Lung Volumes and Capacities Definitions

Generation of a Pressure Gradient

• A pressure difference is generated between the atmosphere and alveoli during breathing
• The diaphragm and intercostal muscles contract to increase intrathoracic volume, making intrapleural pressure (IPP) more negative and expanding the lungs
• Decreased external pressure dilates the superior vena cava and inferior vena cava, and heart chambers, increasing the pressure gradient for venous return to the right heart
• During inspiration, the diaphragm descends, causing an increased intra-abdominal pressure, which forces blood flow towards the heart

Pressure-Volume Relationships in the Respiratory System

• The lung and chest wall have mechanical interactions, with the lung tending to recoil and the chest wall tending to expand
• The interaction between the lung and chest wall results in a negative intrapleural pressure (IPP) at functional residual capacity (FRC)
• Pulmonary surfactant and alveolar interdependence play roles in the recoil and expansion of the lung

Airways Resistance

• Airways resistance is the resistance to airflow in the airways
• Factors that contribute to airways resistance include airway diameter, length, and friction
• Airways resistance can be altered by factors such as airway constriction or dilation, and changes in airway length and diameter

Work of Breathing

• The work of breathing is the energy required to overcome the resistance to airflow in the airways and the elastic resistance of the lung and chest wall
• Factors that contribute to the work of breathing include airways resistance, lung and chest wall compliance, and respiratory rate
• The work of breathing can be increased in certain disease states, such as obstructive and restrictive lung diseases

Pressures Involved in Respiration

• Alveolar pressure is the pressure inside the alveoli and is equal to the intrapulmonary pressure and airway pressure
• Intrapleural pressure (IPP) is the pressure inside the pleural cavity and is normally negative
• Transpulmonary pressure is the transmural pressure difference between the alveolar pressure and intrapleural pressure

Muscles of Respiration

• The muscles of respiration include the diaphragm, external intercostals, scalenes, abdominal muscles, and internal intercostal muscles
• The diaphragm is the primary muscle of inspiration and accounts for half of the tidal inhalation when upright and up to two-thirds when supine
• The diaphragm descends during inspiration, increasing the intrathoracic volume and decreasing the intrapleural pressure

Lung Volumes and Capacities

• Lung volumes and capacities are measured using a spirometer and include vital capacity (VC), total lung capacity (TLC), residual volume (RV), and forced vital capacity (FVC)
• Forced expiratory volume in the first second (FEV1) is the volume of air that can be expired in one second after maximal inhalation

Flow-Volume Curves

• Flow-volume curves demonstrate the relationship between airflow and lung volume during forced expiration
• The curves can be used to diagnose obstructive and restrictive lung diseases
• Peak expiratory flow is the maximal flow rate that can be attained during forced expiration

Clinical Applications

• Cardiopulmonary resuscitation (CPR) can be affected by changes in intrathoracic pressure
• Pneumothorax can cause an increase in intrapleural pressure, leading to decreased venous return and cardiac output
• Expiratory tension pneumothorax can cause air to enter the pleural cavity during inspiration, but not to leave during expiration, leading to increased intrapleural pressure and decreased venous return.