Mechanics of Respiration – Pulmonary Ventilation

Learning Outcomes

• By the end of this topic you should be able to:

  • Name and locate every muscle of respiration and state its role in inspiration/expiration.

  • Explain step‐by‐step how pressure gradients drive airflow in and out of the lungs and how the gradients are generated.

  • Reconstruct the full sequence of events (muscular, volumetric, pressure) during quiet inspiration, quiet expiration, forced inspiration, and forced expiration.

  • List and describe the three major physical factors that hinder or facilitate pulmonary ventilation.

Integrated Functions of the Respiratory System

• Primary purpose: supply O₂ and remove CO₂.

• Five inter-related steps (see figure reference):

  1. Pulmonary ventilation — mechanical movement of air between atmosphere ↔ alveoli.

  2. External respiration — gas exchange lung ↔ blood (O₂ into blood, CO₂ out).

  3. Gas transport — circulation of gases: O₂ carried to tissues, CO₂ carried to lungs.

  4. Internal respiration — exchange blood ↔ cells (O₂ into cells, CO₂ into blood).

  5. Cellular respiration — mitochondrial use of O₂ to generate ATP ("ENERGY!").

• Pulmonary ventilation is the entry point that makes all subsequent steps possible.

Muscles of Respiration & Their Roles

• Quiet inspiration (ACTIVE):

  • Diaphragm (most important) – contracts & flattens ↓, increases superior-inferior dimension.

  • External intercostals – contract, lifting ribs & sternum (pump-handle & bucket-handle motions) → increases anterior-posterior & lateral dimensions.

• Quiet expiration (PASSIVE):

  • No active muscle contraction; relaxation of diaphragm & external intercostals + elastic recoil of lungs & thoracic cage.

• Forced inspiration (during exercise or COPD):

  • Accessory muscles: scalenes, sternocleidomastoid, pectoralis minor → further enlarge thoracic cavity.

• Forced expiration (ACTIVE):

  • Abdominal wall muscles (obliques, transversus abdominis, rectus abdominis) → push viscera up against diaphragm.

  • Internal intercostals → depress rib cage.

Basic Physics: Pressure, Volume & Boyle’s Law

• Atmospheric pressure P_{atm} = 760\,\text{mmHg} (sea level) is reference (0).

• Respiratory pressures expressed relative to P_{atm}:

  • Negative pressure < 0 ⇒ lower than atmospheric; e.g. -4\,\text{mmHg} \Rightarrow 756\,\text{mmHg absolute}.

  • Positive pressure > 0 ⇒ higher than atmospheric; e.g. +4\,\text{mmHg} \Rightarrow 764\,\text{mmHg absolute}.

  • Zero pressure = 0 ⇒ equal to atmospheric =760\,\text{mmHg}.

• Boyle’s Law: for a fixed amount of gas at constant temperature, P \propto \frac{1}{V}.

  • Decreasing container volume → pressure rises.

  • Increasing container volume → pressure falls.

Key Respiratory Pressures & Relationships

• Intrapulmonary (intra-alveolar) pressure P_{pul}

  • Pressure inside alveoli.

  • Fluctuates with breathing; equals P_{atm} at end-inspiration & end-expiration.

• Intrapleural pressure P_{ip}

  • Pressure within pleural cavity between visceral & parietal pleurae.

  • Always negative (≈ -4\,\text{mmHg}) under normal conditions.

  • Maintained by lymphatic drainage of pleural fluid.

  • Negative value results from opposing forces:

  • Inward (lung recoil + alveolar surface tension).

  • Outward (elasticity of chest wall).

• Transpulmonary pressure

  • Keeps airways open; the larger P_{tp}, the larger & more inflated the lungs.

  • If P_{ip}= P_{pul} or P_{atm} → lung collapse (atelectasis).

Sequence of Events: Quiet Inspiration

1. Inspiratory muscles contract (diaphragm descends; external intercostals lift ribs).

2. Thoracic cavity volume ↑.

3. Lung surface follows thoracic wall (pleural coupling) → V_{pul} ↑.

4. P_{pul} drops to ≈ -1\,\text{mmHg}.

5. Air flows into lungs down its pressure gradient until P{pul}=P{atm}.

Sequence of Events: Quiet Expiration

1. Inspiratory muscles relax.

2. Thoracic cavity volume ↓ (elastic recoil of costal cartilages & lungs).

3. V_{pul} ↓.

4. P_{pul} rises to ≈ +1\,\text{mmHg}.

5. Air flows out until P_{pul} =P_{atm}

Forced Breathing Variations

• Forced inspiration: accessory muscles enlarge thorax further, P_{pul} may fall to -2 to -3\,\text{mmHg} ⇒ larger tidal volume.

• Forced expiration: active contraction of abdominals & internal intercostals sharply ↑ intrathoracic pressure, driving rapid airflow.

Homeostatic Imbalances Related to Pressure

• Atelectasis (lung collapse):

  • Causes: plugged bronchiole → alveolar collapse; pneumothorax (air in pleural cavity) from chest wound or visceral pleura rupture.

  • Treatment: remove intrapleural air via chest tube so pleural membranes reseal & lung reinflates.

• Asthma: bronchoconstriction + mucus → ↑ airway resistance; treated with bronchodilators (e.g. albuterol, epinephrine).

Physical Factors Influencing Pulmonary Ventilation

1. Airway Resistance

   • Friction in airways; major non-elastic resistance.

   • Relationship: F = \frac{\Delta P}{R}.

   • Normally, \Delta P is only 1–2 mmHg; small radius changes (e.g. mucus, tumors, smooth muscle spasm) greatly ↑ R (Poiseuille’s law: R \propto \frac{1}{r^4}).

   • Medium-sized bronchi contribute most resistance; at terminal bronchioles, huge cross-sectional area → negligible R.

2. Alveolar Surface Tension

   • Water molecules lining alveoli attract, creating inward collapsing force.

   • Surfactant (produced by Type II pneumocytes) lowers surface tension, preventing collapse & stabilizing alveoli.

   • Deficiency → Infant Respiratory Distress Syndrome (IRDS); each breath requires re-inflation.

3. Lung Compliance ("stretchability")

   • Defined: CL = \frac{\Delta V}{\Delta P{tp}}.

   • High compliance = lungs expand easily.

   • Decreased by: fibrosis (scar tissue), low surfactant, thoracic cage stiffness (kyphosis, costal cartilage ossification), paralysis of intercostals.

Applied Questions Discussed in Slides (With Answers)

• Absolute pressures when P_{atm}=760\,\text{mmHg}:

  • Respiratory pressure -4 → 756\,\text{mmHg}.

  • Respiratory pressure 0 → 760\,\text{mmHg}.

  • Respiratory pressure +4 → 764\,\text{mmHg}.

• Figure labels (slide 20):

  • A = Transpulmonary pressure (4 mmHg).

  • B = Intrapleural pressure (−4 mmHg).

  • C = Intrapulmonary pressure (0 mmHg).

• Boyle’s law container question: container B (smaller volume) has higher pressure.

• "Most important muscle for inspiration?" — the diaphragm.

Ethical / Practical Connections

• Premature infants often lack surfactant → requires exogenous surfactant therapy and positive pressure ventilation.

• COPD patients rely on accessory muscles; energy expenditure for breathing increases dramatically → caloric considerations for care.

• Use of rescue inhalers (albuterol) & epinephrine auto-injectors illustrates pharmacological manipulation of airway smooth muscle tone.

Key Equations & Relationships (Quick Reference)

• Boyle’s Law: P1 V1 = P2 V2 (for a closed system at constant T).

• Transpulmonary pressure: P{tp} = P{pul} - P_{ip}.

• Compliance: CL = \frac{\Delta V}{\Delta P{tp}}.

• Airflow: F = \frac{\Delta P}{R}.

Self-Assessment Checklist

• [ ] I can diagram pressure changes during one full respiratory cycle.

• [ ] I can explain why P_{ip} must remain negative and predict consequences if it becomes positive.

• [ ] I can list every structure and muscle involved in forced breathing.

• [ ] I can articulate how surfactant works and why its absence is devastating.

• [ ] I can solve problems involving Boyle’s law and compliance calculations.

Further Reading

• Marieb & Hoehn: Human Anatomy & Physiology, 13th ed.

• Saladin: Anatomy & Physiology, 9th ed.