Acid-Base Balance, Respiration, and Renal-Pulmonary Regulation

Reference Textbooks and Core Principles

• Core Reference Texts:

  • Bt's Chapter 11 and Egan's Chapter 13 serve as the primary instructional references for acid-base physiology.

• Role of Chemistry in Respiratory Care:

  • Chemistry is a fundamental prerequisite for respiratory care because acid-base physiology relies on balancing chemical equations.

  • Chemical reactions in physiological systems are dynamic and reversible, shifting direction based on reactant and product concentrations.

Cellular Respiration and the Hydrolysis Reaction

• Cellular Respiration (Krebs Cycle):

  • Takes place within the mitochondria of cellular structures.

  • Utilizes oxygen ($O_2$) to produce adenosine triphosphate ($ATP$) for cellular energy.

  • Produces carbon dioxide ($CO_2$) and water ($H_2O$) as metabolic waste products.

• The Hydrolysis Reaction:

  • The reversible acid-base reaction is expressed as:     $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

  • Carbon dioxide ($CO_2$) combines with water ($H_2O$) to yield carbonic acid ($H_2CO_3$).

  • Carbonic acid ($H_2CO_3$) dissociates into one positively charged hydrogen ion ($H^+$) (cation) and one negatively charged bicarbonate ion ($HCO_3^-$) (anion).

  • Shift Dynamics (Le Chatelier's Principle):

  • An increase in $CO_2$ content at the tissue level drives the reaction to the right, increasing carbonic acid ($H_2CO_3$), hydrogen ions ($H^+$), and bicarbonate ions ($HCO_3^-$).

  • An accumulation of $H^+$ and $HCO_3^-$ drives the reaction to the left, recombining into $H_2CO_3$ and subsequently breaking down into $CO_2$ and $H_2O$.

• Acid-Base Roles of Reaction Components:

  • Carbon Dioxide ($CO_2$):

  • $CO_2$ is not an acid by itself, but functions as an acid precursor because its hydration produces carbonic acid ($H_2CO_3$) and free $H^+$ ions.

  • High levels of blood $CO_2$ lead to elevated $H^+$ concentrations, which depresses blood $\text{pH}$.

  • Bicarbonate ($HCO_3^-$):

  • Bicarbonate ($HCO_3^-$) acts as the primary plasma base in the blood.

  • High levels of blood $HCO_3^-$ elevate blood $\text{pH}$.

Internal and External Respiration Dynamics

• Internal Respiration (Systemic Capillaries and Tissues):

  • Oxygenated blood travels from the aorta through systemic arteries and arterioles down to systemic capillaries supplying body tissues (e.g., fingers, toes).

  • Oxygen ($O_2$) bound to hemoglobin inside red blood cells ($RBCs$) dissociates, leaves the erythrocytes, and diffuses into tissue cells for cellular respiration via the Krebs cycle.

  • Metabolic waste $CO_2$ generated by tissue cells leaves the tissue and enters systemic capillary blood.

  • $CO_2$ combines with $H_2O$ in systemic capillary plasma to form carbonic acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$

• External Respiration (Pulmonary Capillaries and Alveoli):

  • Deoxygenated blood carrying $H^+$ ions and $HCO_3^-$ returns through systemic veins to pulmonary capillaries surrounding the pulmonary alveoli.

  • In pulmonary capillaries, the reaction reverses: $H^+$ and $HCO_3^-$ recombine into $H_2CO_3$, which breaks down into $H_2O$ and $CO_2$ gas.

  • $CO_2$ gas diffuses across the pulmonary interstitial space into the alveoli and is eliminated from the body via exhalation.

  • Inhaled atmospheric $O_2$ crosses the alveolar-capillary membrane into pulmonary capillaries, binding to hemoglobin inside RBCs.

Classification of Acids: Volatile vs. Fixed Acids

• Volatile Acids:

  • Definition: Acids that can convert from a liquid state into a gaseous state.

  • Primary Physiological Volatile Acid: Carbonic acid ($H_2CO_3$).

  • Excretion Route: Eliminated exclusively by the lungs through pulmonary ventilation by exhaling gaseous $CO_2$

  • Ventilatory Impact: Changes in alveolar ventilation modulate volatile acid levels. Hyperventilation increases $CO_2$ clearance (lowering acid content), whereas hypoventilation decreases $CO_2$ clearance (accumulating acid content).

• Fixed (Non-Volatile) Acids:

  • Definition: Acids that cannot convert into a gas and must remain in liquid solution within blood plasma and body fluids.

  • Excretion Route: Excreted exclusively by the kidneys via urinary output.

  • Key Examples:

  • Lactic Acid:

    • Generated via anaerobic metabolism when cellular oxygen supply is depleted and cells break down glucose without oxygen.

    • Localized anaerobic metabolism occurs during severe physical exertion, resulting in local muscle cramps or side stitches.

    • Widespread systemic anaerobic metabolism occurs during pathology such as septic shock or global hypoxia, generating massive amounts of circulating lactic acid (lactic acidosis).

  • Ketones (Ketoacids):

    • Produced during diabetic metabolic crises (Diabetic Ketoacidosis / DKA) due to abnormal fat and glucose metabolism, releasing fixed ketoacids into the blood.

  • Sulfuric Acid ($H_2SO_4$).

  • Phosphoric Acid ($H_3PO_4$).

  • Uric Acid.

Physiological Blood Gas Parameters and Normal Ranges

• Primary Homeostatic Objective: Maintain arterial blood $\text{pH}$ within strict normal physiological ranges.

• Standard Reference Values:

  • $\text{pH}$:

  • Normal Range: $7.35 \text{ to } 7.45$

  • Absolute Median Normal Target: $7.40$

  • Acidosis: $\text{pH} < 7.35$

  • Alkalosis: $\text{pH} > 7.45$

  • $\text{PaCO}_2$ (Arterial Partial Pressure of Carbon Dioxide):

  • Normal Range: $35\,mmHg \text{ to } 45\,mmHg$

  • Represents the respiratory/volatile acid component of acid-base balance.

  • $\text{HCO}_3^-$ (Arterial Bicarbonate Concentration):

  • Normal Range: $22\,mEq/L \text{ to } 26\,mEq/L$

  • Represents the renal/metabolic base component of acid-base balance.

Organ Systems in Acid-Base Regulation and Compensation

• Renal Mechanisms of Acid-Base Regulation (Kidneys):

  • Kidneys control base ($HCO_3^-$) levels and fixed acid balance in body fluids.

  • Excretion of Hydrogen Ions ($H^+$): Eliminates fixed liquid acids into urine, causing blood $\text{pH}$ to rise.

  • Reabsorption/Retention of Bicarbonate ($HCO_3^-$): Retains base ions in blood circulation, causing blood $\text{pH}$ to rise.

• Pulmonary Mechanisms of Acid-Base Regulation (Lungs):

  • Lungs regulate volatile acid ($H_2CO_3$ / $CO_2$) concentrations through alterations in alveolar ventilation.

  • Increased Ventilation: Exhales higher amounts of $CO_2$, driving arterial $PaCO_2$ down and raising blood $\text{pH}$.

  • Decreased Ventilation: Retains $CO_2$, driving arterial $PaCO_2$ up and lowering blood $\text{pH}$.

• Inter-Organ Acid-Base Compensation Mechanics:

  • Respiratory Compensation for Metabolic Acidosis:

  • Clinical Scenario: A patient in septic shock presents with lactic acidosis ($\text{pH} = 7.10$, blood lactate $6\,mmol/L$, low $HCO_3^-$).

  • Compensatory Response: When kidneys cannot rapidly excrete the fixed acid load, the respiratory system compensates within hours. The brain signals hyperventilation (such as deep, rapid Kussmaul's respirations seen in DKA) to reduce $PaCO_2$. Lowering volatile acid levels compensates for the excess fixed acid, raising blood $\text{pH}$ back toward normal.

  • Renal Compensation for Respiratory Acidosis:

  • Clinical Scenario: A patient with Chronic Obstructive Pulmonary Disease (COPD) has chronic impaired ventilation, causing persistent retention of $CO_2$ ($PaCO_2 = 60\,mmHg$) and an initial drop in blood $\text{pH}$ ($7.10$).

  • Compensatory Response: Because damaged lungs cannot increase ventilation to remove $CO_2$, the kidneys compensate over several days by reabsorbing and retaining $HCO_3^-$ in the blood. Elevated base levels offset high $PaCO_2$ levels, returning blood $\text{pH}$ back toward standard homeostatic thresholds.

• Metabolic Impact of Fever:

  • Fever increases metabolic rate, causing a $10\%$ increase in oxygen ($O_2$) consumption and carbon dioxide ($CO_2$) production per degree elevation.

  • Elevated cellular $CO_2$ production accelerates the hydrolysis reaction, producing excess carbonic acid.

  • The normal biological reflex to fever-induced hypercapnia is tachypnea (rapid breathing) to exhale the excess $CO_2$.

Questions and Discussion

• Ventilatory Response to Low Blood $\text{pH}$:

  • Question: How does ventilation adjust when blood $\text{pH}$ drops low?

  • Response: When blood $\text{pH}$ drops (acidosis), ventilation increases. Hyperventilation lowers $PaCO_2$ (volatile acid), reducing systemic acid concentration and driving blood $\text{pH}$ back up toward normal.

• Recommended Supplemental Learning Resources:

  • Ninja Nerd: Recommended for detailed anatomy and physiology (A&P) explanations of foundational concepts.

  • Respiratory Coach: Recommended for respiratory therapy clinical applications, ABG interpretations, and clinical therapies.