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🫁 MECHANICS OF BREATHING

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1. WHAT IS IT?

• Study of physical properties of lungs, airways, and chest wall involved in breathing
• Two conditions studied:
  • Static lung = no airflow
  • Dynamic lung = air flowing in/out

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2. MUSCLES OF RESPIRATION

🔵 Thoracic Cavity Structure

• Lungs sit inside thoracic cavity
• Separated from abdomen by the diaphragm
• Chest wall = sternum + 12 pairs of ribs + intercostal muscles
• Rib cage is raised during inspiration, lowered during expiration

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🔵 Muscles of INSPIRATION (breathing IN)

Primary Muscles:

• Dome-shaped muscle attached to lower ribs + xiphoid process
• On contraction → dome flattens
• Pushes abdominal contents downward → thoracic cavity enlarges vertically
• Also pushes rib cage outward → increases anteroposterior + lateral diameter
• Moves ~1-2 cm in quiet breathing; up to 10-12 cm in deep breathing
• Innervated by phrenic nerve (C3, C4, C5)
• Damage to phrenic nerve → that side of diaphragm moves UP (paradoxical)

• Run obliquely between ribs (forward + downward)
• Contraction → lower ribs pulled upward and forward
• Two effects:
  • Bucket-handle effect → increases transverse diameter (2nd to 10th ribs rotate outward)
  • Pump-handle effect → increases vertical diameter (upper 4 ribs push sternum outward)

Accessory (Used in Forced Inspiration):

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🔵 Muscles of EXPIRATION (breathing OUT)

• Quiet expiration = PASSIVE (no muscles needed)
  • Elastic recoil of lungs pushes air out
  • Inspiratory muscles simply relax → chest falls
• Forced expiration = ACTIVE (muscles used):
  • Abdominal muscles (internal oblique, external oblique, transverse abdominis) → push diaphragm up → compress thorax
  • Internal intercostals → pull rib cage downward → decrease thoracic volume
  • Neck + back muscles → lower pectoral girdle

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3. HOW INSPIRATION HAPPENS (Step by Step)

Diaphragm + external intercostals CONTRACT
        ↓
Thoracic cage EXPANDS
        ↓
Intrapleural pressure becomes MORE NEGATIVE
        ↓
Transpulmonary pressure INCREASES
        ↓
Lungs INFLATE
        ↓
Alveolar pressure DROPS (below atmospheric = subatmospheric)
        ↓
Air flows INTO lungs
        ↓
Inspiratory muscles stop contracting → inspiration ends

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4. HOW EXPIRATION HAPPENS (Step by Step)

Inspiratory muscles RELAX
        ↓
Rib cage DROPS
        ↓
Intrapleural pressure becomes LESS NEGATIVE
        ↓
Transpulmonary pressure DECREASES
        ↓
Stretched lungs DEFLATE (elastic recoil)
        ↓
Alveolar pressure RISES (above atmospheric)
        ↓
Air pushed OUT of lungs
        ↓
Continues until alveolar pressure = atmospheric pressure

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5. PRESSURE CHANGES DURING BREATHING

Three Key Pressures:

Key Points:

• Intrapleural pressure is always negative (acts like a vacuum)
• This negative pressure keeps lungs expanded against chest wall
• If chest wall is perforated → air enters pleural space → pneumothorax → lung collapses
• Transmural pressure = pressure difference across a wall
• Transairway pressure = keeps airways open during forced expiration

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6. LUNG VOLUMES & CAPACITIES

4 Lung Volumes:

4 Lung Capacities (combinations of volumes):

FRC is important - maintains stable gas tension so gas exchange continues even between breaths

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7. ELASTIC PROPERTIES

Lung Compliance

• = Ability to stretch (distensibility)
• Formula: Compliance = ΔV / ΔP
• Low compliance (stiff lung) = more work needed → e.g., pulmonary fibrosis
• High compliance (too stretchy) = hard to get air out → e.g., emphysema

Surface Tension & Surfactant

• Air-liquid interface in alveoli creates surface tension → tends to collapse alveoli
• Surfactant (made by Type II pneumocytes) reduces surface tension
• Main component: DPPC (dipalmitoylphosphatidylcholine)
• Surfactant functions:
  1. Prevents lung collapse
  2. Maintains alveolar stability (smaller alveoli don't empty into bigger ones)
  3. Prevents pulmonary edema
  4. Decreases work of breathing
• Surfactant synthesis starts at 34 weeks of pregnancy
• Premature babies lack surfactant → IRDS (Infant Respiratory Distress Syndrome)

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8. TYPES OF BREATHING

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9. AIRWAY RESISTANCE

• Resistance = Driving pressure (ΔP) / Airflow (V)
• Normal = 1-3 cm H₂O/L/sec (at FRC)
• Main site of resistance = medium bronchi (segmental + lobar)
• Small airways contribute only 10-15% of total resistance (aligned in parallel)
• Resistance is higher at low lung volumes, lower at high lung volumes

Airflow Types:

• Laminar flow = slow, silent, streamlined → in small peripheral airways
• Turbulent flow = fast, noisy → in large airways (trachea, bronchi)

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QUICK MEMORY TRICK 🧠

"Diaphragm DOWN → Lungs IN → Pressure DOWN → Air IN" "Diaphragm UP → Lungs OUT → Pressure UP → Air OUT"

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