Dynamics: Setting the System in Motion

Learning Objectives

• To differentiate the characteristics of the respiratory system when there is air flow and change in volume (dynamics) from the respiratory system in the absence of air flow (statics).
• To describe the role of the different elements of the ventilatory pump when the respiratory system is placed in motion.
• To define the principles associated with the flow of gas through tubes.
• To delineate the factors that contribute to airway and tissue resistance in the lungs.
• To define the concept of flow limitation and the principles that lead to this phenomenon.
• To define and integrate the physiological principles that are necessary to understand the flow–volume loop.

Having read about the characteristics of the respiratory system under static conditions, you are now ready to examine what happens when we set the system in motion—that is, when we breathe in and out. We continue to focus on the ventilatory pump in this chapter. Chapter 3 outlined how the elements of the pump work together to generate a negative alveolar pressure that serves as the driving force for the movement of gas into the lungs. At the end of an inhalation, the pump, during resting breathing, relaxes. The recoil forces of the lungs (and of the chest wall at higher lung volumes) cause leftover pressure to become positive so exhalation may follow and the system can be reset for the next breath. We will now examine what happens when the flow of air begins. We described the study of statics as "snapshots" of the respiratory system. Dynamics, the examination of the respiratory system during conditions of flow of gas into and out of the lungs, is the motion picture version of our story.

To begin the study of dynamics, we must examine a series of physical principles that characterize the flow of fluids through tubes. We will build these principles, one upon another, until we are able to construct the flow-volume loop, the most common graphical representation used by clinicians to understand the respiratory system in the dynamíc state. A key to understanding the flow-volume loop and the physiology of obstructive lung diseases such as asthma and emphysema is the concept of flow limitation. By the time you complete this chapter, you will have a firm understanding of these principles and a glimpse of their application in the evaluation of patients with diseases of the airways.