Acid-Base Balance – Comprehensive Bullet-Point Study Notes

Learning Objectives

  • How Lungs & Kidneys Control Acid:

    • Lungs: Get rid of gas-form acid (\text{CO}_2) and adjust breathing rate.

    • Kidneys: Get rid of non-gas acids, and either keep or remove bicarbonate (\text{HCO}_3^-).

  • Acid Strength: Understand how an acid's strength relates to how much it breaks apart (its ext{K}_a).

  • Buffer Systems (Open vs. Closed): Learn the difference between these types of buffer systems and how they handle gas-form vs. non-gas acids.

  • Henderson–Hasselbalch (H-H) Equation: Learn to use this formula for real patient problems.

  • Kidney-Lung Teamwork: Understand how kidneys and lungs help each other when one isn't working well.

  • Kidney's Role in Electrolytes: Explain how the kidneys handle salts and how this affects acid-base balance.

  • Reading Blood Gas Results (ABG): Learn to identify, understand, and act on readings from an arterial blood gas test.

  • Quick ext{CO}_2 Changes: Explain how sudden changes in carbon dioxide (\text{PaCO}_2) affect bicarbonate (\text{HCO}_3^-) levels in the blood.

  • Anion Gap (AG): Calculate the AG to find the cause of metabolic acidosis.

  • Standard Bicarbonate & Base Excess: Use these to figure out the kidney's part in acid-base problems.

Hydrogen Ion (H^+) Regulation in Body Fluids

  • Normal pH: The goal for pH (acidity) outside cells is 7.35 \text{--} 7.45. (This means about 40 \text{ nEq}\,\text{L}^{-1} of H^+ ions).

  • Two Main Sources of H^+:

    • Volatile Acids (Gas-Form): These can turn into a gas. The most important one in the body is carbonic acid (\text{H}_2\text{CO}_3).

    • Body processes create a lot of carbon dioxide (\text{CO}_2).

    • Inside red blood cells, an enzyme (carbonic anhydrase) helps this reaction: \text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow H^+ + \text{HCO}_3^-

    • Isohydric Buffering: Any H^+ released is immediately soaked up by deoxygenated hemoglobin (Hb) in red blood cells.

    • Fixed Acids (Non-Gas Form): These do not turn into a gas.

    • Breaking down proteins produces sulfuric and phosphoric acids.

    • When the body doesn't have enough oxygen, it creates lactic acid.

  • How the Body Keeps pH Constant (despite \text{CO}_2 production):

    • Isohydric Buffering: Hemoglobin inside red blood cells quickly absorbs H^+ ions.

    • Ventilation: Breathing removes \text{CO}_2 from the body.

Buffer Systems

  • What Buffers Do: A mix of a weak acid and its partner chemical (conjugate base) helps prevent big pH changes when strong acids or bases are added.

    • Example in Blood:

    • Adding strong acid (\text{HCl}) causes: \text{HCl} + \text{NaHCO}_3 \rightarrow \text{NaCl} + \text{H}_2\text{CO}_3 (pH drops only a little)

    • Adding strong base (\text{NaOH}) causes: \text{NaOH} + \text{H}_2\text{CO}_3 \rightarrow \text{NaHCO}_3 + \text{H}_2\text{O} (pH rises only a little)

  • Types of Whole-Blood Buffers:

    • Open Systems: Can get rid of acid as a gas, so waste products don't build up.

    • Example: The bicarbonate system in blood plasma and red blood cells.

    • Closed Systems: Do not have a gas component, so their buffering capacity is limited.

    • Examples: Non-bicarbonate buffers like hemoglobin, organic and inorganic phosphates, and plasma proteins.

Bicarbonate (Open) System
  • Parts: Bicarbonate (\text{HCO}_3^-) and carbonic acid (\text{H}_2\text{CO}_3).

  • Because carbonic acid (\text{H}_2\text{CO}_3) can break down into \text{CO}_2 and \text{H}_2\text{O}, and the lungs constantly exhale \text{CO}_2, the reaction keeps moving forward:
    H^+ + \text{HCO}_3^- \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}_2\text{O} + \text{CO}_2\uparrow

  • This system removes a huge amount of acid (about 24{,}000 \text{ mmol day}^{-1} as \text{CO}_2).

  • It cannot buffer its own acid (e.g., too much carbonic acid from slow breathing). For that, it needs the help of non-bicarbonate buffers.

Non-Bicarbonate (Closed) System
  • Parts: Phosphate and proteins (hemoglobin is the most important).

  • How it Works: H^+ + \text{Buf}^- \leftrightarrow \text{HBuf} This reaction eventually reaches a balance, meaning its ability to buffer is limited. Once the partner chemical (\text{Buf}^-) is used up, buffering stops.

  • Hemoglobin is the most important non-bicarbonate buffer because it's in high concentration in red blood cells.

  • It can buffer both fixed and volatile acids, but because it's a closed system, the buildup of products can slow the process.

Henderson–Hasselbalch Equation

  • This equation shows the link between pH, bicarbonate (\text{HCO}_3^-), and dissolved carbon dioxide (\text{CO}_2): pH = 6.1 + \log \left(\dfrac{[\text{HCO}_3^-]}{\text{PaCO}_2 \times 0.03}\right)

    • 6.1 is related to the acid's strength (\text{log K}_a) for carbonic acid.

    • 0.03 \text{ mmol} \cdot \text{L}^{-1} \cdot \text{mmHg}^{-1} is a conversion factor for \text{PaCO}_2 into \text{H}_2\text{CO}_3 levels.

  • Blood gas machines directly measure pH and \text{PaCO}_2; the bicarbonate ([\text{HCO}_3^-]) is then calculated using this equation.

  • Clinical Tip: pH is mostly determined by the ratio of bicarbonate to \text{CO}_2, not their individual amounts.

    • Normal Ratio: \tfrac{[\text{HCO}_3^-]}{[\text{CO}_2]} = \tfrac{24}{40\times0.03} = 20:1 (This ratio gives a normal pH of 7.40).

Acid Excretion Mechanisms

  • First Defense: Buffer systems. Long-term Removal: Lungs and kidneys.

Pulmonary Excretion (Volatile Acid)
  • Our body constantly produces \text{CO}_2 through normal metabolism; the lungs remove about 24,000 \text{ mmol day}^{-1}.

  • Lungs also indirectly help remove fixed acids after they are neutralized by bicarbonate, which then produces \text{CO}_2 + \text{H}_2\text{O}.

  • Response Time: Very fast, seconds to minutes.

Renal Excretion (Fixed Acid & \text{HCO}_3^- Balance)
  • Daily Excretion: The kidneys are crucial for getting rid of non-gas acids and managing bicarbonate levels. They can:

    • Reabsorb (keep) nearly all the filtered bicarbonate to stop the body from losing this key buffer.

    • Produce new bicarbonate to replace what was used up buffering fixed acids.

    • Excrete hydrogen ions (H^+) in the urine, often by combining them with phosphate or ammonia to safely remove them.

  • Response Time: Slower than lungs, taking hours to days.

Acid–Base Disturbances: General Framework

  • Normal Levels to Remember:

    • Bicarbonate ([\text{HCO}_3^-]): 22\text{–}26 \text{ mEq} \cdot \text{L}^{-1}

    • Carbon Dioxide (\text{PaCO}_2): 35\text{–}45 \text{ mmHg}

    • pH: 7.35\text{–}7.45 (maintaining a about 20:1 ratio of bicarbonate to \text{CO}_2)

  • Four Main Imbalances:

    1. Respiratory Acidosis: Too much \text{PaCO}_2 (breathing too slowly).

    2. Respiratory Alkalosis: Too little \text{PaCO}_2 (breathing too fast).

    3. Metabolic Acidosis: Too little bicarbonate ([\text{HCO}_3^-]) (kidney/metabolism problem).

    4. Metabolic Alkalosis: Too much bicarbonate ([\text{HCO}_3^-]) (kidney/metabolism problem).

  • How to Read an ABG (Step-by-Step):

    1. Check pH: Is it acidic (low), basic (high), or normal?

    2. Check Breathing: Does the \text{PaCO}_2 level explain the pH issue?

    3. Check Metabolism/Kidneys: Does the bicarbonate ([\text{HCO}_3^-]) level explain the pH issue?

    4. Look for Help (Compensation): Is the other body system (lungs or kidneys) trying to fix the problem?

  • Quick Change in \text{PaCO}_2 Affects \text{HCO}_3^-: When \text{PaCO}_2 changes suddenly, it affects bicarbonate levels (due to the \text{CO}_2 changing to acid in the blood).

    • Rough Rule: For every 10 \text{ mmHg} change in \text{PaCO}_2, bicarbonate ([\text{HCO}_3^-]) changes by about 1 \text{ mEq} \cdot \text{L}^{-1}.

Respiratory Acidosis (Slow Breathing = Hypoventilation)

  • Definition: \text{PaCO}_2 is higher than 45 \text{ mmHg} and the pH is usually low (blood is too acidic), because breathing is too slow or shallow.

  • Causes: Lung diseases (like COPD, asthma), issues with the brain controlling breathing, weak breathing muscles, or certain drugs.

  • Compensation: The kidneys try to help by holding onto more bicarbonate.

Respiratory Alkalosis (Fast Breathing = Hyperventilation)

  • Definition: Lower than normal \text{PaCO}_2 reduces carbonic acid, making the pH higher than 7.45 (blood is too basic).

  • Causes: Lack of oxygen (most common), anxiety, pain, fever, brain injuries, pregnancy, or taking too much aspirin (salicylate intoxication).

  • Symptoms: Tingling around the mouth/in limbs, light-headedness, hand/foot spasms, severe muscle twitching (tetany) if very serious.

  • Compensation: The kidneys try to help by getting rid of more bicarbonate (can be partial or complete).

  • Fix: Remove what's causing the fast breathing (e.g., give oxygen for low oxygen, anxiety medication, pain relief).

Alveolar Hyperventilation Superimposed on Compensated Chronic Respiratory Acidosis

  • This happens when someone with long-term breathing problems (chronic respiratory acidosis) suddenly starts breathing too fast.

  • Typical Chronic Problem ABG (before the new problem):

    • pH 7.38 (almost normal, because kidneys have compensated)

    • \text{PaCO}_2 = 58 (still high, but the body has adjusted)

    • \text{HCO}_3^- = 33 (kidneys have made more bicarbonate to compensate)

  • What happens with Sudden Fast Breathing: Acute low oxygen can make them breathe faster, lowering their \text{PaCO}_2 (e.g., to 50), which raises their pH (e.g., to 7.44), even if their bicarbonate doesn't change.

    • Confusion: This can look like metabolic acidosis that has been compensated for, unless you know the patient's full medical history.

Metabolic Acidosis

  • Definition: Low bicarbonate ([\text{HCO}_3^-]) and a low pH (blood is too acidic).

  • Causes:

    1. Too Much New Acid: Body makes too much acid, or you ingest it (e.g., lactic acid from poor circulation, ketoacids from diabetes, certain poisons).

    2. Kidneys Not Working: Kidneys can't get rid of enough acid (renal failure).

    3. Losing Bicarbonate: Body loses too much bicarbonate (e.g., severe diarrhea, certain kidney problems).

  • Anion Gap (AG) Evaluation: This helps figure out the cause of metabolic acidosis. \text{AG} = [\text{Na}^+] + [\text{K}^+] - ( [\text{Cl}^-] + [\text{HCO}_3^-] )

    • Normal AG: 9\text{–}14 \text{ mEq} \cdot \text{L}^{-1}.

    • High AG: Means there are extra, unmeasured acids in the blood (like lactate, ketoacids, or poisons).

    • Normal AG with Acidosis: Means the low bicarbonate is balanced by high chloride (often due to bicarbonate loss).

  • Compensation: The body's immediate response is to breathe faster and deeper (called Kussmaul breathing in severe cases like diabetic ketoacidosis) to blow off \text{CO}_2 and raise pH.

  • Symptoms: Shortness of breath (from fast breathing), brain function slowing down (from mild tiredness to coma).

  • Treatment:

    • If pH is above 7.20: Focus on treating the underlying cause, and let the fast breathing compensate.

    • If pH is below 7.20: Bicarbonate might be given intravenously, but it must be done carefully to avoid complications. Treating the underlying cause is always the priority.

Metabolic Alkalosis

  • Definition: High bicarbonate ([\text{HCO}_3^-]) and a high pH (blood is too basic).

  • Causes:

    • Losing stomach acid (e.g., severe vomiting, stomach suction tubes).

    • Water pills (diuretics) that cause fluid loss and chloride loss.

    • Getting too much alkali (base) medication, or certain hormone imbalances.

  • Compensation: The body tries to slow down breathing to keep more \text{CO}_2 (acid). However, this slowing is limited because the body still needs oxygen; sometimes, oxygen levels might fall to about 50 \text{ mmHg}.

  • Management:

    • Restore body fluids, potassium, and chloride levels.

    • For severe cases that don't respond to usual treatment, dilute hydrochloric acid can be given carefully through a central IV line.

Diagnostic Indicators beyond Standard ABG

  • Standard Bicarbonate: A machine adjusts the blood sample's \text{PaCO}_2 to 40 \text{ mmHg} and then measures the bicarbonate.

    • Purpose: To remove any breathing influence and only look at the metabolic (kidney) part of the acid-base balance.

    • Limitation: It's not perfect because a test tube can't fully copy how the body buffers and uses proteins inside a living person.

  • Base Excess / Base Deficit:

    • Purpose: This number tells you exactly how much strong acid or base is needed to bring the blood's pH back to 7.40 at body temperature (37 \text{ °C}) and with a normal \text{PaCO}_2 (40 \text{ mmHg}).

Mixed Acid–Base States

  • These happen when a person has two or more primary acid-base problems at the same time. In these cases, the usual compensation rules don't apply.

    • Example: pH 7.62, \text{PaCO}_2 = 32, \text{HCO}_3^- = 29

    • The high pH is caused by both low \text{PaCO}_2 (respiratory alkalosis) and high \text{HCO}_3^- (metabolic alkalosis).

    • Normal compensation can't explain this double problem.

  • Why it Matters: Common in complex patients like those with severe injuries, widespread infection (septic shock), or chronic lung disease with vomiting.

Clinical Decision Algorithm (Step-by-Step for Doctors)

  1. Check the Patient: Look at their breathing, oxygen levels, blood flow, and medical history.

  2. Get Tests: Order an ABG (arterial blood gas) and electrolyte levels.

  3. Use 4-Step ABG Approach: Systematically interpret the ABG results.

  4. Calculate Anion Gap: Do this if the patient has metabolic acidosis.

  5. Figure Out the Problem: Is it a simple problem, or are there multiple acid-base issues?

  6. Start Treatment: Begin specific treatment for the affected organs (lungs or kidneys) while being careful not to cause more harm (e.g., slowly correct \text{CO}_2 in people who always have high \text{CO}_2, use bicarbonate cautiously).

Ethical & Practical Implications

  • Quick Action Needed: It's vital to quickly spot and fix very high or low pH levels (above 7.55 or below 7.20) to prevent serious heart problems, seizures, or death.

  • Avoid Over-Correction: Correcting the problem too aggressively can be just as dangerous as not correcting it enough (e.g., overly fast mechanical breathing for someone with chronic high \text{CO}_2, or giving bicarbonate indiscriminately for lactic acidosis).

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