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Chapter 6: Respiratory System

Sarah E. A. McConnell

OBJECTIVES

Objectives

After studying this chapter, you should be able to:

  • Trace the development of the respiratory system from its origins in the tubiform embryo.

  • Describe how the lung bud branches to form the lower respiratory system.

  • Relate the four stages of lung development to the capacity to support gas exchange.

  • Describe the formation of the pleural cavities.

ORIGINS OF THE RESPIRATORY SYSTEM

  • The respiratory system is derived from the splanchnopleure of the tubiform embryo.

    • The endoderm gives rise to the epithelium and glands of the airways.

    • The splanchnic (visceral) mesoderm gives rise to the smooth muscle and connective tissue surrounding the airways.

  • During the fourth week of development, the lung bud (respiratory diverticulum) grows from the ventral surface of the cranial foregut. This diverticulum will form the lower respiratory system, consisting of the larynx, trachea, bronchi, bronchioles, and alveoli (Figure 6-1; Clinical Box 6-1).

    • The upper respiratory system (consisting of the nasal passages, oral cavity, and pharynx) develops from the frontonasal prominence and pharyngeal arches (see Chapter 9).

  • Two tracheoesophageal folds grow into the lumen of the gut tube from the left and right sides (Figure 6-2). They fuse together in the midline to form the tracheoesophageal septum, separating the trachea ventrally from the esophagus dorsally while maintaining an open connection at the laryngeal orifice (Clinical Box 6-2).

Figure 6-1.

Lung bud formation. A midsagittal section of the embryo at the end of the fourth week shows the lung bud growing from the ventral surface of the upper foregut.

Source: Based on Figure 6.17 in T. W. Sadler, Chapter 6, “Third to Eighth Weeks: The Embryonic Period,” in Langman's Medical Embryology, 14th ed. Wolters Kluwer. 2019.

A diagram depicts the midsagittal view of an embryo. The lung bud grows from the ventral surface of the upper foregut at the end of the fourth week.
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CLINICAL BOX 6-1. Ectopic Lung Lobes

  • An additional lung bud may grow from the foregut.

  • This extra bud may develop into ectopic lung lobes attached to the trachea or esophagus.

Figure 6-2.

Tracheoesophageal septum formation. (A) A lateral view illustrates the respiratory diverticulum (lung bud) growing from the ventral aspect of the cranial foregut. (B–C) Ventral views show the tracheoesophageal folds extending from the lateral walls toward each other to fuse in the midline, forming the tracheoesophageal septum, which separates the trachea ventrally from the esophagus dorsally.

Source: Based on Figure 15.6 in T. W. Sadler, Chapter 15, “Digestive System,” in Langman's Medical Embryology, 14e. Wolters Kluwer. 2019.

Three diagrams of the formation of the tracheoesophageal septum, separating the trachea from the esophagus.
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CLINICAL BOX 6-2. Tracheoesophageal Fistula

  • A tracheoesophageal fistula (TEF) is an abnormal connection between the trachea and esophagus that results from improper partitioning of the foregut by the tracheoesophageal septum.

  • Different types of TEF involve varying degrees of tracheal and esophageal patency and different points of connection between the trachea and esophagus.

    • TEFs are frequently ...

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