Acid-Base Balance and Buffer Systems

Course Context and Interactions

• Personnel and News Partnership: The instructor mentions that she and Miss Michelle are news partners. They are occasionally put on the news together for segments such as "Health Watch 3."

• Anecdotes and Classroom Environment:

  • A student or child (referenced as 16 years old) was discussed regarding picking up "Crumbl" cookies that were ordered on a phone.

  • A humorous story was shared about a friend's child who ordered $400 worth of Legos on Amazon.

  • The class noted that the CNA (Certified Nursing Assistant) class next door was very "lively" and "stomping," which might cause slight distractions.

  • The instructor shared a joke about a student falling asleep to the sound of her voice, describing it as the "stuff nightmares are made from."

Introduction to Acid-Base Chemistry: Strong vs. Weak

• Terminology and Definitions:

  • Strong Acids and Strong Bases: These substances dissociate completely in a mixture. Once broken apart, they have difficulty reforming into their original state or do not go back together well.

  • Weak Acids and Weak Bases: These substances dissociate in a fluid mixture (like water) but do not do so completely. This process is reversible, allowing them to reform and push chemical equations in either direction (left or right).

• Examples in Physiology:

  • Weak Acid: Carbonic acid ($H_2CO_3$), which is described as volatile and unstable.

  • Weak Base: Bicarbonate ($HCO_3^-$).

  • Strong Acid: Hydrogen ($H^+$) when dissociated.

The CO2 Hydration Reaction and Transport

• The Chemical Equation:   $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$

• Step-by-Step Path from Tissue to Lungs:

  1. Tissue Metabolism: Tissues (e.g., a leg muscle) create $CO_2$ as a byproduct of metabolism. Under normal resting conditions, this production rate is $200\,ml/min$.

  2. Capillary Entry: $CO_2$ flows into the systemic capillary bed and enters the venous system.

  3. Reaction with Water: Upon hitting the water in the blood plasma (which is over 90% water), $CO_2$ and $H_2O$ form carbonic acid ($H_2CO_3$).

  4. Dissociation: Because carbonic acid is a weak, volatile acid, it cannot stay together. It dissociates into bicarbonate ($HCO_3^-$) and a hydrogen ion ($H^+$).

  5. Transport: This is the primary way $CO_2$ is carried through the blood to the lungs.

  6. Reversal at Lungs: Once it reaches the lungs, the reaction reverses, turning back into $CO_2$ and $H_2O$, and the $CO_2$ is exhaled.

Chemical Buffer Systems: The First Line of Defense

• Overview: The chemical buffer systems are the first line of defense in regulating acid-base balance because they respond the fastest.

• First System: Carbonic Acid-Bicarbonate Buffer System:

  • Location: Operates primarily in the blood plasma.

  • Significance: It is the most powerful chemical buffer in the body because of its flexibility (the ability to flux either way based on the environment).

• Second System: Phosphate Buffer System:

  • Efficiency: Only about $1/6$ as effective as the bicarbonate system in blood plasma.

  • Primary Role: It is highly effective in the intracellular fluid and the kidneys (specifically within the glomerular filtrate, which eventually becomes urine).

• Third System: Protein Buffer System:

  • Distribution: Found in both plasma and cells.

  • Power: Provides 75% of the total buffering power of body fluids, primarily through intracellular proteins.

  • Mechanism: Proteins are polymers of amino acids with exposed carboxyl groups that can dissociate to add hydrogen ions if the pH is elevated.

  • Amphoteric Molecules: These are molecules that can function as either an acid or a base depending on the environment. Hemoglobin is a prime example.

  • Hemoglobin Flux: When hemoglobin loses oxygen at the tissue level (becoming reduced), it carries a negative charge. This allows it to bind with free hydrogen ions from the $CO_2$ hydration reaction, effectively decreasing the acidity of the plasma.

The Henderson-Hasselbalch Equation and pH Ratios

• Conceptual Application: This equation is used to mathematically prove that under normal conditions, the pH of the blood should be 7.4.

• The Formula Components:

  • $pH = pK + \log\left(\frac{[Base]}{[Acid]}\right)$

  • pK: A specific mathematical constant associated with an acid. For carbonic acid in the blood, this value never changes.

• The 20:1 Ratio:

  • The normal ratio of bases ($HCO_3^-$) to acids ($H_2CO_3$) is $20:1$.

  • Decreased Ratio (e.g., 15:1): Results in a more acidic state; the pH will drop below 7.4.

  • Increased Ratio (e.g., 30:1): Results in a more alkaline state; the pH will rise above 7.4.

The Respiratory System: The Second Line of Defense

• Mechanism: Regulates acid-base balance by altering the elimination of $CO_2$.

• Power Potential: It responds slower than chemical buffers but has twice the buffering power of all chemical buffer systems combined.

• Equilibrium Shifts:

  • Right Shift: Caused by an increase in $CO_2$ (due to high metabolic demand or hypoventilation).

  • Left Shift: Caused by a decrease in $CO_2$ (due to hyperventilation or blowing off too much $CO_2$).

Ventilation, CO2 Production, and Capnia States

• Eucapnia (Normal State):

  • Produced via a balance where the amount of $CO_2$ eliminated matches the $200\,ml/min$ produced at the tissues.

  • Normal Alveolar Ventilation ($V\dot{A}$): Approximately $4\,L/min$.

  • Normal $P_aCO_2$ matches Alveolar $P_ACO_2$ (roughly $40\,mmHg$).

• Hypoventilation and Hypercapnia:

  • Definition: A decrease in minute ventilation or alveolar ventilation that leads to a buildup of $CO_2$.

  • Relationship: If alveolar ventilation is cut in half (from $4\,L/min$ to $2\,L/min$), the $CO_2$ in the system doubles.

  • Clinical Marker: High $P_aCO_2$ (Hypercapnia).

  • Causes: Drug overdose (requiring Narcan), slow or shallow breathing (respiratory rate < 12).

• Hyperventilation and Hypocapnia:

  • Definition: An increase in ventilation above normal resting needs.

  • Relationship: Doubling alveolar ventilation results in cutting the plasma $CO_2$ in half.

  • Clinical Marker: Low $P_aCO_2$ (Hypocapnia).

  • Causes: Head trauma, excessive respiratory rate despite resting metabolism.

The Renal System: The Third Line of Defense

• Fixed Acid Removal: The kidneys rid the body of fixed acids, including:

  • Phosphoric acid.

  • Uric acid.

  • Lactic acid.

  • Ketones.

• Alkaline Regulation: The renal system regulates alkaline substances by determining whether to retain or excrete bicarbonate ($HCO_3^-$) to manage hydrogen ion levels in extracellular fluid.

Arterial Blood Gas (ABG) Analysis and Clinical Application

• Introduction: ABG analysis is the most basic yet essential test of lung function.

• Sample Requirements:

  • Anaerobic: The sample must have no air bubbles. Air bubbles allow gases to move from areas of high concentration (blood $CO_2$) to low concentration (atmospheric air), skewing results.

  • Invasive: It is a painful puncture ("Big stick") with inherent risks.

• Normal pH Ranges:

  • Ideal Range: $7.35 - 7.45$.

  • Physiologic Survival Range: $6.9 - 7.8$.

• Normal Partial Pressures:

  • $P_aCO_2$: $35 - 45\,mmHg$ (Torr).

  • $P_vCO_2$ (Mixed Venous): $40 - 46\,mmHg$.

  • $P_aO_2$: $80 - 100\,mmHg$ (Torr).

  • $P_vO_2$ (Mixed Venous): Approximately $40\,mmHg$.

Oxygen Content, Delivery, and Shunting

• Oxygen Content Calculations:

  • Arterial Oxygen Content ($C_aO_2$): Represents the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma.

  • Formula: $C_aO_2 = (Hb \times 1.34 \times S_aO_2) + (P_aO_2 \times 0.003)$

• Oxygen Delivery ($DO_2$):

  • Depends on the arterial oxygen content and the cardiac output ($QT$).

  • Formula: $DO_2 = C_aO_2 \times QT \times 10$ (where 10 is a conversion factor).

• The A-a Gradient:

  • The difference between alveolar oxygen ($P_AO_2$) and arterial oxygen ($P_aO_2$).

  • Normal Gradient: Usually less than $20\,mmHg$.

  • Normal Anatomical Shunts: Caused by bronchial circulation (AV anastomosis) and thebesian veins, which drain venous blood directly into the left side of the heart.

• Factors Affecting $P_aO_2$:

  • Increased via: Hyperventilation or elevated $F_IO_2$ (supplemental oxygen therapy).

  • Decreased via:

    • Hypoventilation.

    • Diffusion Defects (based on Fick's Law: decreased surface area, decreased pressure gradient, or increased membrane thickness).

    • V/Q Mismatch (Ventilation-Perfusion mismatch, shunts, or dead space).

    • High altitude.

Questions & Discussion

• Student Question: "Oh, the hydrogen is… would that be considered a strong acid?"

• Instructor Response: "Uh-huh. That's considered a strong acid. Yes."

• Student Question: "So there's the significance behind a weak acid is the ability to, like, work that… To go either way?"

• Instructor Response: "Yes. Uh-huh. Yep. And you're gonna find out more about that… there's a flux there."

• Student Concern: A student mentioned that they feel comfortable with the concepts but are nervous about "naming it correctly" when identifying ABG results.

• Instructor Response: Reassured the students that they will work through it more during the slide set and emphasized that understanding the relationship between the base-to-acid ratio and the pH direction is the key.