Respiratory System Introduction PDF

Page 893 23.1 Introduction to the Respiratory System The respiratory system consists of the respiratory tract (nose, nasal cavity, pharynx, larynx, trachea, and bronchiole tree that ends at air sacs called alveoli) and the lungs. Here, we examine how the respiratory system serves numerous functions...

Page 893 23.1 Introduction to the Respiratory System The respiratory system consists of the respiratory tract (nose, nasal cavity, pharynx, larynx, trachea, and bronchiole tree that ends at air sacs called alveoli) and the lungs. Here, we examine how the respiratory system serves numerous functions in the body, is organized both structurally and functionally, and its respiratory tract is lined internally and protected with a mucous membrane. 23.1a General Functions of the Respiratory System LEARNING OBJECTIVE 1. List and explain the functions of the respiratory system. The primary, and perhaps only, function most individuals would associate with the respiratory system is breathing. However, the respiratory system has several purposes that include the following: Air passageway. The respiratory tract is a passageway for air between the external environment and the alveoli of the lungs. Air is moved from the atmosphere to alveoli as we breathe in and then expelled into the atmosphere as we breathe out. Site for the exchange of oxygen and carbon dioxide. A thin membrane formed by each alveolus and associated pulmonary capillaries provides the site for exchange of oxygen and carbon dioxide. Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli. Detection of odors. Olfactory receptors located in the superior regions of the nasal cavity detect odors as air moves past them. Sensory input from these receptors is then relayed to various regions of the brain for interpretation (see section 16.3a). Sound production. The vocal cords of the larynx (voice box) vibrate as air moves across them to produce sounds; these sounds then resonate in upper respiratory structures. WHAT DID YOU LEARN? 1 Which respiratory structure is associated with the exchange of respiratory gases? 23.1b General Organization of the Respiratory System LEARNING OBJECTIVE 2. Compare and contrast the structural organization and the functional organization of the respiratory system. The respiratory system is organized structurally into two regions: an upper respiratory tract and a lower respiratory tract ( figure 23.1). The nose, nasal cavity, pharynx, and larynx form the upper respiratory tract. The trachea and the air passageways within the lungs— bronchi, bronchioles (including terminal and respiratory bronchioles), alveolar ducts, and alveoli—are the components of the lower respiratory tract. Figure 23.1 General Anatomy of the Respiratory System. Structurally, the respiratory system is organized into the upper respiratory tract and lower respiratory tract. Functionally, the respiratory system is divided into the conducting zone and respiratory zone. Respiratory System Overview Page 894 The structures of the respiratory system are also categorized based on function. Passageways that transport or conduct air are part of the conducting zone; these structures include the passageways from the nose to the end of the terminal bronchioles. Structures that participate in gas exchange with the blood—including the respiratory bronchioles, alveolar ducts, and alveoli—are part of the respiratory zone. WHAT DID YOU LEARN? 2 What four structures compose the upper respiratory tract? What three structures compose the respiratory zone? INTEGRATE LEARNING STRATEGY 23.1 You may find these hints helpful to remember the boundary structures in the general organization of the respiratory system: Structural organization: The Lower respiratory tract begins inferior to the Larynx and includes all structures to the alveoli. Functional organization: The terminal bronchioles are the end of or final portion of the conducting zone. 23.1c Respiratory Mucosa LEARNING OBJECTIVES 3. Describe the structure of the mucosa that lines the respiratory tract and the structural changes observed along its length. 4. Explain the function of mucus produced by the mucosa. The respiratory tract is exposed to the external environment and is lined internally by a mucosa, also called a mucous membrane (see section 5.5b). This mucosa is more specifically called the respiratory mucosa. In general, the respiratory mucosa is composed of an epithelium resting upon a basement membrane, and an underlying lamina propria composed of areolar connective tissue. The epithelium is ciliated (having cilia) in most portions of the respiratory tract conducting zone. A general pattern of structural change is observed in the epithelium along the length of the respiratory tract. The epithelium becomes progressively thinner from the nasal cavity to the alveoli; it changes from pseudostratified ciliated columnar to simple ciliated columnar to simple cuboidal to simple squamous (see section 5.1c). Exceptions to this general pattern occur in selected regions of the respiratory tract. These exceptions include (a) portions of the pharynx that serve as a passageway for both air and food (i.e., oropharynx and laryngopharynx) and (b) components of the larynx that include the vocal folds and the area immediately superior to them. These areas are lined by a nonkeratinized stratified squamous epithelium (instead of ciliated pseudostratified columnar epithelium) to withstand abrasion. Figure 23.2 summarizes the types of epithelia found in the mucosa along the respiratory tract. Figure 23.2 Respiratory Mucosa. A mucosa forms the inner lining of the respiratory tract and is called the respiratory mucosa. (a) The general structure of the respiratory mucosa includes three major layers: an epithelium, a basement membrane, and an underlying lamina propria. (b) The types of epithelia in the mucosa become progressively thinner along the length of the respiratory tract. (c) Exceptions to the general thinning pattern occur in regions subject to abrasion. APR Module 11: Respiratory: Histology: Respiratory Epithelium: LM High Magnification: Goblet Cell of Respiratory Epithelium Page 895 The epithelium lining most of the respiratory tract contains goblet cells (unicellular exocrine glands; see section 5.1d), and the underlying lamina propria houses both mucous and serous glands. Mucus is produced from the combined secretions of these cells and glands. The amount of mucus produced daily is approximately 1 to 7 tablespoons, but that amount increases with exposure to irritants. Mucous secretions contain mucin, a protein that increases the viscosity of mucus to more effectively trap inhaled dust, dirt particles, microorganisms, and pollen. The secretions also contain specific substances to help defend the body against infectious agents (see section 22.1), including lysozyme (an antibacterial enzyme), defensins (antimicrobial proteins), and immunoglobulin A (antibodies) (see section 22.3a). Both mucus and saliva entrap materials, which may be coughed up together as a viscous substance called sputum. Physicians may request sputum samples from their patients to diagnose potential respiratory infections. WHAT DID YOU LEARN? 3 In what ways does the epithelium of the upper respiratory tract differ from the epithelium in the alveoli? INTEGRATE CLINICAL VIEW 23.1 Cystic Fibrosis Cystic fibrosis (sis΄tik f Ī-brō΄sis) is the most common serious genetic disease in Caucasians, occurring with a frequency of approximately 1 in 3500 births in the United States. The condition is rare among people of Asian and African descent. The symptoms of cystic fibrosis involve multiple body systems, but all result from defective chloride channels. The defective channel prevents chloride ion from being pumped from epithelial cells lining the respiratory tract into the lumen of the respiratory tract. Without chloride on the cell surface, water does not move to hydrate the cells’ surface. The mucus covering the epithelial surface becomes thick and sticky, and the cells’ cilia are not able to move freely to effectively sweep the mucus (and its trapped particulates) upward in the respiratory tract. Thus, the thick and sticky mucus blocks the respiratory tract. Pulmonary infections, secondary to airway obstruction, are common and can be life-threatening. The ducts of the pancreas and salivary glands also become blocked. In the case of the pancreas, the obstructed ducts lead to a backup of digestive enzymes, which eventually destroys the pancreas (see Clinical View 26.17: “Cystic Fibrosis and the Pancreas”).