4.3 Mechanics of Breathing

Breathing is a complex process driven by pressure differences in the body. This section explores how the lungs and chest work together to move air in and out. We'll look at the mechanics of inhalation and exhalation, and how pressure changes affect lung volume.

Understanding airway resistance and lung compliance is crucial for grasping respiratory function. We'll dive into factors that impact these properties and how they influence breathing mechanics. This knowledge is key for diagnosing and treating respiratory disorders.

Pressure gradients in breathing

Pressure differences drive airflow

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• Breathing involves the movement of air into and out of the lungs driven by pressure gradients between the atmosphere and the alveoli
• The magnitude of the pressure gradient determines the rate of airflow during inhalation and exhalation
• Larger pressure differences lead to faster airflow rates (high altitude breathing)
• Smaller pressure differences result in slower airflow rates (shallow breathing)

Inhalation mechanics

• During inhalation, the diaphragm and external intercostal muscles contract increasing the volume of the thoracic cavity
• Increased thoracic volume decreases the pressure within the lungs relative to the atmosphere creating a pressure gradient
• The pressure gradient causes air to flow from the high-pressure atmosphere into the low-pressure lungs (sucking action)
• Inhalation continues until the pressure equilibrates between the atmosphere and alveoli

Exhalation mechanics

• During exhalation, the diaphragm and external intercostal muscles relax decreasing the volume of the thoracic cavity
• Decreased thoracic volume increases the pressure within the lungs relative to the atmosphere creating a pressure gradient
• The pressure gradient causes air to flow from the high-pressure lungs to the low-pressure atmosphere (pushing action)
• Exhalation continues until the pressure equilibrates between the alveoli and atmosphere
• Elastic recoil of the lungs and chest wall also contributes to exhalation by compressing the alveoli (passive process)

Lung volume and intrapleural pressure

Intrapleural pressure changes

• Intrapleural pressure is the pressure within the pleural cavity, the space between the visceral and parietal pleura surrounding the lungs
• During inhalation, as lung volume increases, intrapleural pressure becomes more negative relative to atmospheric pressure
  • Allows the lungs to expand and draw in air by counteracting the elastic recoil of the lungs
  • Negative intrapleural pressure "pulls" the lungs open as the chest expands
• During exhalation, as lung volume decreases, intrapleural pressure becomes less negative or slightly positive relative to atmospheric pressure
  • Causes the lungs to recoil and expel air as the elastic forces of the lungs are unopposed
  • Less negative or positive intrapleural pressure allows the lungs to collapse as the chest wall recoils

Pressure-volume relationship

• The relationship between lung volume and intrapleural pressure can be visualized using a pressure-volume curve demonstrating the compliance of the lungs and chest wall
• Compliance is the slope of the pressure-volume curve and represents the ease of lung expansion (ΔV/ΔP)
• A steep pressure-volume curve indicates high compliance where small pressure changes lead to large volume changes (young, healthy lungs)
• A flat pressure-volume curve indicates low compliance where large pressure changes lead to small volume changes (fibrotic lungs)

Pathological conditions affecting intrapleural pressure

• Conditions that affect intrapleural pressure can impair the mechanics of breathing by altering the pressure gradient between the lungs and pleural cavity
• Pneumothorax: Air enters the pleural space, equalizing the intrapleural pressure with atmospheric pressure and causing lung collapse
• Pleural effusion: Fluid accumulates in the pleural space, increasing the intrapleural pressure and compressing the lungs
• Atelectasis: Alveolar collapse due to decreased intrapleural pressure, often caused by airway obstruction or prolonged shallow breathing

Compliance and elastance in lung function

Compliance

• Compliance refers to the ease with which the lungs and chest wall can expand and is defined as the change in volume per unit change in pressure (ΔV/ΔP)
• High compliance indicates easily distensible lungs and chest wall, requiring less pressure to achieve a given change in volume (emphysema)
• Low compliance indicates stiffer, less distensible lungs and chest wall, requiring more pressure to achieve a given change in volume (pulmonary fibrosis)
• Factors affecting lung compliance include age, disease states (emphysema, fibrosis), and surfactant production

Elastance

• Elastance is the reciprocal of compliance (1/compliance) and refers to the tendency of the lungs and chest wall to recoil to their original shape after being stretched
• High elastance indicates a strong tendency for the lungs and chest wall to recoil, requiring more pressure to maintain a given volume (restrictive lung diseases)
• Low elastance indicates a weak tendency for the lungs and chest wall to recoil, requiring less pressure to maintain a given volume (obstructive lung diseases)
• Factors affecting lung elastance include the amount and composition of elastic fibers (elastin, collagen) in the lung tissue

Impact on breathing mechanics

• Changes in compliance and elastance can impact the work of breathing and the distribution of ventilation within the lungs
• Decreased compliance or increased elastance requires greater pressure changes to achieve adequate ventilation, increasing the work of breathing (respiratory muscle fatigue)
• Uneven distribution of compliance and elastance can lead to ventilation-perfusion mismatching, where some areas of the lung are overventilated while others are underventilated (hypoxemia, hypercapnia)
• Measuring compliance and elastance using techniques like spirometry or lung volumes can help assess lung function and diagnose respiratory disorders

Factors influencing airway resistance

Factors increasing airway resistance

• Bronchoconstriction: Narrowing of airways due to smooth muscle contraction triggered by stimuli like cold air, irritants, or allergens (asthma)
• Mucus secretion: Excessive or thick mucus production can obstruct airways and increase resistance to airflow (chronic bronchitis)
• Inflammation: Swelling of airway walls due to inflammatory processes reduces luminal diameter and increases resistance (bronchitis, pneumonia)
• Structural changes: Conditions like asthma, chronic obstructive pulmonary disease (COPD), or tumors can cause permanent alterations in airway structure that increase resistance

Factors decreasing airway resistance

• Bronchodilation: Relaxation of airway smooth muscle induced by medications like beta-2 agonists (albuterol) or anticholinergics (ipratropium)
• Posture: Sitting upright can optimize airway diameter and reduce resistance compared to supine positions by reducing compression of the airways
• Breathing techniques: Pursed-lip breathing or diaphragmatic breathing can help maintain open airways and reduce resistance (COPD)
• Airway clearance: Techniques like coughing, huffing, or chest physiotherapy can help remove mucus and debris from the airways, reducing obstruction

Effects on breathing

• Increased airway resistance requires greater pressure gradients to maintain adequate airflow, leading to increased work of breathing and potential ventilation-perfusion mismatching
• Decreased airway resistance allows for easier airflow and reduced work of breathing, improving ventilation and gas exchange
• Measuring airway resistance using techniques like spirometry, plethysmography, or forced oscillation provides valuable information about lung function and the presence of obstructive airway diseases (asthma, COPD)
• Understanding factors influencing airway resistance guides therapeutic interventions aimed at optimizing breathing mechanics and improving patient outcomes