Respiratory Physiology: Mechanics of Breathing and Pulmonary Ventilation

Mechanics of Breathing

Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs, enabling gas exchange with the external environment. It consists of two main phases:

• Inspiration: Gases flow into the lungs.

• Expiration: Gases exit the lungs.

Pressure Relationships in the Thoracic Cavity

Atmospheric and Respiratory Pressures

• Atmospheric pressure (Patm): Pressure exerted by the air surrounding the body; at sea level, this is 760 mm Hg (1 atmosphere).

• Respiratory pressures are described relative to Patm:

  • Negative respiratory pressure: Less than Patm

  • Positive respiratory pressure: Greater than Patm

  • Zero respiratory pressure: Equal to Patm

Intrapulmonary and Intrapleural Pressures

• Intrapulmonary pressure (Ppul): Pressure in the alveoli (also called intra-alveolar pressure). Fluctuates with breathing and always eventually equalizes with Patm.

• Intrapleural pressure (Pip): Pressure in the pleural cavity. Fluctuates with breathing but is always negative (usually 4 mm Hg less than Ppul).

• Fluid level in the pleural cavity must be kept minimal; excess fluid is pumped out by the lymphatic system. If fluid accumulates, positive Pip pressure develops, causing lung collapse.

Forces Affecting Intrapleural Pressure

• Two inward forces promote lung collapse:

  1. Lungs' natural tendency to recoil due to elasticity.

  2. Surface tension of alveolar fluid, which pulls on alveoli to reduce their size.

• One outward force tends to enlarge the lungs: elasticity of the chest wall pulls the thorax outward.

• Negative Pip is maintained by the adhesive force between parietal and visceral pleurae.

Transpulmonary Pressure

• Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressures: $P_{pul} - P_{ip}$

• This pressure keeps lung spaces open and prevents lung collapse. The greater the transpulmonary pressure, the larger the lungs.

• Lungs will collapse if:

  • Ppul = Pip or

  • Ppul = Patm

  Negative Pip must be maintained to keep lungs inflated.

Clinical Correlate: Homeostatic Imbalance 22.7

• Atelectasis: Lung collapse due to plugged bronchioles (collapse of alveoli) or pneumothorax (air in pleural cavity).

• Pneumothorax: Can occur from wounds in parietal pleura or rupture of visceral pleura. Treated by removing air with chest tubes; lungs reinflate when pleurae heal.

Pulmonary Ventilation: Inspiration and Expiration

Boyle's Law

Boyle's law describes the relationship between pressure and volume of a gas:

• Gases fill their container; if the container's volume decreases, pressure increases (and vice versa).

• Mathematically: $P_1 V_1 = P_2 V_2$

Inspiration

• Active process involving inspiratory muscles (diaphragm and external intercostals).

• Action of the diaphragm: Contracts and flattens, increasing thoracic volume.

• Action of intercostal muscles: External intercostals contract, lifting the rib cage and increasing thoracic volume.

• As thoracic cavity volume increases, lungs are stretched, causing intrapulmonary pressure to drop (Ppul < Patm), so air flows into the lungs.

• Forced (deep) inspirations occur during exercise or in certain diseases, involving accessory muscles (scalenes, sternocleidomastoid, pectoralis minor, erector spinae).

Expiration

• Normally a passive process: inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil.

• Volume decrease causes intrapulmonary pressure to rise (Ppul > Patm), so air flows out of the lungs.

• Forced expiration is active, using oblique and transverse abdominal muscles and internal intercostals.

Nonrespiratory Air Movements

• Other processes can move air into or out of the lungs, such as coughing, sneezing, crying, laughing, hiccups, and yawns.

• Most are reflex actions, but some are voluntary.

Physical Factors Influencing Pulmonary Ventilation

Airway Resistance

• Friction is the major nonelastic source of resistance to gas flow in airways.

• Relationship: $F = \frac{\Delta P}{R}$ Where F = flow, ΔP = pressure gradient, R = resistance.

• Gas flow changes inversely with resistance; resistance is usually insignificant due to large airway diameters and extensive branching.

• Resistance is highest in medium-sized bronchi and disappears at terminal bronchioles (where diffusion drives gas movement).

Clinical Correlate: Homeostatic Imbalance 22.8

• Increased airway resistance (e.g., in asthma) makes breathing more strenuous and can prevent ventilation.

• Epinephrine dilates bronchioles, reducing resistance.

Alveolar Surface Tension

• Surface tension: Attraction of liquid molecules at a gas-liquid interface draws molecules together and resists expansion.

• Water coats alveolar walls, tending to cause alveoli to shrink (collapse).

• Surfactant: A detergent-like lipid and protein complex produced by type II alveolar cells that reduces surface tension and prevents alveolar collapse.

Clinical Correlate: Homeostatic Imbalance 22.9

• Insufficient surfactant in premature infants causes infant respiratory distress syndrome (IRDS)—alveoli collapse after each breath, requiring much energy to reinflate.

• Treatment includes spraying surfactant into air passages and using positive pressure devices; severe cases may require mechanical ventilation.

Lung Compliance

• Lung compliance: Measure of the change in lung volume that occurs with a given change in transpulmonary pressure.

• High compliance means lungs expand easily; determined by distensibility of lung tissue and surfactant.

• Mathematically: $C_L = \frac{\Delta V}{\Delta (P_{pul} - P_{ip})}$ Where CL = compliance, ΔV = change in lung volume, Δ(Ppul - Pip) = change in transpulmonary pressure.

• Compliance can be diminished by:

  • Nonelastic scar tissue (fibrosis)

  • Reduced surfactant production

  • Decreased flexibility of the thoracic cage

Clinical Correlate: Homeostatic Imbalance 22.10

• Conditions such as chronic inflammation, infections (e.g., tuberculosis), or decreased surfactant lower lung compliance, making breathing more difficult and energy-demanding.

Summary Table: Pressure Relationships in the Thoracic Cavity

Pressure	Location	Normal Value	Role
Atmospheric (Patm)	Outside body	760 mm Hg	Reference for other pressures
Intrapulmonary (Ppul)	Alveoli	Fluctuates, equalizes with Patm	Drives air movement
Intrapleural (Pip)	Pleural cavity	~4 mm Hg less than Ppul	Keeps lungs expanded
Transpulmonary	Ppul - Pip	Varies	Prevents lung collapse

Additional info: This summary expands on the provided slides by including definitions, equations, and clinical context for a comprehensive understanding of respiratory physiology relevant to Anatomy & Physiology students.