical neutrality, intracellular $K^+$ shifts out of cells into serum.

  • Serves as a temporary buffering mechanism to remove $H^+$ from serum.

  • Reverse process occurs automatically as pH returns toward normal.

• Secondary Renal Risk:

  • If renal function is preserved, the kidneys will excrete the shifted extracellular serum $K^+$.

  • Can lead to severe total-body potassium depletion (hypokalemia) over time.

Physiological Systems Regulating Acid-Base Balance

• Respiratory Mechanisms:

  • Regulates volatile acid concentrations by altering the elimination of $CO_2$ via lungs.

  • Equation:

  • $H_2CO_3 \rightleftharpoons H_2O + CO_2$

  • Exhalation clears gaseous $CO_2$, effectively lowering circulating $H_2CO_3$ levels.

  • Does NOT affect fixed non-volatile acids (such as lactic acid).

  • pH adjustments are executed by altering respiratory rate and/or depth.

  • Essential operational components:

  • Central and peripheral chemoreceptors

  • Brainstem respiratory neurons

  • Motor neurons

  • Diaphragm and accessory respiratory muscles

  • Intact chest wall and airway structures

• Renal Mechanisms:

  • The single MOST EFFECTIVE biological regulator of systemic pH balance.

  • Key functions:

  • Eliminates large quantities of non-volatile (fixed) acids (cannot eliminate carbonic acid directly).

  • Excretes excess base ($HCO_3^-$).

  • Conserves and synthesizes new Bicarbonate ($HCO_3^-$) ions.

  • Failure of renal function directly leads to systemic acid-base failure.

• Timeframes / Rates of Systemic Correction:

  • Chemical Buffers: Almost instantaneous.

  • Respiratory System: Minutes to several hours.

  • Renal System: Hours to several days.

Arterial Blood Gas (ABG) Analysis and Clinical Parameters

• Diagnostic Assessment Goals:

  • Evaluates clinical acid-base balance status.

  • Evaluates pulmonary ventilation efficiency ($CO_2$ elimination capability).

• Normal Reference Values:

  • pH: $7.35 - 7.45$ (Indicates overall balance of $H^+$)

  • $PaCO_2$: $35 - 45\,\text{mmHg}$ (Respiratory component; indicates dissolved $CO_2$ and carbonic acid state)

  • $HCO_3^-$: $22 - 26\,\text{mEq/L}$ (Metabolic component; calculated value on standard ABG)

  • $PaO_2$: $80 - 100\,\text{mmHg}$

  • $O_2\,\text{Saturation}$: $95 - 100\%$

• Chemistry Panel Distinction:

  • ABG calculates $HCO_3^-$.

  • Serum chemistry panels measure $HCO_3^-$ and report it under Total $CO_2$ / Serum $CO_2$

Primary Acid-Base Disorders and the ROME Mnemonic

• Classification of Primary Acid-Base Disturbances:

  • Respiratory Acidosis: Primary increase in $PaCO_2$ from inadequate pulmonary ventilation.

  • Respiratory Alkalosis: Primary decrease in $PaCO_2$ from hyperventilation.

  • Metabolic Acidosis: Primary decrease in $HCO_3^-$ or accumulation of non-volatile $H^+$ ions.

  • Metabolic Alkalosis: Primary increase in $HCO_3^-$ or depletion of $H^+$ ions.

• The ROME Mnemonic:

  • Respiratory Opposite:

  • Alkalosis: pH increases ($\uparrow$), $PaCO_2$ decreases ($\downarrow$).

  • Acidosis: pH decreases ($\downarrow$), $PaCO_2$ increases ($\uparrow$).

  • Metabolic Equal:

  • Alkalosis: pH increases ($\uparrow$), $HCO_3^-$ increases ($\uparrow$).

  • Acidosis: pH decreases ($\downarrow$), $HCO_3^-$ decreases ($\downarrow$).

• Primary Disturbance and Secondary Response Matrix:

  • Metabolic Acidosis:

  • Systemic pH: Decreased ($\downarrow$)

  • Serum $[H^+]$: Increased ($\uparrow$)

  • Primary Disturbance: Decreased $HCO_3^-$ ($\downarrow$)

  • Secondary Compensatory Response: Decreased $PaCO_2$ ($\downarrow$)

  • Metabolic Alkalosis:

  • Systemic pH: Increased ($\uparrow$)

  • Serum $[H^+]$: Decreased ($\downarrow$)

  • Primary Disturbance: Increased $HCO_3^-$ ($\uparrow$)

  • Secondary Compensatory Response: Increased $PaCO_2$ ($\uparrow$)

  • Respiratory Acidosis:

  • Systemic pH: Decreased ($\downarrow$)

  • Serum $[H^+]$: Increased ($\uparrow$)

  • Primary Disturbance: Increased $PaCO_2$ ($\uparrow$)

  • Secondary Compensatory Response: Increased $HCO_3^-$ ($\uparrow$)

  • Respiratory Alkalosis:

  • Systemic pH: Increased ($\uparrow$)

  • Serum $[H^+]$: Decreased ($\downarrow$)

  • Primary Disturbance: Decreased $PaCO_2$ ($\downarrow$)

  • Secondary Compensatory Response: Decreased $HCO_3^-$ ($\downarrow$)

Compensation Mechanisms and Patterns

• Systemic Compensation States:

  • Compensation Definition: Physiological adjustment by a non-primary organ system to normalize pH.

  • Partial Compensation:

  • Systemic pH remains outside the normal range ($< 7.35$ or $> 7.45$).

  • Both $PaCO_2$ and $HCO_3^-$ display abnormal laboratory values.

  • Complete Compensation:

  • Systemic pH is restored within normal limits ($7.35 - 7.45$).

  • Both $PaCO_2$ and $HCO_3^-$ remain abnormal, reflecting ongoing active compensation.

• Cross-System Compensation Dynamics:

  • Metabolic Primary Problem $\rightarrow$ Respiratory Compensation:

  • Organ Involved: Lungs

  • Mechanism: Hyperventilation (excretes excess $CO_2$) or Hypoventilation (retains $CO_2$)

  • Speed of Activation: Rapid (minutes to hours)

  • Respiratory Primary Problem $\rightarrow$ Metabolic Compensation:

  • Organ Involved: Kidneys

  • Mechanism: Secretion or retention of $H^+$ and retention or excretion of $HCO_3^-$

  • Speed of Activation: Slow (takes several days to become fully effective)

Practice Questions and Clinical Evaluation Scenarios

• Blood Buffer Systems:

  • Primary Blood Buffer System: The bicarbonate-carbonic acid buffer system serves as the primary buffer system in the blood to maintain systemic acid-base balance.

  • Enzyme Catalysis: Carbonic anhydrase is the enzyme located in red blood cells responsible for converting carbon dioxide ($CO_2$) and water ($H_2O$) into carbonic acid ($H_2CO_3$) within the bicarbonate buffering system.

• Physiological System Roles and Compensation Responses:

  • Renal Regulation Mechanism:

    • When systemic pH is high (alkalemia), the kidneys maintain acid-base balance by excreting bicarbonate ($HCO_3^-$).

    • Patients suffering from kidney failure lose the capacity to clear fixed acids and retain bicarbonate, placing them at direct risk for metabolic acidosis.

  • Respiratory Regulation Mechanism:

    • The respiratory system regulates acid-base balance by rapidly altering the rate and depth of $CO_2$ exhalation.

    • When carbon dioxide ($CO_2$) levels rise in circulating blood, the physiological response is an increase in respiratory rate to blow off excess gas.

    • The primary respiratory compensatory response to metabolic acidosis is increased ventilation (hyperventilation).

  • Compensatory Shifts in Specific Acid-Base Disorders:

    • An increase in blood bicarbonate ($HCO_3^-$) concentration occurs as a renal compensatory mechanism in response to respiratory acidosis.

    • In a patient who develops metabolic alkalosis due to prolonged vomiting (loss of gastric acid), the primary respiratory compensation is a decrease in respiratory rate (hypoventilation) to retain $CO_2$.

• Diagnostic Interpretations and Clinical Etiologies:

  • Inverse Hydrogen Ion and pH Relationship: An increase in hydrogen ion concentration ($[H^+]$) directly leads to a decrease in systemic pH.

  • Etiology of High Anion Gap Metabolic Acidosis:

    • Diabetic Ketoacidosis (DKA) is a primary cause of high anion gap metabolic acidosis.

    • Diarrhea and renal tubular acidosis present as non-anion gap metabolic acidosis, whereas vomiting causes metabolic alkalosis.

  • Etiology of Respiratory Alkalosis:

    • Anxiety triggering hyperventilation is a primary condition that leads to acute respiratory alkalosis via excessive blowing off of $CO_2$.

  • System Attribution in Acid-Base Disturbances:

    • A patient presenting with an acidic pH of $7.28$ and an elevated $PaCO_2$ level is experiencing primary respiratory acidosis, identifying the respiratory system as the organ system responsible for the imbalance.