Physiology 5th Ed.

STRUCTURE OF THE RESPIRATORY SYSTEM

Airways

The respiratory system includes the lungs and a series of airways that connect the lungs to the external environment. The structures of the respiratory system are subdivided into a conducting zone (or conducting airways), which brings air into and out of the lungs, and a respiratory zone lined with alveoli, where gas exchange occurs. The functions of the conducting and respiratory zones differ, and the structures lining them also differ (Fig. 5-1).

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Figure 5–1 Structure of the airways. The number of the various structures is reported for two lungs.

Conducting Zone

The conducting zone includes the nose, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles. These structures function to bring air into and out of the respiratory zone for gas exchange and to warm, humidify, and filter the air before it reaches the critical gas exchange region.

The trachea is the main conducting airway. The trachea divides into two bronchi, one leading into each lung, which divide into two smaller bronchi, which divide again. Ultimately, there are 23 such divisions into increasingly smaller airways.

The conducting airways are lined with mucus-secreting and ciliated cells that function to remove inhaled particles. Although large particles usually are filtered out in the nose, small particles may enter the airways, where they are captured by mucus, which is then swept upward by the rhythmic beating of the cilia.

The walls of the conducting airways contain smooth muscle. This smooth muscle has both sympathetic and parasympathetic innervation, which have opposite effects on airway diameter: (1) Sympathetic adrenergic neurons activate β2 receptors on bronchial smooth muscle, which leads to relaxation and dilation of the airways. In addition, and what is more important, these β2 receptors are activated by circulating epinephrine released from the adrenal medulla and by β2-adrenergic agonists such as isoproterenol. (2) Parasympathetic cholinergic neurons activate muscarinic receptors, which leads to contraction and constriction of the airways.

Changes in diameter of the conducting airways result in changes in their resistance, which produce changes in airflow. Thus, the effects of the autonomic nervous system on airway diameter have predictable effects on airway resistance and airflow. The most notable effects are those of β2-adrenergic agonists (e.g., epinephrine, isoproterenol, albuterol), which are used to dilate the airways in the treatment ofasthma.

Respiratory Zone

The respiratory zone includes the structures that are lined with alveoli and, therefore, participate in gas exchange: the respiratory bronchioles, alveolar ducts, and alveolar sacs. The respiratory bronchiolesare transitional structures. Like the conducting airways, they have cilia and smooth muscle, but they also are considered part of the gas exchange region because alveoli occasionally bud off their walls. Thealveolar ducts are completely lined with alveoli, but they contain no cilia and little smooth muscle. The alveolar ducts terminate in alveolar sacs, which also are lined with alveoli.

The alveoli are pouchlike evaginations of the walls of the respiratory bronchioles, the alveolar ducts, and the alveolar sacs. Each lung has a total of approximately 300 million alveoli. The diameter of each alveolus is approximately 200 µm. Exchange of oxygen (O2) and carbon dioxide (CO2) between alveolar gas and pulmonary capillary blood can occur rapidly and efficiently across the alveoli because alveolar walls are thin and have a large surface area for diffusion.

The alveolar walls are rimmed with elastic fibers and lined with epithelial cells, called type I and type II pneumocytes (or alveolar cells). Type II pneumocytes synthesize pulmonary surfactant (necessary for reduction of surface tension of alveoli) and have regenerative capacity for the type I and type II pneumocytes.

The alveoli contain phagocytic cells called alveolar macrophages. Alveolar macrophages keep the alveoli free of dust and debris because the alveoli have no cilia to perform this function. Macrophages fill with debris and migrate to the bronchioles, where the beating cilia carry debris to the upper airways and the pharynx, where it can be swallowed or expectorated.

Pulmonary Blood Flow

Pulmonary blood flow is the cardiac output of the right heart. It is ejected from the right ventricle and delivered to the lungs via the pulmonary artery (see Chapter 4Fig. 4-1). The pulmonary arteries branch into increasingly smaller arteries and travel with the bronchi toward the respiratory zones. The smallest arteries divide into arterioles and then into the pulmonary capillaries, which form dense networks around the alveoli.

Because of gravitational effects, pulmonary blood flow is not distributed evenly in the lungs. When a person is standing, blood flow is lowest at the apex (top) of the lungs and highest at the base (bottom) of the lungs. When the person is supine (lying down), these gravitational effects disappear. The physiologic significance of regional variations in blood flow is discussed later in the chapter.

As in other organs, regulation of pulmonary blood flow is accomplished by altering the resistance of the pulmonary arterioles. Changes in pulmonary arteriolar resistance are controlled by local factors, mainly O2.

Bronchial circulation is the blood supply to the conducting airways (which do not participate in gas exchange) and is a very small fraction of the total pulmonary blood flow.