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\uparrowThis 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:
Respiratory Acidosis: Too much \text{PaCO}_2 (breathing too slowly).
Respiratory Alkalosis: Too little \text{PaCO}_2 (breathing too fast).
Metabolic Acidosis: Too little bicarbonate ([\text{HCO}_3^-]) (kidney/metabolism problem).
Metabolic Alkalosis: Too much bicarbonate ([\text{HCO}_3^-]) (kidney/metabolism problem).
How to Read an ABG (Step-by-Step):
Check pH: Is it acidic (low), basic (high), or normal?
Check Breathing: Does the \text{PaCO}_2 level explain the pH issue?
Check Metabolism/Kidneys: Does the bicarbonate ([\text{HCO}_3^-]) level explain the pH issue?
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:
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).
Kidneys Not Working: Kidneys can't get rid of enough acid (renal failure).
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)
Check the Patient: Look at their breathing, oxygen levels, blood flow, and medical history.
Get Tests: Order an ABG (arterial blood gas) and electrolyte levels.
Use 4-Step ABG Approach: Systematically interpret the ABG results.
Calculate Anion Gap: Do this if the patient has metabolic acidosis.
Figure Out the Problem: Is it a simple problem, or are there multiple acid-base issues?
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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