Lesson 1, Topic 1
In Progress

Anatomy of the Lower Respiratory Tract

April 11, 2024

Learning Objective: Differentiate among the structures of the lower respiratory tract.
      The lower respiratory tract consists of the trachea, bronchial tubes, and lungs (see FIGURE 26.1). These structures are lined with mucous membranes and cilia. The trachea (windpipe) lies in the space between the lungs, called the mediastinum. Air travels from the larynx through the trachea, and then the trachea branches into the right and left bronchi. The right bronchus is wider than the left bronchus.

Bronchioles and Alveoli
Learning Objective: Describe the anatomy of the bronchioles and alveoli.
      The bronchi divide into smaller branches, called bronchioles. These bronchioles end in microscopic ducts capped by air sacs, called alveoli. Each thin-walled alveolus is in contact with a blood capillary. This contact between the two structures allows the exchange of gases. It is at this point that oxygen (O2) from the inspired air moves across the one-cell membrane into the blood cells. Carbon dioxide (CO2) moves in the other direction, from the blood into the air to be expired. Each alveolus is coated with a substance called surfactant, which keeps it from collapsing. Without surfactant, the alveoli stick together during exhalation (breathing out) and deflate. Inhalation (breathing in) becomes more difficult, and less O2 can move into the bloodstream. This condition is life-threatening. Babies born before 37 to 39 weeks of gestation are at risk of not having enough surfactant. If time permits before delivery, steroids can be given to the mother to help mature the baby’s lungs.

FIGURE 26.2  Lobes of the lung.

Learning Objective: Describe the anatomy of the right and left lungs.
      The bronchial tree and alveoli are the major structures in the right and left lungs. The lungs are soft and spongy because of the air sacs that make up most of their mass. They hang on the right and left sides of the chest, separated by the pericardial sac, which contains the heart. Each lung is composed of sections called lobes. The right lung consists of three lobes, whereas the left has only two lobes (FIGURE 26.2).
      Because each lobe has its own bronchus and blood supply, the removal of one lobe (lobectomy) results in little or no damage to the rest of the lung. The left lung is longer and narrower. It has a distinct indentation in its center, known as the cardiac notch. This is where the left ventricle of the heart is located and where an apical pulse is heard.

Pleura and Pleural Cavity
Learning Objective: Describe the pleura and pleural cavity.
      Pleura is a thin serous membrane found in the thoracic cavity. The pleura folds back on itself, creating a sac that surrounds the lung. The visceral pleura covers the lungs and adjoining structures. The parietal pleura, the outer portion of the pleura, lines the thoracic cavity and covers the diaphragm and mediastinum. Only the parietal pleura contains pain receptors, making it highly sensitive to pain. The right and left pleural sacs are entirely separate.
The pleural cavity is the space between the visceral and parietal pleurae. A small amount of pleural fluid is in the pleural cavity. When the lungs expand, moving the visceral pleura closer to the parietal pleura, friction is reduced between the tissues due to the pleural fluid.

Respiratory Muscles
Learning Objective: Describe the muscles involved with respiration.
      The muscles responsible for normal, quiet respiration are the diaphragm and the intercostal muscles. On inspiration, the diaphragm is pulled down and flattened as it contracts, and the intercostal muscles expand, pulling air into the lungs. On expiration, the diaphragm relaxes and moves upward, pushing air out of the lungs.