Mechanics of Ventilation
When you take a breath in, air is able to pass into the lungs because the respiratory muscles contract and increases intrathoracic volume. At this moment the intrapleural pressure decreases to about -9cmH20.
The changes in intrapleural pressure causes lungs to expand, generating a negative intra-alveolar pressure as they are pulled open. As the atmospheric pressure rises, air will flow from high to low – approximately 500ml of air is inhaled during quiet ventilation. During exercise, other accessory respiratory muscles are employed and thus generate more negative pressure to allow greater volume of air to be inhaled up to 3L.
While inspiration is an active process requiring exchange of energy into the respiratory muscles to contract – expiration is a passive process due to elastic recoil of the chest wall. When you are at the gym or doing rigorous activities, abdominal muscles and internal intercostals contract to help expel more air quickly.
Surfactant and Surface Tension
As we have now learned, our lungs create different pressures to allow ventilation to happen. It takes considerable pressure increase before there is change in volume. Once adequate pressure is achieved, more pressure results in more expansion of lungs until it reaches maximum capacity. The lungs maintain the volume until the pressure falls about 8cmH20.
These unequal pressures needed to maintain a given lung volume during inspiration and expiration is termed hysteresis. Our lungs are able to function smoothly with the presence of surfactant. Surfactant is a phospholipid-rich detergent produced by type II alveolar cells; it coats the luminal surface of alveoli.
The main function of surfactant is to produce a force called surface tension. This is present at all air-fluid interfaces.
Surface tension occurs because water molecules are more attracted to each other than to gas molecules. In the alveolus, gas is surrounded by water and because of surface tension, it creates an inward pressure. This resists expansion and accounts for almost two-thirds of elastic recoil.
Other functions of surfactant include increasing compliance and reducing work of breathing; prevent fluid accumulation in the lung; and reduces tendency of alveoli to collapse.