Lung Compliance and Function Quiz

Physical Properties of the Lungs

• Compliance: Lung compliance, also known as pulmonary compliance, measures the lung's ability to stretch and expand (distensibility of elastic tissue). It's a change in lung volume per change in transpulmonary pressure (DV/DP). The lung is 100 times more distensible than a balloon. Compliance is determined by elastic forces in the lungs.
• Elastic Forces: These forces have two components: (1) elastic forces of lung tissue (determined mainly by elastin and collagen fibers in the parenchyma) and (2) elastic forces from surface tension of fluid lining the alveoli.
• Surface Tension: Fluid surface tension decreases lung compliance. A lung with low compliance needs more force for breathing, while a lung with high compliance requires less force.
• Surfactant: Surfactant, a mixture of lipids and proteins secreted by Type II alveolar epithelial cells (about 10% of alveolar surface area), decreases surface tension in alveoli. It disrupts cohesive forces between water molecules in alveolar fluid, making it easier for alveoli to expand and stay open during inhalation and exhalation. Compressed surfactant phospholipid molecules decrease surface tension to near-zero levels at the end of exhalation.
• Mobility of the Thoracic Cage: Any musculoskeletal disease decreases lung compliance.
• Compliance Reduction: Compliance reduces when pulmonary venous pressure increases, or alveolar edema occurs due to insufficient alveolar inflation, or the lung remains unventilated (atelectasis), or lung diseases cause fibrosis. Conversely, in chronic obstructive pulmonary disease (e.g. emphysema), alveolar walls degenerate, increasing compliance.

Lung Volumes and Capacities

• Lung Volumes: Measurements of air in the lungs at different stages of the breathing cycle.
• Tidal Volume (TV): The amount of air inhaled or exhaled with each breath during normal breathing. Typical value: 500 mL for an average adult.
• Expiratory Reserve Volume (ERV): The amount of air that can be exhaled after a normal tidal exhalation. Typical value: 1100 mL.
• Residual Volume (RV): The amount of air remaining in the lungs after maximum exhalation, preventing lung collapse. Typical value: 1200 mL.
• Lung Capacities: Combinations of two or more lung volumes, giving a broader picture of lung function.
• Total Lung Capacity (TLC): The total amount of air the lungs can hold. Formula: TLC = TV + IRV + ERV + RV. Typical value: 6000 mL.
• Vital Capacity (VC): The maximum amount of air exhaled after maximum inhalation. Formula: VC = IRV + TV + ERV. Typical value: 4800 mL.
• Inspiratory Capacity (IC): The maximum amount of air that can be inhaled after a normal tidal exhalation. Formula: IC = TV + IRV. Typical value: 3600 mL.
• Functional Residual Capacity (FRC): The amount of air remaining in the lungs after a normal tidal exhalation. Formula: FRC = ERV + RV. Typical value: 2400 mL.

Pulmonary Function Tests

• Pulmonary Function Tests (PFTs): A group of tests measuring how well lungs function, assessing lung volume, capacity, flow, and efficiency of oxygen exchange between lungs and blood. Used to diagnose and monitor conditions like asthma and COPD.
• Spirometry: The most common PFT, measuring how much and how quickly air can be breathed in and out.
• Spirometer Goals: Predicting pulmonary dysfunction, differentiating obstructive and restrictive lung diseases, assessing disease severity and progression, evaluating treatment response, and assessing lung impairment from occupational hazards.

Measurements Obtained from Spirometer

• Forced Expiratory Volume in the First Second (FEV1): The volume of air forcefully exhaled in the first second after a deep breath. Usually >70% of FVC (FEV1/FVC > 70%).
• Obstructive Lung Disease: In obstructive conditions (e.g., asthma, COPD), FEV1 is reduced more than FVC, resulting in FEV1/FVC < 70%.
• Restrictive Lung Disease: In restrictive conditions (e.g., fibrosis), both FEV1 and FVC are reduced, resulting in a normal or increased FEV1/FVC.

Factors Influencing Vital Capacity (VC):

• Physiological: Directly proportional to height, more in males due to greater chest size and muscle power (increased surface area). Decreases with age and in supine position. Affected by strength of respiratory muscles and pregnancy (unchanged or increase by 10%).
• Pathological: Disease of respiratory muscles and abdominal conditions (e.g., pain, splinting).

Dead Space

• Dead Space: Air in the airways and lungs that does not participate in gas exchange.
• Anatomic Dead Space: The volume of air in conducting airways (nose, mouth, trachea, bronchi). Approximately 150 mL in a healthy adult.
• Alveolar Dead Space: The volume of air reaching alveoli but not participating in gas exchange due to poor perfusion. Can increase in pathological conditions.
• Total Dead Space: The sum of anatomic and alveolar dead space. An increase indicates respiratory system inefficiency. In healthy individuals, physiologic dead space equals anatomic dead space.

Ventilation

• Total Ventilation: Total volume of gas leaving the lungs per unit time.
• Alveolar Ventilation: Volume of gas reaching the respiratory airways. Not all total ventilation reaches the alveoli. Anatomic dead space (150ml of tidal volume) is not involved in gas exchange.