7.2 Mechanics of breathing

Breathing is a complex process that relies on pressure changes within the lungs. As we inhale, our diaphragm and intercostal muscles contract, expanding the chest cavity and lowering lung pressure. This draws air in. Exhaling reverses this process, pushing air out.

Understanding lung mechanics is crucial for grasping respiratory function. We'll explore how muscles work together to facilitate breathing, examine lung volumes and capacities, and discuss factors affecting lung function. This knowledge forms the foundation for understanding respiratory physiology and disorders.

Pressure Changes in Breathing

Inspiration and Expiration

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• Inspiration (inhalation) occurs when the pressure inside the lungs (intrapulmonary pressure) becomes lower than the atmospheric pressure, causing air to flow into the lungs
• Expiration (exhalation) occurs when the intrapulmonary pressure becomes higher than the atmospheric pressure, causing air to flow out of the lungs
• The pressure changes during breathing are caused by the contraction and relaxation of respiratory muscles, which alter the volume of the thoracic cavity and the lungs

Muscle Actions During Breathing

• During inspiration, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity and decreasing the intrapulmonary pressure
  • The increased volume lowers the pressure inside the lungs (Boyle's Law: $PV = k$), creating a pressure gradient that drives air into the lungs
• During expiration, the diaphragm and external intercostal muscles relax, decreasing the volume of the thoracic cavity and increasing the intrapulmonary pressure
  • The decreased volume raises the pressure inside the lungs, creating a pressure gradient that drives air out of the lungs
  • Expiration is usually passive, relying on the elastic recoil of the lungs and chest wall to return to their resting positions

Respiratory Muscle Function

Diaphragm

• The diaphragm is the primary muscle of inspiration, located at the base of the thoracic cavity, separating it from the abdominal cavity
• During inspiration, the diaphragm contracts and flattens, increasing the vertical dimension of the thoracic cavity and decreasing the intrapulmonary pressure
  • This action is responsible for about 75% of the air movement during quiet breathing
• The diaphragm is innervated by the phrenic nerves (C3-C5) and is under involuntary control by the respiratory centers in the brainstem

Other Inspiratory Muscles

• The external intercostal muscles, located between the ribs, assist in inspiration by contracting and lifting the ribs upward and outward, increasing the anteroposterior and transverse dimensions of the thoracic cavity
  • This action is responsible for about 25% of the air movement during quiet breathing
• Accessory muscles of inspiration, such as the sternocleidomastoid and scalene muscles, are used during deep or labored breathing to further expand the thoracic cavity
  • These muscles are typically only recruited during exercise or respiratory distress (dyspnea)

Expiratory Muscles

• During forced expiration, the internal intercostal muscles and the abdominal muscles (rectus abdominis, transverse abdominis, and obliques) contract, pushing the diaphragm upward and compressing the abdominal contents, which increases the intrapulmonary pressure and forces air out of the lungs
  • These muscles are important for activities that require rapid or forceful expiration, such as coughing, sneezing, or singing
• The expiratory muscles are also used during exercise to increase the rate and depth of breathing to meet the increased metabolic demands of the body

Lung Volumes and Capacities

Lung Volumes

• Tidal volume (TV) is the amount of air that moves into and out of the lungs during a normal, quiet breath, typically around 500 mL in adults
• Inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled beyond the tidal volume during a deep breath, usually around 3,000 mL
• Expiratory reserve volume (ERV) is the additional amount of air that can be exhaled beyond the tidal volume during a forceful expiration, typically around 1,100 mL
• Residual volume (RV) is the amount of air that remains in the lungs after a maximal expiration, usually around 1,200 mL, and cannot be expelled due to the closure of small airways and the inherent elasticity of the lung tissue

Lung Capacities

• The sum of the tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume equals the total lung capacity (TLC), which is the maximum volume of air the lungs can hold, typically around 6,000 mL in adult males
  • TLC = TV + IRV + ERV + RV
• Other lung capacities include:
  • Inspiratory capacity (IC) = TV + IRV
  • Functional residual capacity (FRC) = ERV + RV
  • Vital capacity (VC) = TV + IRV + ERV
• These lung volumes and capacities can be measured using spirometry, which is an important tool for assessing lung function and diagnosing respiratory disorders

Factors Affecting Lung Function

Lung Compliance

• Lung compliance refers to the ease with which the lungs can expand and contract during breathing, determined by the elasticity of the lung tissue and the surface tension of the alveolar fluid
• Surfactant, a mixture of phospholipids and proteins secreted by type II alveolar cells, reduces the surface tension of the alveolar fluid, increasing lung compliance and preventing alveolar collapse
  • Surfactant deficiency, as seen in premature infants with respiratory distress syndrome (RDS), can lead to alveolar collapse and respiratory failure
• Elastin and collagen fibers in the lung tissue provide elastic recoil, which helps to expel air during expiration and maintain the structure of the lungs
• Conditions that decrease lung compliance, such as pulmonary fibrosis or acute respiratory distress syndrome (ARDS), make it harder to inflate the lungs and can lead to respiratory failure

Airway Resistance

• Airway resistance refers to the opposition to airflow through the respiratory tract, primarily determined by the diameter of the airways
• Factors that increase airway resistance include bronchoconstriction (narrowing of the airways), mucus secretion, inflammation, and structural abnormalities such as tumors or foreign objects
  • Bronchoconstriction can be triggered by irritants (smoke), allergens (pollen), or cold air
• Conditions that increase airway resistance, such as asthma, chronic obstructive pulmonary disease (COPD), or cystic fibrosis, make it harder to move air in and out of the lungs, leading to increased work of breathing and reduced gas exchange
• Medications such as bronchodilators can be used to reduce airway resistance by relaxing the smooth muscle in the airways, while corticosteroids can reduce inflammation and mucus secretion
  • Examples of bronchodilators include beta-2 agonists (albuterol) and anticholinergics (ipratropium)