What are the mechanisms and factors influencing lung volumes and capacities, elasticity, and resistance in respiration?

From the Guidelines

Respiration is a vital physiological process that involves the exchange of gases between an organism and its environment, with lung volumes, capacities, elasticity, and resistance playing crucial roles in maintaining efficient breathing. The human respiratory system is remarkably efficient, with lungs designed to maximize gas exchange through various volumes and capacities. Lung volumes refer to the different amounts of air moved during breathing, including tidal volume (approximately 500 mL during normal breathing), inspiratory reserve volume (about 3000 mL of additional air that can be inhaled beyond tidal volume), expiratory reserve volume (roughly 1200 mL of air that can be exhaled beyond normal exhalation), and residual volume (about 1200 mL of air remaining in the lungs after maximal exhalation) 1. These individual volumes combine to form lung capacities: total lung capacity (about 6000 mL), vital capacity (approximately 4800 mL), inspiratory capacity (around 3500 mL), and functional residual capacity (about 2400 mL). Lung elasticity is crucial for efficient breathing and is determined by two opposing forces: elastic recoil of lung tissue that promotes lung deflation and elastic recoil of the chest wall that promotes expansion. This elasticity comes from elastin fibers in lung tissue and surface tension forces at the air-liquid interface in alveoli, which is reduced by pulmonary surfactant. Without surfactant, alveoli would collapse during exhalation, requiring tremendous effort to reinflate. Airway resistance also significantly impacts breathing efficiency and represents the opposition to airflow through the respiratory tract. Most airway resistance occurs in medium-sized bronchi, with factors like airway radius (most significant factor, as resistance is inversely proportional to the fourth power of the radius), airway length, and gas viscosity all contributing. Bronchoconstriction increases resistance while bronchodilation decreases it, which is why bronchodilators like albuterol (typical dose 2-4 puffs every 4-6 hours as needed) are used to treat conditions like asthma by relaxing airway smooth muscle. Understanding these concepts is essential for diagnosing and treating respiratory disorders, as abnormalities in lung volumes, reduced elasticity, or increased resistance can significantly impair respiratory function and require specific therapeutic interventions.

Key Factors in Respiration

• Lung volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume
• Lung capacities: total lung capacity, vital capacity, inspiratory capacity, functional residual capacity
• Lung elasticity: elastic recoil of lung tissue, elastic recoil of the chest wall, surface tension forces, pulmonary surfactant
• Airway resistance: airway radius, airway length, gas viscosity, bronchoconstriction, bronchodilation

Clinical Implications

In patients with chronic obstructive pulmonary disease (COPD), lung volumes and capacities are affected, leading to impaired gas exchange and respiratory function 1. The use of bronchodilators can improve airway physiology and symptoms, even in the absence of a significant flow response measured by FEV1 1. Exercise testing can help diagnose desaturation and assess the severity of COPD, with changes in operating lung volumes and expiratory flow limitation being important indicators of disease severity 1. Cardiopulmonary exercise testing can also provide valuable information on ventilatory constraint and the elastic load to breathing, helping to guide therapeutic interventions 1.

Therapeutic Interventions

• Bronchodilators: albuterol, salbutamol
• Pulmonary rehabilitation: exercise training, education, behavioral modification
• Oxygen therapy: supplemental oxygen, oxygen therapy during exercise
• Surgery: lung transplantation, bullectomy

In conclusion, understanding the complex relationships between lung volumes, capacities, elasticity, and resistance is crucial for diagnosing and treating respiratory disorders, and therapeutic interventions should be tailored to address the specific needs of each patient, with the goal of improving morbidity, mortality, and quality of life. However, the most recent and highest quality study 1 suggests that the use of pre- and post-bronchodilator spirometry is recommended for the diagnosis of COPD, and that bronchodilators can improve airway physiology and symptoms, even in the absence of a significant flow response measured by FEV1.

From the Research

Lung Volumes and Capacities

• Lung volumes are either dynamic or static, and are measured at different degrees of inspiration or expiration 2
• Static lung volumes are further subdivided into four standard volumes: tidal, inspiratory reserve, expiratory reserve, and residual volumes 2
• Static lung volumes are also subdivided into four standard capacities: inspiratory, functional residual, vital, and total lung capacities 2
• Dynamic lung volumes are mostly derived from vital capacity and are essential for diagnosis and follow-up of obstructive lung diseases 2
• Static lung volumes are equally important for evaluation of obstructive as well as restrictive ventilatory defects 2

Elasticity

• The elasticity of the lungs refers to their ability to expand and contract during breathing
• There is no direct evidence in the provided studies to discuss lung elasticity, however, the use of inhaled corticosteroids (ICS) and long-acting beta2-agonists (LABA) can affect lung function and may have an impact on lung elasticity 3, 4, 5, 6

Resistance

• Resistance in the lungs refers to the opposition to airflow during breathing
• The use of LABAs can help reduce airway resistance and improve lung function in patients with chronic obstructive pulmonary disease (COPD) 4, 5
• Anticholinergics, such as tiotropium and ipratropium, can also reduce airway resistance and improve lung function in patients with COPD 5
• The combination of ICS and LABA can have complementary, additive, and synergistic effects on reducing airway resistance and improving lung function 6

Clinical Significance

• Lung volume measurements are important for diagnosis and follow-up of obstructive lung diseases, as well as for evaluation of restrictive ventilatory defects 2
• The use of ICS and LABA combinations can improve lung function, reduce exacerbations, and improve quality of life in patients with COPD 3, 4, 6
• However, the use of beta2-agonists can increase respiratory deaths in COPD, possibly due to tolerance that develops to their bronchodilator and bronchoprotective effects 5