Respiratory Physiology: Mechanics of Breathing and Pulmonary Ventilation

Mechanics of Breathing

Pulmonary Ventilation

Pulmonary ventilation is the process by which air moves into and out of the lungs, enabling gas exchange with the blood. It consists of two phases:

• Inspiration: Gases flow into the lungs.

• Expiration: Gases exit the lungs.

Pressure Relationships in the Thoracic Cavity

Atmospheric Pressure (Patm)

Atmospheric pressure is the pressure exerted by the air surrounding the body. At sea level, it is typically 760 mm Hg, which equals 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 Pressure (Ppul)

Intrapulmonary pressure is the pressure within the alveoli (also called intra-alveolar pressure). It fluctuates with breathing and always eventually equalizes with atmospheric pressure.

Intrapleural Pressure (Pip)

Intrapleural pressure is the pressure within the pleural cavity. It also fluctuates with breathing but is always a negative pressure (usually 4 mm Hg less than Ppul).

• Fluid level in the pleural cavity must be kept at a minimum; excess fluid is pumped out by the lymphatic system.

• If fluid accumulates, positive Pip pressure develops, leading to 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 lungs: Elasticity of the chest wall pulls the thorax outward.

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

Transpulmonary Pressure

Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressures:

• Formula: $P_{pul} - P_{ip}$

• This pressure keeps lung spaces open and prevents lung collapse.

• The greater the transpulmonary pressure, the larger the lungs will be.

• Lungs will collapse if Pip = Ppul or Pip = Patm.

Diagram: Intrapulmonary and Intrapleural Pressure Relationships

Figure 22.14 illustrates the relationships between atmospheric, intrapulmonary, and intrapleural pressures in the thoracic cavity.

Pneumothorax

Pneumothorax is the presence of air in the pleural cavity, which can lead to lung collapse. It may result from wounds in the parietal pleura or rupture of the visceral pleura. Treatment involves removing air with chest tubes; when the pleurae heal, the lung reinflates.

Pulmonary Ventilation

Volume and Pressure Changes

Pulmonary ventilation depends on volume changes in the thoracic cavity, which lead to pressure changes. These pressure changes cause the flow of gases to equalize pressure.

Boyle's Law

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

• Gases always fill the container they are in.

• If the amount of gas is constant and container size is reduced, pressure increases.

• Pressure varies inversely with volume.

• Formula: $P_1 V_1 = P_2 V_2$

Inspiration

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

• Action of the diaphragm: When the dome-shaped diaphragm contracts, it moves inferiorly and flattens out, increasing thoracic volume.

• Action of intercostal muscles: When external intercostals contract, the rib cage is lifted up and out, increasing thoracic volume.

Sequence of Events During Inspiration and Expiration

As thoracic cavity volume increases, lungs are stretched and pulled out with the thoracic cage, causing intrapulmonary pressure to drop by 1 mm Hg (Ppul < Patm). Air flows into the lungs down the pressure gradient until Ppul = Patm. During the same period, Pip lowers to about 6 mm Hg less than Patm.

Forced (Deep) Inspiration

• Occurs during vigorous exercise or in people with COPD.

• Accessory muscles (scalenes, sternocleidomastoid, pectoralis minor, erector spinae) are activated to further increase thoracic cage size and create a larger pressure gradient.

Expiration

• Quiet expiration is normally a passive process.

• Inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil.

• Volume decrease causes intrapulmonary pressure to increase by +1 mm Hg.

• Air flows out of lungs down its pressure gradient until Ppul = Patm.

• Forced expiration is an active process using oblique and transverse abdominal muscles, as well as internal intercostal muscles.

Nonrespiratory Air Movements

Various processes can move air into or out of the lungs besides breathing. These may modify normal respiratory rhythm and most result from reflex action, though some are voluntary.

• Examples: coughing, sneezing, crying, laughing, hiccups, yawns.

Physical Factors Influencing Pulmonary Ventilation

Overview

Three physical factors influence the ease of air passage and the amount of energy required for ventilation:

• Airway resistance

• Alveolar surface tension

• Lung compliance

Airway Resistance

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

• Relationship between flow (F), pressure (ΔP), and resistance (R):

• Formula: $F = \frac{\Delta P}{R}$

• ΔP is the pressure gradient between atmosphere and alveoli (2 mm Hg or less during normal quiet breathing).

• Gas flow changes inversely with resistance.

• Resistance in the respiratory tree is usually insignificant because:

  1. Diameters of airways in the first part of the conducting zone are large.

  2. Progressive branching of airways increases total cross-sectional area.

• Any resistance usually occurs in medium-sized bronchi; resistance disappears at terminal bronchioles where diffusion drives gas movement.

Alveolar Surface Tension

• Surface tension is the attraction of liquid molecules to one another at a gas-liquid interface.

• Tends to draw liquid molecules closer together and reduce contact with dissimilar gas molecules.

• Resists any force that tends to increase surface area of liquid.

• Water, with high surface tension, coats alveolar walls in a thin film and tends to cause alveoli to shrink to the smallest size (collapse).

Surfactant

• Surfactant is a detergent-like lipid and protein complex that helps reduce surface tension of alveolar fluid.

• Prevents alveolar collapse.

• Produced by type II alveolar cells.

Clinical: Infant Respiratory Distress Syndrome (IRDS)

• Insufficient quantity of surfactant in premature infants causes IRDS.

• Increased surface tension results in collapse of alveoli after each breath; alveoli must be completely reinflated during each inspiration, using a tremendous amount of energy.

• Common in premature babies; fetal lungs do not produce adequate surfactant until the last two months of development.

• Treatment: spraying natural or synthetic surfactant into newborn's air passages; positive pressure devices help keep alveoli open between breaths; severe cases may require mechanical ventilation.

• Survivors of mechanical ventilation may develop bronchopulmonary dysplasia, a chronic childhood lung disease.

Lung Compliance

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

• It reflects how much "stretch" the lung has.

• Normally high because of the distensibility of lung tissue and surfactant, which decreases alveolar surface tension.

• Higher lung compliance means it is easier to expand lungs.

• Formula: $C_L = \frac{\Delta V}{\Delta (P_{pul} - P_{ip})}$

• Compliance can be diminished by:

  • Nonelastic scar tissue replacing lung tissue (fibrosis)

  • Reduced production of surfactant

  • Decreased flexibility of thoracic cage

Clinical: Decreased Lung Compliance

• Any decrease in the normal presence of gas diminishes lung compliance.

• Chronic inflammation or infections (such as tuberculosis) can cause nonelastic scar tissue to replace normal lung tissue.

• Decreased production of surfactant also impairs lung compliance.

• The lower the lung compliance, the more energy is needed just to breathe.

• Compliance of the respiratory system is also affected by the compliance (distensibility) of the thoracic wall, which can be decreased by deformities of the thorax, ossification of costal cartilage, or paralysis of intercostal muscles (common in old age).

Example Table: Pressure Relationships in the Thoracic Cavity

Pressure Type	Location	Normal Value (mm Hg)	Role
Atmospheric Pressure (Patm)	Outside body	760	Reference for other pressures
Intrapulmonary Pressure (Ppul)	Alveoli	Varies, equalizes with Patm	Drives air movement
Intrapleural Pressure (Pip)	Pleural cavity	~756	Keeps lungs inflated
Transpulmonary Pressure	Difference between alveoli and pleural cavity	~4	Prevents lung collapse

Additional info: Academic context and definitions have been expanded for clarity and completeness. Figures referenced in the notes correspond to textbook diagrams illustrating pressure relationships and volume changes during breathing.