SECTION 04: LUNG VOLUMES

By the end of Section 04, you should be able to:

• Describe the different lung volumes and why they are important.

• Describe how lung volumes are affected in obstructive and restrictive lung diseases.

• Using the formula for minute ventilation, make an argument as to whether it is better to increase frequency of breathing or tidal volume in order to increase ventilation.

• Describe what is meant by the “work” of breathing

🫁 LUNG VOLUMES & CAPACITIES (Spirometry)

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🔹 VT – Tidal Volume

• 💨 Normal breath in/out

• ~500 mL at rest
  🟢 Everyday breathing

🔹 IRV – Inspiratory Reserve Volume

• ⬆ Extra air you can inhale after a normal breath in

• ~3000 mL
  🟢 "Big inhale" on top of a normal breath

🔹 IC – Inspiratory Capacity

• Total air you can inhale from a normal breath out

• IC = VT + IRV = ~3500 mL
  🟢 Breath in as much as possible after a normal exhale

🔹 ERV – Expiratory Reserve Volume

• Extra air you can breathe out after a normal breath out

• ~1000 mL
  🟠 Push out as much as you can after a normal exhale

🔹 RV – Residual Volume

• Air that’s left in lungs after full exhale

• ~1200 mL
  🔴 Can’t be measured by spirometry
  🛑 Keeps lungs from collapsing

🔹 FRC – Functional Residual Capacity

• Air left in lungs after normal breath out

• FRC = ERV + RV = ~2200 mL
  🟣 “Resting” air in lungs

🔹 VC – Vital Capacity

• Max amount you can breathe out after a full breath in

• VC = IRV + VT + ERV = ~4500 mL
  🔵 Big breath in → big breath out

🔹 TLC – Total Lung Capacity

• All the air lungs can hold

• TLC = VC + RV = ~5700 mL
  ⚫ Total capacity

🔹 FEV₁ – Forced Expiratory Volume in 1 Second

• How much air you can forcefully exhale in 1 second

• Used to test lung function (esp. in asthma/COPD)
  ✅ Normally ~80% of VC

Graph summarizing the above

🫁 Obstructive vs. Restrictive Lung Disease

🟥 Obstructive Lung Disease

Examples: Asthma, COPD (Chronic Bronchitis, Emphysema)

🫁 Obstructive vs. Restrictive Lung Disease - 🔹 What’s Happening?

📉 Spirometry Results:

• ↓ FEV₁ (can't blow out much air in 1 sec)

• ↓ VC (Vital Capacity)

• ↑ RV & FRC (air gets trapped → hyperinflation)

• ↓ FEV₁ / VC ratio (< 80%)

❗ Common Symptoms:

• Shortness of breath

• Wheezing

• Breath stacking

🟦 Restrictive Lung Disease

Examples: Pulmonary fibrosis, obesity-related lung restriction, scoliosis

🔹 What’s Happening?

• Lungs are stiff or small

• Hard to get air in

📉 Spirometry Results:

• ↓ FEV₁ (less air overall)

• ↓ VC

• ↓ RV & FRC (lungs hold less air overall)

• **FEV₁ / VC ratio is normal or high (≥ 80%)

❗ Common Symptoms:

• Shallow breathing

• Feeling like you can’t take a deep breath

✅ Key Differences:

Master and understand this labelling

Review terms

🫁 Using Expiratory Flow to Assess Lung Function

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📈 What’s Measured?

We analyze forced expiration — breathing out as fast and fully as possible — to see how the lungs perform.

🔹 Why Use Expiration (Not Inhalation)?

🧪 Try this:

• Inhale fast: Takes <0.5 sec

• Exhale fully: Takes ~5 sec
  ➡ Expiration is harder → better reveals lung limitations

📉 How It’s Measured:

1. 

1. Patient exhales forcefully

2. Curve on left shows volume over time

3. Slope = flow rate

4. This data builds flow-volume curves (right graph)

🟢 Normal Curve

• Starts at full lung (TLC)

• High peak flow (~7 L/s)

• Linear drop in flow with decreasing volume

• Ends at residual volume

🔴 Obstructive Disease Curve

• Starts at higher lung volume (due to air trapping)

• Lower peak flow

• Scooped or "caved-in" shape

• Ends at higher residual volume
  ➡ Example: Asthma, COPD

🔵 Restrictive Disease Curve

• Starts at lower lung volume

• Lower peak flow (but sharp curve)

• Ends at lower residual volume
  ➡ Example: Pulmonary fibrosis

💡 Key Takeaway:

Forced expiration reveals more about lung function than inspiration
➡ It’s where limitations appear most clearly

🌬 VENTILATION & DEAD SPACE

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🔹 Minute Ventilation (VE)

🧠 But Not All Air is Useful for Gas Exchange!

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🔸 Anatomical Dead Space

📊 Gas Exchange Breakdown (Per Breath):

• 500 mL total inhaled (VT)

  • 150 mL → stays in airways = dead space

  • 350 mL → reaches alveoli = effective for gas exchange

🔁 Exhalation Works the Same:

• First 150 mL of air you breathe out = from the airways (not alveoli)

• So, 150 mL of fresh oxygen-rich air from the last breath never reached the alveoli at all

💡 Key Takeaway:

Even if your minute ventilation is normal, the actual gas exchange is lower
→ You need to account for anatomical dead space (150 mL)

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🫁 VENTILATION: What Improves Gas Exchange?

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🔹 Key Question:

Is it better to breathe faster or deeper to get more oxygen into your blood?

📊 Minute Ventilation Formula:

🛑 The Problem: Dead Space (150 mL)

• Every breath: first 150 mL of air = wasted (just fills airways, doesn’t reach alveoli)

• If VT = 150 mL, then alveolar ventilation = 0

🔁 Compare the Two Options:

➤ Breathing Faster (↑ f):

• Many small breaths

• More total air moved — BUT most of it gets trapped in dead space

• → Poor alveolar ventilation

➤ Breathing Deeper (↑ VT):

• Fewer, larger breaths

• More air goes beyond dead space

• → Better gas exchange

✅ Best Strategy:

✅ Best Strategy:

Slow, deep breathing
→ Maximizes alveolar ventilation
→ More oxygen in, more CO₂ out

🧠 Takeaway:

To improve gas exchange, it's better to increase tidal volume, not just breathing rate.

🫁 Alveolar Ventilation (V̇A)

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📘 Key Concept:

• Dead space (VD) = ~0.15 L (150 mL) → doesn’t participate in gas exchange

• Effective breathing = VT > VD

• Alveolar Ventilation (V̇A) = the air actually reaching alveoli per minute

📊 Formula Review:

• Minute Ventilation (V̇E) = VT × f

• Alveolar Ventilation (V̇A) = (VT - VD) × f

• V̇A/V̇E = % of total ventilation used for gas exchange
  → Higher = more efficient breathing

🛌 At Rest Examples (All V̇E = 6 L/min)

🏃‍♂ During Exercise

✅ Main Takeaways

• Deep > Fast for gas exchange

• Best efficiency = when VD/VT is low

• Breathing too shallow and fast → wastes air in dead space

• Exercise increases both depth and rate → improves gas exchange

💨 Work of Breathing

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🧠 Definition:

Work of Breathing = Energy your body uses to move air in and out of your lungs

🛑 Normal breathing uses < 3% of total body energy at rest

📈 Two Types of Work:

1. Elastic Work

   • Effort to stretch the lung and chest wall

   • ⬆ When tidal volume is high (deep breaths)

2. Flow-Resistive Work

   • Effort to overcome airway resistance

   • ⬆ When breathing frequency is high (fast breaths)

📊 Graph Breakdown:

• X-axis = Respiratory Frequency (breaths/min)

• Y-axis = Work of Breathing

🔵 Elastic work = Higher at low frequencies
🔴 Flow-resistive work = Higher at high frequencies
⚫ Total work = U-shaped curve

✅ Minimum total work occurs at moderate frequency
→ That’s the optimal breathing rate your body naturally uses at rest

⚖ What Happens at Extremes?

• Low frequency → deep breaths = ↑ elastic work

• High frequency → shallow, fast breaths = ↑ flow resistance

🟢 Best strategy = balance both for efficient breathing

🫁 Work of Breathing: COPD vs. Exercise

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🔴 COPD (Chronic Obstructive Pulmonary Disease)

🚫 What Happens:

• Airways are narrowed → ↑ flow-resistive work

• Breathing out becomes hard and slow

📉 Tidal Volume (VT): ↓ (lower, because lungs are already full—air trapping) 📈 Respiratory Frequency (f): ↑ (faster, shallow breaths)

⚠ Problem: More shallow breaths = more dead space = less gas exchange efficiency

🏃‍♂ During Exercise

💪 What Happens:

• Body needs more oxygen and must remove more CO₂

📈 Tidal Volume (VT): ↑ (deeper breaths) 📈 Respiratory Frequency (f): ↑ (more breaths per minute)

✅ Strategy: Take deep and faster breaths → improves alveolar ventilation
→ Flow-resistive + elastic work both increase, but this is tolerated short-term

🧠 Summary Chart:

🫁 Factors That Increase the Work of Breathing (

🔹 2. ↑ Airway Resistance

• Airways are narrowed or blocked

• Example: COPD, asthma

• ❗ Effect:

  • ↑ Flow-resistive work

  • ↓ Breathing Rate (f)

  • VT stays roughly the same
    ➡ Takes longer to breathe out

🔹 3. ↓ Elastic Recoil

• Lungs don’t spring back easily

• Example: Emphysema (damaged alveoli)

• ❗ Effect:

  • Expiration needs muscle effort

  • ↓ Breathing Rate (f)

  • VT stays the same
    ➡ Air gets trapped → harder to exhale

🔹 4. ↑ Demand for Ventilation

• Body needs more oxygen

• Example: Exercise

• ❗ Effect:

  • ↑ Tidal Volume (VT)

  • ↑ Breathing Rate (f)
    ➡ More work but beneficial

✅ Quick Summary Table: