Summary for final exam
Reading File
Running Command
cd /tmp/workspace/attachments/e1d98b7f-6a22-4827-a188-c8429a6e57b0/ && python3 -c " import pdfplumber pdf_path = 'John Edward Hall_ Michael Edward Hall - Guyton and Hall textbook of medical physiology-Elsevier Inc. (2021)-387-404.pdf' with pdfplumber.open(pdf_path) as pdf: print(f'Total pages: {len(pdf.pages)}') for i, page in enumerate(pdf.pages): text = page.extract_text() print(f'--- PAGE {i+1} ---') print(text) print() "
I now have all the content. Here is a comprehensive final-exam summary of Guyton & Hall Chapter 31: Acid-Base Regulation.
Acid-Base Regulation - Final Exam Summary
Guyton & Hall Medical Physiology, Chapter 31 (pp. 387-404)
1. Core Definitions
| Term | Definition |
|---|---|
| Acid | Molecule that releases H+ (e.g., HCl, H₂CO₃) |
| Base | Ion/molecule that accepts H+ (e.g., HCO₃⁻, HPO₄²⁻, proteins) |
| Acidemia | Blood pH significantly below 7.4 |
| Alkalemia | Blood pH above 7.4 |
| Acidosis/Alkalosis | Processes that lead to acidemia/alkalemia |
Normal values:
- Arterial blood pH: 7.40 (H⁺ = 40 nEq/L)
- Venous blood / interstitial fluid pH: 7.35
- Intracellular pH: 6.0-7.4
- Urine pH: 4.5-8.0
- Gastric HCl pH: 0.8
- Survivable range: pH 6.8 to 8.0
2. Three Lines of Defense Against pH Changes
| System | Speed | Mechanism |
|---|---|---|
| Chemical buffers | Seconds | Bind/release H+ instantly |
| Respiratory system | Minutes (3-12 min) | Adjusts CO₂ elimination |
| Kidneys | Hours to days | Most powerful; permanently corrects imbalance |
3. Buffer Systems
Bicarbonate Buffer System (Most Important Extracellular Buffer)
- Components: H₂CO₃ (weak acid) + NaHCO₃ (bicarbonate salt)
- Reaction chain: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- Henderson-Hasselbalch Equation:
pH = 6.1 + log [HCO₃⁻ / (0.03 × PCO₂)]
- Normal: pH = 6.1 + log (24 / 1.2) = 6.1 + 1.3 = 7.4
- pK of bicarbonate system = 6.1 (far from body pH of 7.4)
- Despite this, it is the most powerful extracellular buffer because its two components (HCO₃⁻ and CO₂) are independently regulated by kidneys and lungs
Phosphate Buffer System
- Components: H₂PO₄⁻ and HPO₄²⁻
- pK = 6.8 (closer to body pH, more efficient per molecule)
- Minor role in extracellular fluid (~8% of bicarbonate buffering power)
- Important in: (1) renal tubular fluid (phosphate is concentrated there), (2) intracellular fluid
Protein Buffers (Intracellular - Most Important Overall)
- ~60-70% of all chemical buffering in the body
- Hemoglobin (Hb) is especially important in red blood cells: Hb + H⁺ ⇌ HHb
- Slow to equilibrate (several hours) but powerful
Isohydric Principle
- All buffer systems in a solution are in equilibrium with the same H⁺ concentration
- A change in one buffer system shifts all others simultaneously
4. Respiratory Regulation of Acid-Base Balance
Mechanism:
- ↑H⁺ → stimulates respiratory center → ↑alveolar ventilation → ↓PCO₂ → ↓H⁺
- ↓H⁺ → depresses respiratory center → ↓ventilation → ↑PCO₂ → ↑H⁺
Quantitative effects:
- Doubling alveolar ventilation raises pH by ~0.23 (from 7.40 to 7.63)
- Reducing ventilation to ¼ normal lowers pH by ~0.45 (from 7.40 to 6.95)
- Efficiency: 50-75% correction of a metabolic pH disturbance (feedback gain of 1-3)
- Buffering power: 1-2× all chemical buffers combined
Note: Respiratory compensation for alkalosis is less effective because hypoxemia limits the degree to which ventilation can be reduced.
5. Renal Regulation of Acid-Base Balance
The kidneys regulate H⁺ via three mechanisms:
- Secretion of H⁺ into tubular fluid
- Reabsorption of filtered HCO₃⁻
- Generation of new HCO₃⁻
Daily Quantities
- HCO₃⁻ filtered: ~4,320 mEq/day
- H⁺ secreted to reabsorb filtered HCO₃⁻: ~4,000 mEq/day (95% in proximal tubule)
- Additional H⁺ for nonvolatile acid excretion: ~80 mEq/day
- Total H⁺ secreted daily: ~4,400 mEq/day
H⁺ Secretion Mechanisms by Segment
| Segment | Mechanism | % of H⁺ secreted | Minimum pH achieved |
|---|---|---|---|
| Proximal tubule | Na⁺-H⁺ counter-transport (secondary active) | ~80% | ~6.7 |
| Thick ascending loop of Henle | Na⁺-H⁺ counter-transport | ~10% | ~6.7 |
| Early distal tubule | Na⁺-H⁺ counter-transport | small | ~6.7 |
| Late distal + collecting tubules | Primary active (H⁺-ATPase; H⁺-K⁺-ATPase in type A intercalated cells) | ~5% | ~4.5 |
How HCO₃⁻ is Reabsorbed (Not Direct!)
HCO₃⁻ does not cross the luminal membrane directly. Instead:
- H⁺ secreted into lumen combines with filtered HCO₃⁻ → H₂CO₃
- H₂CO₃ → CO₂ + H₂O (via carbonic anhydrase on brush border)
- CO₂ diffuses into tubular cell → forms new H₂CO₃ → H⁺ + HCO₃⁻
- HCO₃⁻ exits across basolateral membrane into blood; H⁺ is re-secreted
Urinary Buffers - Generation of "New" HCO₃⁻
When H⁺ secretion exceeds filtered HCO₃⁻, excess H⁺ combines with urinary buffers:
Phosphate buffer (titratable acid):
- H⁺ + HPO₄²⁻ → H₂PO₄⁻ (excreted as NaH₂PO₄)
- Each H⁺ excreted this way adds one new HCO₃⁻ to the blood
- Provides ~30-40 mEq/day
Ammonia buffer (NH₄⁺):
- Glutamine → 2 NH₄⁺ + 2 HCO₃⁻ (in proximal tubule cells)
- NH₄⁺ excreted via Na⁺-NH₄⁺ counter-transport; HCO₃⁻ enters blood
- In collecting tubule: H⁺ secreted → combines with NH₃ → NH₄⁺ (trapped in lumen)
- Each NH₄⁺ excreted adds one new HCO₃⁻ to the blood
- Normal: ~50 mEq/day; in chronic acidosis: up to 500 mEq/day
- Most important mechanism for new HCO₃⁻ generation in chronic acidosis
Net Acid Excretion Formula:
Net acid excretion = NH₄⁺ excretion + Titratable acid - HCO₃⁻ excretion
6. Factors Regulating H⁺ Secretion
| Factor | Effect on H⁺ Secretion | Effect on HCO₃⁻ Reabsorption |
|---|---|---|
| ↑PCO₂ | ↑ | ↑ |
| ↑H⁺ (↓HCO₃⁻) | ↑ | ↑ |
| ↓ECF volume | ↑ (via Ang II, aldosterone) | ↑ |
| ↑Angiotensin II | ↑ | ↑ |
| ↑Aldosterone | ↑ | ↑ |
| Hypokalemia | ↑ | ↑ |
| Hyperkalemia | ↓ | ↓ |
Note: ECF volume depletion → ↑Ang II + ↑aldosterone → excess H⁺ secretion → metabolic alkalosis
7. The Four Primary Acid-Base Disorders
| Disorder | Primary Change | pH | H⁺ | PCO₂ | HCO₃⁻ |
|---|---|---|---|---|---|
| Respiratory acidosis | ↑PCO₂ | ↓ | ↑ | ↑↑ | ↑ (renal compensation) |
| Respiratory alkalosis | ↓PCO₂ | ↑ | ↓ | ↓↓ | ↓ (renal compensation) |
| Metabolic acidosis | ↓HCO₃⁻ | ↓ | ↑ | ↓ (respiratory compensation) | ↓↓ |
| Metabolic alkalosis | ↑HCO₃⁻ | ↑ | ↓ | ↑ (respiratory compensation) | ↑↑ |
Double arrows (↑↑ or ↓↓) = primary disturbance
8. Renal Correction of Acidosis vs. Alkalosis
In acidosis:
- Complete reabsorption of all filtered HCO₃⁻
- Excess H⁺ secreted → combines with phosphate and ammonia buffers
- Large amounts of new HCO₃⁻ added to blood
- NH₄⁺ excretion increases dramatically (up to 500 mEq/day)
In alkalosis:
- H⁺ secretion reduced → cannot reabsorb all filtered HCO₃⁻
- Excess HCO₃⁻ excreted in urine (same as adding H⁺ to blood)
- No titratable acid or NH₄⁺ excretion (no excess H⁺ available)
9. Clinical Causes of Acid-Base Disorders
Respiratory Acidosis (↓ventilation → ↑PCO₂)
- Severe emphysema, pneumonia, airway obstruction
- Respiratory center depression (drugs, trauma)
- Decreased pulmonary membrane surface area
Respiratory Alkalosis (↑ventilation → ↓PCO₂)
- High altitude (hypoxia stimulates breathing)
- Psychogenic hyperventilation
- Fever, early salicylate toxicity
Metabolic Acidosis (primary ↓HCO₃⁻)
- Diarrhea (most common) - loss of HCO₃⁻ in feces
- Diabetes mellitus - ketoacidosis (ketoacids overwhelm buffers)
- Renal failure - impaired H⁺ secretion, ↓NH₄⁺ production
- Renal tubular acidosis - defect in H⁺ secretion or HCO₃⁻ reabsorption
- Lactic acidosis, aspirin poisoning, methanol/ethylene glycol (↑anion gap)
Metabolic Alkalosis (primary ↑HCO₃⁻)
- Vomiting of gastric contents - loss of HCl
- Diuretics (except carbonic anhydrase inhibitors) - ↑Na⁺ reabsorption → ↑H⁺ secretion
- Excess aldosterone (Conn syndrome) - ↑H⁺ secretion
- NaHCO₃ ingestion - direct HCO₃⁻ loading
10. Clinical Diagnosis of Acid-Base Disorders
Step-by-Step Approach (3 values needed)
- pH < 7.4 = acidosis; > 7.4 = alkalosis
- PCO₂ (normal 40 mmHg): ↑ = respiratory component to acidosis; ↓ = respiratory component to alkalosis
- HCO₃⁻ (normal 24 mEq/L): ↓ = metabolic component to acidosis; ↑ = metabolic component to alkalosis
Expected Compensations (Simple Disorders)
| Disorder | Expected Compensation |
|---|---|
| Respiratory acidosis | ↑HCO₃⁻ (renal, takes 3-5 days) |
| Respiratory alkalosis | ↓HCO₃⁻ (renal, takes 3-5 days) |
| Metabolic acidosis | ↓PCO₂ (respiratory, within 3-12 min) |
| Metabolic alkalosis | ↑PCO₂ (respiratory, limited by hypoxemia) |
Mixed acid-base disorder: Compensatory response is markedly different from expected → two simultaneous primary disorders (e.g., diarrhea + emphysema = mixed metabolic + respiratory acidosis)
11. Anion Gap
Anion gap = [Na⁺] - [HCO₃⁻] - [Cl⁻] = 144 - 24 - 108 = ~12 mEq/L (normal: 8-16 mEq/L)
Increased anion gap (normochloremic metabolic acidosis):
- Diabetic ketoacidosis, lactic acidosis, chronic renal failure, aspirin/methanol/ethylene glycol poisoning, starvation
Normal anion gap (hyperchloremic metabolic acidosis):
- Diarrhea, renal tubular acidosis, carbonic anhydrase inhibitors, Addison disease
12. Treatment of Acid-Base Disorders
| Condition | Treatment |
|---|---|
| Acidosis | NaHCO₃ (oral/IV), sodium lactate, sodium gluconate |
| Alkalosis | Ammonium chloride (oral/IV - caution: NH₄⁺ is toxic); isotonic saline for volume depletion-associated alkalosis |
| Best approach | Correct the underlying cause |
High-Yield Exam Points
- Normal arterial pH = 7.40; venous = 7.35
- Henderson-Hasselbalch: pH = 6.1 + log(HCO₃⁻ / 0.03 × PCO₂)
- Kidneys are slowest but most powerful acid-base regulators
- ~85% of HCO₃⁻ reabsorption occurs in the proximal tubule
- Minimum urine pH achievable = 4.5 (collecting duct, primary active H⁺-ATPase)
- Free H⁺ excretion in urine is negligible; most acid is excreted as NH₄⁺ and titratable acid
- In chronic acidosis: NH₄⁺ excretion can reach 500 mEq/day
- Hypokalemia → alkalosis; Hyperkalemia → acidosis (via effects on renal H⁺ secretion)
- ECF volume depletion → metabolic alkalosis (through Ang II and aldosterone)
- Respiratory compensation for metabolic acidosis takes 3-12 minutes; renal compensation for respiratory disorders takes 3-5 days
This is a shared conversation. Sign in to Orris to start your own chat.