After studying this chapter, you should be able to:
List the passages through which air passes from the exterior to the alveoli, and describe the cells that line each of them.
List the major muscles involved in respiration, and state the role of each.
Define the basic measures of lung volume and give approximate values for each in a normal adult.
Define lung compliance and airway resistance.
Compare the pulmonary and systemic circulations, and list some major differences between them.
Describe basic lung defense and metabolic functions.
Define partial pressure and calculate the partial pressure of each of the important gases in the atmosphere at sea level.
The structure of the respiratory system is uniquely suited to its primary function, the transport of gases in and out of the body. The respiratory system provides a large volume of tissue that is constantly exposed to the outside environment, and thus, to potential infection and injury. The pulmonary system includes a unique circulation that must handle the blood flow. This chapter begins with the basic anatomy and cellular physiology that contribute to the respiratory system and some of their unique features. The chapter also includes discussion of how the anatomic features contribute to the basic mechanics of breathing, as well as some highlights of nonrespiratory physiology in the pulmonary system.
REGIONS OF THE RESPIRATORY TRACT
Airflow through the respiratory system can be broken down into three interconnected regions: the upper airway, the conducting airway, and the alveolar airway. The upper airway consists of the entry systems, the nose/nasal cavity and mouth that lead into the pharynx. The larynx extends from the lower part of the pharynx to complete the upper airway. The nose is the primary point of entry for inhaled air; therefore, the mucosal epithelium lining the nasopharyngeal airways is exposed to the highest concentration of inhaled allergens, toxicants, and particulate matter. With this in mind, it is easy to understand that in addition to olfaction, the nose and upper airway provides two additional crucial functions in airflow—(1) filtering out large particulates to prevent them from reaching the conducting and alveolar airways and (2) serving to warm and humidify air as it enters the body. Particulates larger than 30–50 μm in size tend to not to be inhaled through the nose whereas particulates on the order of 5–10 μm impact on the nasopharynx and do enter the conducting airway. Most of these latter particles settle on mucous membranes in the nose and pharynx. Because of their momentum, they do not follow the airstream as it curves downward into the lungs, and they impact on or near the tonsils and adenoids, large collections of immunologically active lymphoid tissue in the back of the pharynx.