Ganong’s Review of Medical Physiology, 24th Edition
SECTION VI Respiratory Physiology
Respiration, or the uptake of O2 and removal of CO2 from the body as a whole, is the primary goal of the lung. At rest, a normal human breathes 12–15 times a minute. With each breath containing ∼500 mL of air, this translates to 6-8 L of air that is inspired and expired every minute. Once the air reaches the depths of the lung in the alveoli, simple diffusion allows O2 to enter the blood in the pulmonary capillaries and CO2 to enter the alveoli, from where it can be expired. Using some basic math, on average, 250 mL of O2 enters the body per minute and 200 mL of CO2 is excreted. In addition to the O2 that enters the respiratory system, inspired air also contains a variety of particulates that must be properly filtered and/or removed to maintain lung health. Finally, although we have a certain amount of control over breathing, most of the minute to minute function, including the fine adjustments necessary for proper lung function, are accomplished independent of voluntary control. The goal of this section is to review basic concepts that underlie important aspects of the control and outcome of breathing as well as to highlight other important functions in respiratory physiology.
The respiratory system is connected to the outside world by the upper airway that leads down a set of conduits before reaching the gas-exchanging areas (the alveoli). The function of the lungs is supported by a variety of anatomical features that serve to inflate/deflate the lung, thereby allowing the movement of gases to and from the rest of the body. Supporting features include the chest wall; the respiratory muscles (which increase and decrease the size of the thoracic cavity); the areas in the brain that control the muscles; and the tracts and nerves that connect the brain to the muscles. Finally, the lung supports the pulmonary circulation, which allows for movement of gases to other organs and tissues of the body. In the first chapter of this section we will explore the unique anatomical and cellular makeup of the respiratory system and how the intricate structure of the lung contributes to respiratory physiology. This examination will lead into basic lung measurements that both define and allow for lung inflation/deflation, as well as some of the nonrespiratory functions essential for lung health.
Our discussion will continue with an overview of the primary function of the respiratory system-the capture of O2 from the outside environment and its delivery to tissues, as well as the simultaneous removal of CO2 from the tissues to the outside environment. During this discussion, the critical role of pH in gas exchange as well as the ability of the lung to contribute to pH regulation of the blood is examined. A discussion of respiratory responses to altered O2 or CO2 concentrations, caused by environmental and/or physiological changes, is used to better understand the overall control of coordinated uptake of O2 and excretion of CO2.
Control of breathing is quite complex, and includes not only the repetitive neuronal firing that controls muscle movements that inflate/deflate the lung, but also a series of feedback loops that increase/decrease deflation depending on the gas content of the blood. The final chapter in this section begins with an overview of some of the key factors that aid in this regulation of respiration. Specific examples of common respiratory abnormalities and how they relate to altered regulation of breathing are also discussed to better understand the intricate feedback loops that help to regulate breathing.
Due to the complexity of the lung and thus the variety of working parts that can be compromised, there is a wide-ranging list of diseases that impact its function. Such diseases include common (and uncommon) respiratory infections, asthma, chronic obstructive pulmonary disease (COPD), acute respiratory distress syndrome, pulmonary hypertension, lung cancer and many more. The health burden from such a diverse collection of disorders cannot be overstated. Using COPD as an example, current conservative estimates hold that over 12 million adults in the United States suffer from the condition. In fact, COPD is the fourth leading cause of death (and rising) and a contributory factor in an equal number of non-COPD deaths. Although treatment strategies for COPD, largely based on continuing research efforts and understanding, have contributed to an improved lifestyle, the underlying causes are, as of yet, untreatable. The continued and improved understanding of respiratory physiology and lung function (and its dysfunction) will provide opportunities to develop new strategies for treatment of COPD, as well as the myriad of other lung diseases.