Getting Started: The Approach to Respiratory Physiology

A Systems Approach to Respiratory Physiology

In teaching physiology to medical students for the past decade, we have often heard a plea for assistance in making the different things they were learning "fit together." For knowledge to be both meaningful and useful, it is important to have a superstructure upon which to hang individual concepts. Similarly, in teaching interns and residents for the past 20 years, we have seen many mistakes made in the care of patients with respiratory disease because of the exclusive focus on the lungs as the physician tried to analyze the problem at hand. The answer to both of these problems is a systems approach to respiratory physiology.

The respiratory system is composed of all of the elements needed to move air from the atmosphere down to the alveoli, exchange oxygen and carbon dioxide in the blood flowing through the pulmonary capillaries, and then move carbon dioxide back to the atmosphere. The core components of this system are the respiratory controller, ventilatory pump, and gas exchanger.

The respiratory controller consists of the elements of the central nervous system (CNS) that tell us how often and how deeply to breathe. The ventilatory pump is composed of multiple structures that perform the bellows function of the respiratory system.

The muscles, bones, cartilage, and related soft tissue of the chest wall are necessary to move the chest wall and create changes in intrathoracic pressure. The peripheral nerves that connect the CNS to the muscles are critical to transform the messages from the controller into movement of the chest wall. Airways serve as conduits for flow of gas from the mouth to the alveoli, and the pleura connect the motion of the chest wall to the lungs. Finally, the gas exchanger, the alveoli and the pulmonary capillaries, is the site of the exchange of oxygen and carbon dioxide (Fig. 1-1).

Chapter 2 provides an overview of the anatomy of the respiratory system, emphasizing the function of the anatomic structures. Next the book explores the workings of the ventilatory pump, both under static conditions (no flow of gas; Chapter 3) and dynamic conditions (when the system is set in motion; Chapter 4). Chapter 5 discusses the gas exchanger, and Chapter 6 examines the unique aspects of respiratory control. Acid-base physiology, which plays an important role in ventilatory control, is introduced in Chapter 7. The origins of dyspnea, or shortness of breath, one of the most debilitating symptoms in clinical medicine, are far more complex than was thought even just a few years ago. To help you understand dyspnea when you move on in your studies of pathophysiology, we will explore the physiologic bases of respiratory sensations, which draw on elements of all three components of the respiratory system, in Chapter 8. Finally, Chapter 9 integrates all that you have learned and makes some links from your knowledge of the respiratory system to the cardiovascular system, as we examine the physiology of exercise. As we develop the concepts in each of these chapters, we will continually bring you back to the the superstructure of the respiratory system -- controller, pump, gas exchanger -- will help you understand how the individual pieces fit together.