Statics: Snapshots of the Ventilatory Pump

Learning Objectives

• To describe the forces that determine lung volumes.
• To describe the interactions between chest wall, pleura, and lungs that are responsible for the passive properties of the respiratory system.
• To outline the principles behind the concept of compliance and illustrate their application to the respiratory system.
• To describe the role of surface tension in the inflation and deflation of the lung.
• To define the role of surfactant in the stabilization of alveoli.

Throughout life, the respiratory system is always "on duty" as gas is moved from the atmosphere down to the alveoli and back out again. Every minute of every day, the ventilatory pump moves 5 L of air in and out of the lungs (more during exercise and other activities). This process requires the integrated action of the chest wall, pleura, and lungs. As we begin our study of the ventilatory pump, some questions come to mind. What determines how much air we can breathe into the lungs? Why can't we exhale all the air out of our lungs? What determines the size of our lungs at the end of a normal breath? Although the ventilatory pump is constantly in motion, we begin our examination of its function by freezing it for a moment in time and examining its properties when there is no air moving into or out of the lungs. The study of the respiratory system during these "snapshots" is termed statics.