Besides the pressure difference between the external environment and the lungs, the airflow rate and ease of pulmonary ventilation are also influenced by three other factors: surface tension of the fluid in the alveoli, compliance of the lungs, and airway resistance.
Alveolar Surface Tension
The alveolar fluid lines the luminal surface of the alveoli and exerts a force called surface tension. This force is caused by the polar water molecules in the liquid being more strongly attracted to each other than gas molecules in the air. This results in a tendency for the fluid to collapse the alveoli. This surface tension must be overcome to expand the lungs during inhalation. However, in premature infants with a deficiency of surfactant, the surface tension is significantly increased and can lead to respiratory distress syndrome. This condition requires more significant effort during inhalation to reopen the collapsed alveoli.
Lung Compliance
The next factor affecting pulmonary ventilation is compliance, which refers to how much effort is required to stretch the lungs and chest wall. High compliance implies that the lungs and chest wall expand easily, while low compliance implies resistance to expansion. In the lungs, elasticity and surface tension contribute to changes in compliance. The elastic fibers in lung tissue allow them to expand readily due to the easy stretching, and the surfactant in the alveolar fluid reduces the surface tension. However, certain conditions, such as tuberculosis or pulmonary edema, can decrease lung compliance by scarring lung tissue or filling it with fluid. Conversely, emphysema increases lung compliance due to the destruction of elastic fibers in the alveolar walls, making the lungs more easily expandable but less effective at expelling air.
Airway Resistance
Finally, airway resistance also plays a role in pulmonary ventilation. The airflow rate through the airways depends on the pressure difference and the resistance. As the lungs expand during inhalation, airway resistance decreases due to the enlargement of the bronchioles. However, during exhalation, when bronchioles decrease in diameter, resistance increases and is regulated by smooth muscles in the walls of the airways, which can either constrict or relax depending on signals from the sympathetic or parasympathetic divisions of the ANS. Certain conditions, such as asthma or COPD, can increase airway resistance due to the obstruction or collapse of airways.
In conclusion, the rate of airflow and ease of pulmonary ventilation are affected by the surface tension of alveolar fluid, lung compliance, and airway resistance. Understanding these factors is crucial for maintaining healthy breathing and identifying respiratory disorders that require medical intervention.
