SECTION 02: MECHANICS OF BREATHING

By the end of Section 02, you should be able to:

• Describe the different pressures involved in the mechanics of breathing.

• Applying the law of Laplace, describe why alveoli do not collapse.

• Describe the pressure changes that occur to enable inhalation and expiration.

• Describe the pressure-volume relationship of the lung.

🔢 Key Equation for Air Flow

🎯 What Drives Air Flow?

• A pressure gradient (difference in pressure) between:

  • The nose and the alveoli

🫧 When Does Air Flow In?

• Alveolar pressure < Nose pressure

• Air flows into alveoli (inhalation)

💨 When Does Air Flow Out?

• Alveolar pressure > Nose pressure

• Air flows out of alveoli (exhalation)

🧱 What the Pressure Must Overcome:

1. Elastance = Stiffness of lungs/chest (how stretchy they are)

2. Resistance = Friction in the airways

3. Inertia = Tendency of tissues/air to resist changes in motion (minor)

📚 Respiratory Mechanics =

How pressures, volume, and resistance interact to allow breathing

🌬 PRESSURES IN THE RESPIRATORY SYSTEM

🌎 1. Atmospheric Pressure (PB)

• Also called: Barometric Pressure

• It’s the air pressure around us from the atmosphere

• At sea level = 760 mmHg
  ➝ In respiratory equations, we treat it as 0 cm H₂O

• Why? Because it’s the reference point (same at nose/mouth and alveoli at rest)

🫁 2. Alveolar Pressure (PA)

• Also called: Intrapulmonary Pressure

• Pressure inside the alveoli

• At end of a normal breath (inspiration or expiration): PA = 0 cm H₂O (same as atmosphere)

🟦 3. Pleural Pressure (Ppl)

• Also called: Intrapleural Pressure

• Pressure in the pleural space (between lungs and chest wall)

• Normally around: -5 cm H₂O

• Why negative?

  • Lungs want to collapse inward

  • Chest wall wants to expand outward

  • Creates a slight suction effect between them

↔ 4. Transpulmonary Pressure (Ptp)

Intrathoracic Pressure:

• ➝ Pressure inside the thoracic cavity, closely matches pleural pressure

🧪 Summary Table:

🧪 Units Used in Respiratory Physiology

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📏 1. mmHg (millimeters of mercury)

📌 Key Conversion:

• 1 mmHg = 1.36 cm H₂O

• So 5 cm H₂O ≈ 3.7 mmHg

🌀 3. Pressure Measurements Are Relative

• Respiratory pressures are measured relative to atmospheric pressure (0 cm H₂O)

• Example:
  If atmospheric pressure = 1034 cm H₂O
  And alveolar pressure = 1029 cm H₂O
  Then: Alveolar pressure = -5 cm H₂O

❗ Note:

• “Negative pressure” doesn't mean less than zero — it just means lower than atmospheric pressure

Manometer

• Device that measures pressure (like a ruler for gas or fluid pressure)

🫁 Elastic Recoil of the Lungs

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🔁 What Is Elastic Recoil?

• The lungs have a natural tendency to deflate after they’ve been stretched (like a balloon shrinking after letting go).

• This is due to:

  1. Elastin fibers

  2. Surface tension in alveoli

🧵 1. Elastin Fibers

• Found in lung connective tissue

• Arranged like a mesh to allow stretching

• When stretched during inhalation, they snap back to help push air out

💧 2. Surface Tension

• Caused by a thin liquid lining inside the alveoli

• Makes up about 70% of the recoil force

• Why?

a) Resists Stretching

• Water molecules stick together and don’t like being pulled apart

• Makes alveoli resist expanding

b) Wants to Shrink

• Water molecules pull inward, trying to shrink the alveoli

• Without opposing forces (like inhalation), alveoli would collapse and expel air

⚖ Balance of Forces (Shown in Image)

• Lungs want to collapse inward

• Chest wall wants to spring outward

• The pleural space and fluid in it balance these forces, preventing lung collapse

💡 Summary:

🫁 ALVEOLAR STABILITY: Why Alveoli Don’t Collapse

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💧 1. Pulmonary Surfactant

• What is it?
  A mix of lipids + proteins made by type II alveolar cells

• What does it do?

  • Spreads over the alveolar lining

  • Reduces surface tension by separating water molecules

  • Makes lungs easier to expand (↑ compliance)

  • Prevents alveoli from collapsing (especially small ones)

🔄 2. Alveolar Interdependence

• What is it?
  Alveoli are connected to each other by elastic tissue

• How it works:
  If one alveolus starts to collapse, neighbors stretch and pull it open

• Helps stabilize the lung structure

🌟 Surfactant Effects

👶 Clinical Application: Respiratory Distress Syndrome (RDS)

🍼 In Premature Babies:

• Surfactant production begins ~24 weeks gestation

• Sufficient levels by ~35 weeks

🚨 If born early (<35 weeks):

• Not enough surfactant

• Alveoli stick together & collapse

• Baby’s lungs are less compliant

• Breathing becomes very hard

• Condition = Neonatal Respiratory Distress Syndrome (RDS)

Surfactant

Surfactant is a fluid made by type II alveolar cells in the lungs.

It’s a mix of lipids and proteins that:

• Reduces surface tension in the alveoli

• Prevents alveoli from collapsing after exhalation

• Makes it easier to breathe by increasing lung compliance

Without surfactant, breathing would require much more effort, especially in newborns.

🫁 Symptoms of RDS

• Labored breathing

• Weak cry

• Blue skin tone (cyanosis)

• May require surfactant therapy + oxygen

💬 Summary:

📏 Law of Laplace (for alveoli)

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🧪 The Formula:

🧠 What the terms mean:

• P = Collapsing pressure inside the alveolus

• T = Surface tension

• r = Radius of the alveolus

🔁 How It Works:

The Law of Laplace - 💡 Why this matters:

• Smaller alveoli are more at risk of collapsing due to higher pressure.

• Surfactant reduces T more in small alveoli to equalize pressure across all alveoli.

• Keeps alveolar sizes stable and prevents collapse.

🫁 Summary:

The Law of Laplace explains why smaller alveoli need more help (from surfactant) to stay open.

Activity

Activity pt 2

🧮 Law of Laplace Equation

❓ Question: Compare Collapsing Pressure

💧 What Is Pulmonary Surfactant?

• A lipid + protein mixture secreted by type II alveolar cells

• Spreads over the alveolar surface

• Reduces surface tension by breaking water-water bonds

• Makes lungs more compliant and prevents alveoli from collapsing

🔄 How It Prevents Collapse

• Each alveolus adjusts how much surfactant it makes

• Smaller alveoli get more surfactant → greater reduction in surface tension

• This helps equalize collapsing pressure across alveoli of different sizes

🧱 Other Factor: Alveolar Interdependence

• Alveoli are linked together by elastic tissue

• If one alveolus starts to collapse, neighboring alveoli pull it back open

• This supports alveolar structure

🫁 Summary Table

Activity

💨 Alveolar Pressure (PA) and Airflow

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🧠 Key Idea:

• Air flows from high pressure → low pressure

• So to breathe in, PA < atmospheric pressure

• To breathe out, PA > atmospheric pressure

🌍 Atmospheric Pressure

• Treated as 0 mmHg (a fixed reference point)

📐 Important Equation:

🫁 What Controls PA?

• Lung recoil pressure (Pl) is based on lung volume
  ➝ You can’t change Pl directly to change PA

• So, you must change pleural pressure (Ppl) to change PA

💪 How Do We Change Ppl?

📌 Summary:

• To change alveolar pressure (PA) and move air:

  • You must change pleural pressure (Ppl)

  • And that’s done by using your respiratory muscles

🫁 Inhalation and Exhalation = Pressure-Driven Airflow

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🌬 Before Inhalation (End of Expiration)

🫧 Inhalation Begins

🛑 Inhalation Ends

• Inspiratory muscles stop contracting

• Lung recoil catches up to Ppl

• → PA rises to match atmospheric pressure

• → Airflow stops

💨 Exhalation Begins

• Inspiratory muscles are fully relaxed

• Lung recoil now greater than Ppl

• → PA becomes positive

• → Air flows out of alveoli

🔁 Active Exhalation (Extra Effort)

• Happens during exercise, coughing, etc.

• Expiratory muscles contract

• → Increases Ppl and PA

• → More air pushed out

🌬 Onset of Inhalation

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🧘 Before Inhalation Starts

• Alveolar Pressure (PA) = Atmospheric Pressure

• → No airflow

💪 Inspiration Begins

• Inspiratory muscles contract

• → Pleural Pressure (Ppl) becomes more negative

• → Thoracic cavity expands

🫁 What Happens Next?

• Alveolar pressure (PA) drops slightly (≈ -1 cm H₂O)

• → Creates a pressure gradient

• → Air flows into alveoli

📈 Airflow Continues Until...

• Alveolar pressure rises (as air fills lungs)

• When PA = atmospheric pressure again

• → Inhalation ends

📐 Extra Note:

• The drop in pleural pressure is not linear

  • At the beginning of inhalation, there is increased resistance to overcome

🔁 Graph Quick Guide:

💨 Onset of Exhalation

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🛑 End of Inhalation

• Inspiratory muscles relax

• No more effort to expand the chest

🔁 What Happens Next?

• Pleural pressure (Ppl) becomes less negative

• → Alveolar pressure (PA) increases

• → Now PA > atmospheric pressure

🌬 Air Flows Out

• Air flows out of the lungs
  → Until PA = atmospheric pressure
  → At this point, airflow stops

❌ No Expiratory Muscles Needed

• Normal exhalation is passive

• Caused by elastic recoil of the lungs and chest wall

🧠 Summary:

💨 Active Exhalation

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🧘‍♀ At Rest

• Expiration is passive (lungs recoil, no muscle use)

🏃‍♂ During Exercise or Forceful Breathing

• Active exhalation helps push air out faster and deeper

• Expiratory muscles contract to increase pleural pressure (Ppl)

• This helps push more air out of the lungs

💪 Muscles Involved

📉 Effect on Lung Volumes

• Lowers End-Expiratory Lung Volume (aka FRC)

• → Increases Tidal Volume (more room to inhale)

⚠ During Forced Expiration

• As air flows out, pressure drops due to resistance

• Eventually reaches the Equal Pressure Point (EPP):

  • Airway pressure = Pleural pressure

  • Beyond this: airway compresses

  • → Transpulmonary pressure (Ptp) becomes negative

  • → Further pressure does NOT increase flow

📌 Key Definitions:

🧠 Summary:

• Active exhalation = muscle-driven, used during exercise or forced breathing

• Flow limit happens at the EPP, where trying harder doesn’t help

💨 Active Expiration & Lung Collapse Prevention

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📐 Key Formula:

❓ Why Don’t Lungs Collapse in Active Expiration? (3 reasons)—> ✅ 1. Alveolar Pressure Rises with Pleural Pressure

• During active expiration, Ppl becomes positive (due to abdominal muscle contraction)

• But PA also increases (since PA = Pl + Ppl)

• So alveoli stay inflated → no collapse

✅ 2. Airway Compression Limits Flow

• High Ppl compresses airways, increasing resistance

• Reaches Equal Pressure Point (EPP)

• Beyond that point, more pressure = more compression, not more air flow

✅ 3. You Can’t Exhale Below Residual Volume

• Because of airway compression, you can’t push all air out

• Prevents lungs from fully collapsing or bronchioles from shutting down

🧠 Summary:

🔄 Pressure–Volume Relationships (3 things to note) —> 📈 1. Lung Pressure (Pl)

• As lung volume ↑, lung recoil pressure (Pl) ↑

• Starts near 0 cm H₂O at residual volume (RV)

• Increases to about +30 cm H₂O at total lung capacity (TLC)

• The lungs want to deflate when stretched (positive pressure)

🧱 2. Chest Wall Pressure (Pw)

• Acts like a spring:

  • Below 65% of vital capacity: wants to expand (negative pressure = inflating)

  • At 100% vital capacity: wants to collapse (positive pressure = deflating)

🔄 3. Respiratory System Pressure (Prs)

🧠 Key Concepts

🫁 Visual Notes from the Graph:

• Left side = inward arrows from chest wall = inflating

• Right side = outward arrows = deflating

• FRC (Functional Residual Capacity) = point where lung recoil and chest wall outward force are balanced

📈 Compliance (C)

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💡 Definition:

🔼 High Compliance

• Lungs stretch easily

• Less pressure needed to move air in/out

• Found at Functional Residual Capacity (FRC)

• ➝ Breathing requires less effort

🔽 Low Compliance

• Lungs are stiff or damaged

• More pressure needed to breathe

• Makes inhalation/exhalation harder

• Common in lung diseases (e.g., emphysema)

🫁 Clinical Example: Emphysema

• Damaged alveoli → reduced compliance

• Even at FRC, breathing becomes more work

📌 Summary Table

📏 What Is Compliance?

🫁 Compliance Is Greatest At...

• Functional Residual Capacity (FRC)
  ➝ Means less effort is needed to breathe in or out at this volume

🔼 High Compliance

• Lungs stretch easily

• Requires less pressure to move air

• Found in healthy lungs at FRC

🔽 Low Compliance

• Lungs are stiff

• Requires more pressure to breathe

• Seen in diseases like fibrosis or emphysema

💡 What Affects Compliance?

1. Elasticity (Elastin fibers)

• Lungs recoil after stretch due to elastin in connective tissue

2. Surface Tension

• Water lining alveoli wants to collapse them

• Surfactant ↓ surface tension → ↑ compliance

🌊 Surfactant

• Made by type II alveolar cells

• Reduces surface tension

• Increases compliance

• Makes breathing easier

🔄 Hysteresis

• Pressure–volume curve is different during inspiration and expiration

• Reason: Must overcome surface tension during inhalation

🤝 Lung + Chest Wall Compliance

• Chest wall wants to expand

• Lungs want to collapse

• At FRC, these forces balance

• Together, their combined compliance is lower than either one alone

🔁 Summary Table: