Mechanism of respiration

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Mechanism of Respiration

Respiration involves the movement of air between the atmosphere and the alveoli through cyclical changes in thoracic volume and pressure. It has two phases — inspiration (active) and expiration (passive at rest).

1. Structural Framework

The thoracic cage (ribs, sternum, thoracic vertebrae) and its muscles form the mechanical pump. The pleural cavity — a thin, fluid-filled space between the visceral and parietal pleura — transmits pressure changes from the chest wall to the lungs.

Key anatomical point: The lungs adhere to the thoracic wall through capillary forces of the pleural fluid and negative intrapleural pressure. They passively follow every thoracic movement. If the pleural cavity is breached (pneumothorax), negative pressure is lost, and the elastic lungs collapse to ~⅓ their original volume. — Color Atlas of Human Anatomy Vol. 2

2. Pressure Relationships

Three pressures govern breathing (all referenced to atmospheric pressure = 0):

Pressure	At Rest	During Inspiration	During Expiration
Alveolar (P_alv)	0 cm H₂O	−1 cm H₂O	+1 cm H₂O
Intrapleural (P_ip)	−5 cm H₂O	−8 cm H₂O	−4 cm H₂O
Transmural (P_alv − P_ip)	+5 cm H₂O	+7 cm H₂O	+5 cm H₂O

The transmural pressure (also called transpulmonary pressure) is always positive throughout the breathing cycle, keeping the lungs open. — Costanzo Physiology 7th Ed.

The negative intrapleural pressure at rest exists because two opposing elastic forces pull on the intrapleural space:

The lungs (elastic tissue + surface tension) tend to collapse inward
The chest wall tends to spring outward This creates a "vacuum" between them.

3. Muscles of Breathing

Inspiration (Active)

Diaphragm — the primary muscle; on contraction, it descends, pushing abdominal contents down and expanding the thorax caudally
External intercostals — elevate the ribs, increasing transverse and sagittal diameters
Scalene muscles — accessory muscles that elevate the upper ribs
Sternocleidomastoid — used during forced inspiration

Expiration (Passive at rest)

Driven by elastic recoil of lungs and chest wall — no muscle contraction required at rest
Internal intercostals — pull ribs down and inward during forced expiration
Abdominal muscles (rectus abdominis, transverse abdominis) — compress abdomen, push diaphragm up; active during exercise, coughing, sneezing

— Costanzo Physiology 7th Ed.; Color Atlas of Human Anatomy Vol. 2

4. The Breathing Cycle

At Rest (FRC — Functional Residual Capacity)

No air movement; alveolar pressure = atmospheric pressure (0 cm H₂O)
Intrapleural pressure = −5 cm H₂O
Volume in lungs = FRC (~2.5 L in adults)

Inspiration

The diaphragm contracts → thoracic volume increases
By Boyle's Law (P × V = constant), as volume ↑, pressure ↓
Alveolar pressure drops to −1 cm H₂O (below atmospheric)
Pressure gradient (atmosphere → alveoli) drives air into the lungs
Air flows in until alveolar pressure returns to 0 — flow stops
Volume added = tidal volume (~500 mL); total volume now = FRC + VT
Intrapleural pressure becomes more negative (−8 cm H₂O) due to increased elastic recoil and negative airway pressure

Expiration (quiet)

Diaphragm relaxes → thoracic volume decreases
Elastic recoil of lungs generates positive alveolar pressure (+1 cm H₂O)
Air flows out of lungs (alveoli → atmosphere) down the pressure gradient
Flow stops when alveolar pressure returns to 0; system returns to FRC
Intrapleural pressure returns to −5 cm H₂O

— Costanzo Physiology 7th Ed.

5. Illustration: Thoracic Cage and Diaphragm Movement

Mechanics of respiration showing thoracic cage and diaphragm positions during inspiration (top) and expiration (bottom)

Fig. 3.21 — Positions of the thoracic cage and diaphragm during respiration. During inspiration (A), the ribs rise, increasing transverse (1) and sagittal (2) diameters; the epigastric angle widens (3); the diaphragm descends (4); and the inferior thorax expands (5). Opposite changes occur during expiration (B). — Color Atlas of Human Anatomy Vol. 2

6. Compliance and Resistance

Compliance

Compliance = ΔVolume / ΔPressure — describes how easily the lungs stretch
High compliance (emphysema) → lungs over-distend
Low compliance (fibrosis, pulmonary edema) → stiff lungs, harder to inflate
The chest wall itself also has compliance; the two compliances act in series

Surface Tension & Surfactant

The alveolar air-liquid interface creates surface tension (by Laplace's Law: P = 2T/r), which tends to collapse alveoli
Surfactant (secreted by type II pneumocytes) reduces surface tension, especially in small alveoli, preventing alveolar collapse (atelectasis) and reducing the work of breathing

Airway Resistance

Governed by Poiseuille's Law: Resistance ∝ 1/r⁴ (radius has the greatest effect)
Main site of resistance: medium-sized bronchi (not alveoli, which have too large a combined cross-section)
Increased in asthma and COPD → expiratory muscles must assist passive expiration

7. Types of Breathing

Thoracic (costal) breathing — predominant in adults; relies on rib cage movement
Diaphragmatic (abdominal) breathing — predominant in infants and elderly
Infants use abdominal breathing due to horizontal rib orientation; the elderly due to reduced thoracic elasticity

Summary Table

Phase	Muscle Activity	Thoracic Volume	Alveolar Pressure	Air Flow
Rest	None	Stable (FRC)	0 cm H₂O	None
Inspiration	Diaphragm contracts	Increases	−1 cm H₂O	Into lungs
End of inspiration	Diaphragm plateaus	Max	0 cm H₂O	Stops
Expiration (quiet)	Elastic recoil only	Decreases	+1 cm H₂O	Out of lungs
End of expiration	Muscles relax	FRC	0 cm H₂O	Stops

The entire mechanism rests on Boyle's Law (pressure-volume relationship) operating within a compliant thoracopulmonary system, powered by the diaphragm and accessory muscles, and modulated by the elastic properties of lung tissue and surfactant.

With diagram

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Here is the complete, diagram-supported explanation of the mechanism of respiration:

Mechanism of Respiration — With Diagrams

Diagram 1 — Lung & Chest Wall System

Lung and chest wall schematic showing conducting airways, alveoli, and intrapleural space

The lungs (alveoli) sit inside the chest wall, separated by the intrapleural space. Air enters via the conducting airways. The intrapleural space transmits pressure changes from the thorax to the lung surface. — Costanzo Physiology 7th Ed., Fig. 5.8

Diagram 2 — Thoracic Cage & Diaphragm Movement

Positions of thoracic cage and diaphragm during inspiration (top) and expiration (bottom)

During Inspiration (A): Ribs rise → transverse (1) and sagittal (2) diameters of the thorax increase → epigastric angle widens (3) → diaphragm descends (4) → thorax expands caudally (5). Lung volume increases.

During Expiration (B): All movements reverse. Ribs descend, diaphragm domes rise, thoracic volume decreases. — Color Atlas of Human Anatomy Vol. 2, Fig. 3.21

Diagram 3 — Pressures Throughout the Breathing Cycle

Pressure cycle showing alveolar and intrapleural pressures at rest (A), mid-inspiration (B), end-inspiration (C), and mid-expiration (D)

This is the key diagram. All pressures are in cm H₂O relative to atmospheric (= 0). Yellow arrows = transmural (expanding) pressure. Blue arrows = direction of airflow.

Phase	Alveolar P	Intrapleural P	Transmural P	Airflow
A — Rest	0	−5	+5	None
B — Mid-Inspiration	−1	−6.5	+5.5	Into lungs ↓
C — End-Inspiration	0	−8	+8	Stops
D — Mid-Expiration	+1	−6.5	+7.5	Out of lungs ↑

Key principle (Boyle's Law): As thoracic volume ↑ → alveolar pressure ↓ → air flows in. As volume ↓ → pressure ↑ → air flows out. Transmural pressure remains positive throughout, keeping airways open. — Costanzo Physiology 7th Ed., Fig. 5.14

Diagram 4 — Intrapleural Pressure: Normal vs. Pneumothorax

Normal intrapleural pressure vs pneumothorax showing lung collapse and chest wall expansion

Normal: Intrapleural pressure = −5 cm H₂O (negative), held by opposing elastic forces — lung trying to collapse inward, chest wall trying to spring outward. This negative pressure keeps the lungs inflated.

Pneumothorax: Air enters the intrapleural space → pressure rises to 0 → lungs collapse inward (elastic recoil unopposed) → chest wall springs outward. — Costanzo Physiology 7th Ed., Fig. 5.9

Mechanism Summary

Neural stimulus (brainstem respiratory centres)
        ↓
Diaphragm + external intercostals CONTRACT
        ↓
Thoracic volume INCREASES
        ↓
Intrapleural pressure ↓ (more negative: −5 → −8 cm H₂O)
        ↓
Lung expands → Alveolar volume ↑ → Alveolar pressure ↓ (0 → −1 cm H₂O)
        ↓
Pressure gradient: Atmosphere (0) > Alveoli (−1)
        ↓
AIR FLOWS IN (Inspiration)
        ↓
Alveolar pressure returns to 0 → flow stops
        ↓
Diaphragm RELAXES → Elastic recoil of lung
        ↓
Alveolar pressure ↑ (0 → +1 cm H₂O)
        ↓
Pressure gradient: Alveoli (+1) > Atmosphere (0)
        ↓
AIR FLOWS OUT (Passive Expiration)
        ↓
Return to FRC — cycle repeats

The entire mechanism is driven by Boyle's Law within a compliant thoracopulmonary system, with the negative intrapleural pressure acting as the critical coupling force between the chest wall and the lung. — Costanzo Physiology 7th Ed.

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