Bio 14.1 Respiratory System Structure PDF

Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 14: Respiration 488 Lesson 14.1 **Respiratory System Structure** Introduction The respiratory system includes the lungs and the structures that carry air to and from the lung...

Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 14: Respiration 488 Lesson 14.1 **Respiratory System Structure** Introduction The respiratory system includes the lungs and the structures that carry air to and from the lungs. This system is functionally divided into two zones. In the **respiratory zone**, exchange of respiratory gases (ie, oxygen and carbon dioxide) occurs. In the **conducting zone**, air is transported to and from the respiratory zone, and the air is conditioned to enhance respiratory zone function. This lesson describes the general structure of the components of the respiratory system, including those components that participate in protecting the system from potentially harmful materials in the environment. 14.1.01 Lung Structure Figure 14.1 shows the major components of the respiratory system. The respiratory zone (ie, region responsible for gas exchange) is present within the lungs, which also contain some structures of the conducting zone (ie, passageways through which air is transported). In addition to air-conducting structures within the lungs, the conducting zone includes the nose (ie, **nostrils**, **nasal cavity**), **pharynx**, **larynx**, **trachea**, and **primary bronchi** (singular: bronchus). Many components of the conducting zone, including the nose, larynx, trachea, and bronchi, are reinforced with [cartilage](javascript:void(0)). Cartilage helps maintain open airways by preventing collapse of these structures during breathing. Chapter 14: Respiration 489 **Figure 14.1** Major components of the respiratory system. The primary bronchi connect the trachea to the lungs. Within the lungs, the primary bronchi repeatedly branch to form additional bronchi with smaller and smaller diameters and progressively less cartilage reinforcement. Within the lungs, air passageways with diameters less than approximately 1 mm are called **bronchioles**, the walls of which contain smooth muscle and lack cartilage entirely, allowing bronchiolar diameter to be adjusted to regulate airflow. Bronchioles continue branching to form smaller air passageways, eventually giving rise to **terminal bronchioles**, which constitute the final section of the conducting zone. Terminal bronchioles connect the conducting zone to the respiratory zone, which consists of **respiratory bronchioles**, **alveolar ducts**, and **alveolar sacs** within the lung (see Figure 14.2). Respiratory bronchioles have scattered, thin-walled microscopic air pockets called **alveoli** (singular: alveolus) that bulge from the bronchioles\' surface. Respiratory bronchioles receive air from terminal bronchioles and transfer air into alveolar ducts, which are short tubes that have many individually attached alveoli and less smooth muscle than terminal bronchioles. Alveolar ducts carry air to alveolar sacs, which are dead-end structures composed of small clusters of interconnected alveoli. Alveolar sacs and their constituent alveoli contain no smooth muscle. A diagram of the internal organs of a person Description automatically generated Chapter 14: Respiration 490 **Figure 14.2** Components of the respiratory zone. An average human lung in a healthy adult contains approximately 250 million alveoli, with an internal surface area (ie, surface area exposed to air in the lung) of approximately 50 m2 (incredibly large when compared to an average skin surface area for the entire body of approximately 1.7 m2). The wall (ie, septum) of an alveolus consists of a single layer of epithelial cells. These cells include type I cells, which account for about 95% of the wall\'s structure and allow for gas exchange, and type II cells, which secrete **pulmonary surfactant**, a substance that reduces surface tension within the lung and facilitates lung inflation. Alveoli are surrounded by elastic fibers and capillaries, with adjacent alveoli and capillary walls forming the **respiratory membrane** (see Figure 14.2). This membrane permits rapid exchange of gases between the blood and air in the lungs via [simple diffusion](javascript:void(0)). The lungs are located within the thoracic cavity, the inferior boundary of which is formed by the **diaphragm**. Within the thoracic cavity, each lung is surrounded by a double-layered saclike membrane, or **pleura** (Figure 14.3). The inner layer of this sac, the **visceral pleura**, is attached to the exterior  Chapter 14: Respiration 491 surface of the lung, and the outer layer, the **parietal pleura**, is attached to the thoracic cavity wall. The pleural layers slide easily past one another but typically do not separate because the slit-like **pleural cavity** between them contains a small volume of watery fluid (ie, **pleural fluid**) that keeps the layers in close contact via [hydrogen bonding](javascript:void(0)). **Figure 14.3** Double-layered structure of a pleura. 14.1.02 Mucociliary Escalator Both the wall of an alveolus and the wall of a capillary are primarily composed of a single layer of flat epithelial cells. As a result, the respiratory membrane (ie, structure separating air from blood in the lung) is very thin, approximately 0.5 µm, and does not typically function as an effective barrier against entry of pathogens or other harmful substances (eg, toxic chemicals) into the body. Consequently, the respiratory system employs a variety of mechanisms by which pathogens and other potentially harmful materials are prevented from reaching the lung\'s alveoli or are removed from the alveoli upon arrival. A primary means by which the lungs are protected from pathogens and other potentially harmful inhaled substances is a mechanism called **mucociliary clearance (MCC)**, which occurs via the **mucociliary escalator**. The mucociliary escalator consists of airway **mucus**, which traps foreign materials, and **cilia**, hairlike structures that continuously wave back and forth on the surface of specialized cells to sweep mucus toward the [pharynx](javascript:void(0)). Upon reaching the pharynx, the mucus is expectorated (eg, spit from the mouth) or swallowed (ie, mucus-entrapped pathogens are destroyed via acidic [stomach secretions](javascript:void(0))). The mucociliary escalator is present in most regions of the respiratory system, including the nasal cavity, larynx, trachea, bronchi, and bronchioles. These regions are lined by an [epithelium](javascript:void(0)) that consists of ciliated cells interspersed with secretory cells (eg, mucus-secreting **goblet cells**). In addition, the entrance to the nasal cavity contains hairs that help filter large particles from inhaled air. Pathogens and particles not eliminated by MCC can be removed from alveoli by **alveolar macrophages**, which engulf and destroy foreign materials via [phagocytosis](javascript:void(0)). Figure 14.4 depicts structures that help protect the respiratory system. A diagram of the human body Description automatically generated Chapter 14: Respiration 492 **Figure 14.4** Protective structures in the respiratory system.