The Lungs

Each lung (fig. 22.9) is a somewhat conical organ with a broad, concave base resting on the diaphragm and a blunt peak called the apex projecting slightly superior to the clavicle. The broad costal surface is pressed against the rib cage, and the smaller concave mediastinal surface faces medially. The lungs do not fill the entire rib cage. Inferior to the lungs and diaphragm, much of the space within the rib cage is occupied by the liver, spleen, and stomach (see fig. A.14, p. 45).

The lung receives the bronchus, blood vessels, lymphatic vessels, and nerves through its hilum, a slit in the mediastinal surface (see fig. 22.26a, p. 870). These structures entering the hilum constitute the root of the lung. Because the heart tilts to the left, the left lung is a little smaller than the right and has an indentation called the cardiac impression to accommodate it. The left lung has a superior lobe and an inferior lobe with a deep fissure between them; the right lung, by contrast, has three lobes—superior, middle, and inferior—separated by two fissures.

The Bronchial Tree

The lung has a spongy parenchyma containing the bronchial tree (fig 22.10), a highly branched system of air tubes extending from the primary bronchus to about 65,000 terminal bronchioles. Two primary bronchi (BRONK-eye) arise from the trachea at the level of the angle of the sternum. Each continues for 2 to 3 cm and enters the hilum of its respective lung. The right bronchus is slightly wider and more vertical than the left; consequently, aspirated (inhaled) foreign objects lodge in the right bronchus more often than in the left. Like the trachea, the primary bronchi are supported by C-shaped hyaline cartilages. All divisions of the bronchial tree also have a substantial amount of elastic connective tissue, which is important in expelling air from the lungs.

After entering the hilum, the primary bronchus branches into one secondary (lobar) bronchus for each pulmonary lobe. Thus, there are two secondary bronchi in the left lung and three in the right.

Each secondary bronchus divides into tertiary (segmental) bronchi—10 in the right lung and 8 in the left.

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22. The Respiratory System I Text

848 Part Four Regulation and Maintenance

848 Part Four Regulation and Maintenance

How Far Deep Mucociliary Escalator

Figure 22.7 Anatomy of the Lower Respiratory Tract. (a) Anterior view. (b) Longitudinal section of the trachea showing the action of the mucociliary escalator. (c) Cross section of the trachea showing the C-shaped tracheal cartilage. Why do inhaled objects more often go into the right primary bronchus than into the left?

Figure 22.7 Anatomy of the Lower Respiratory Tract. (a) Anterior view. (b) Longitudinal section of the trachea showing the action of the mucociliary escalator. (c) Cross section of the trachea showing the C-shaped tracheal cartilage. Why do inhaled objects more often go into the right primary bronchus than into the left?

The portion of the lung supplied by each tertiary bronchus is called a bronchopulmonary segment. Secondary and tertiary bronchi are supported by overlapping plates of cartilage, not rings. Branches of the pulmonary artery closely follow the bronchial tree on their way to the alveoli. The bronchial tree itself is nourished by the bronchial artery, which arises from the aorta and carries systemic blood.

Bronchioles are continuations of the airway that are 1 mm or less in diameter and lack cartilage. A well-developed layer of smooth muscle in their walls enables them to dilate or constrict, as discussed later. Spasmodic contractions of this muscle at death cause the bronchioles to exhibit a wavy lumen in most histological sections. The portion of the lung ventilated by one bronchiole is called a pulmonary lobule.

Each bronchiole divides into 50 to 80 terminal bronchioles, the final branches of the conducting division. They measure 0.5 mm or less in diameter and have no mucous glands or goblet cells. They do have cilia, however, so that mucus draining into them from the higher passages can be driven back by the mucociliary escalator, thus preventing congestion of the terminal bronchioles and alveoli.

Each terminal bronchiole gives off two or more smaller respiratory bronchioles, which mark the beginning of the respiratory division. All branches of the respiratory division are defined by the presence of alveoli. The respiratory bronchioles have scanty smooth muscle, and the smallest of them are nonciliated. Each divides into 2 to 10 elongated, thin-walled passages called alveolar ducts that end in alveolar sacs, which are grapelike clusters of alveoli (fig. 22.11). Alveoli also bud from the walls of the respiratory bronchioles and alveolar ducts.

The epithelium of the bronchial tree is pseudostrati-fied columnar in the bronchi, simple cuboidal in the bronchioles, and simple squamous in the alveolar ducts, sacs, and alveoli. It is ciliated except in the distal reaches of the respiratory bronchioles and beyond.


The functional importance of human lung structure is best appreciated by comparison to the lungs of a few other animals. In frogs and other amphibians, the lung is a simple sac lined with blood vessels. This is sufficient to meet the oxygen needs of animals with relatively low metabolic

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Tracheal Epithelium
Figure 22.8 The Tracheal Epithelium Showing Ciliated Cells and Nonciliated Goblet Cells. The small bumps on the goblet cells are microvilli. (Colorized SEM micrograph)

rates. Mammals, with their high metabolic rates, could never have evolved with such a simple lung. Rather than consisting of one large sac, each human lung is a spongy mass composed of 150 million little sacs, the alveoli, which provide about 70 m2 of surface for gas exchange.

An alveolus (AL-vee-OH-lus) (fig. 22.12) is a pouch about 0.2 to 0.5 mm in diameter. Its wall consists predominantly of squamous (type I) alveolar cells—thin cells that allow for rapid gas diffusion between the alveolus and bloodstream. About 5% of the alveolar cells are round to cuboidal great (type II) alveolar cells. They secrete a detergent-like lipoprotein called pulmonary surfactant, which forms a thin film on the insides of the alveoli and bronchioles. Its function is discussed later.

Alveolar macrophages (dust cells) wander the lumens of the alveoli and the connective tissue between them. They are the last line of defense against inhaled matter. Particles over 10 ^m in diameter are usually strained out by the nasal vibrissae or trapped in the mucus of the upper respiratory tract. Most particles 2 to 10 ^m in diameter are trapped in the mucus of the bronchi and bronchioles, where the airflow is relatively slow, and then removed by the mucociliary escalator. Many particles smaller than 2 ^m, however, make their way into the alveoli, where they are phagocytized by the macrophages. In lungs that are infected or bleeding, the macrophages also phagocytize bacteria and loose blood cells. Alveolar

Chapter 22 The Respiratory System 849

macrophages greatly outnumber all other cell types in the lung; as many as 50 million perish each day as they ride up the mucociliary escalator to be swallowed.

Each alveolus is surrounded by a basket of blood capillaries supplied by the pulmonary artery. The barrier between the alveolar air and blood, called the respiratory membrane, consists only of the squamous type I alveolar cell, the squamous endothelial cell of the capillary, and their fused basement membranes. These have a total thickness of only 0.5 ^m.

The pulmonary circulation has very low blood pressure. In alveolar capillaries, the mean blood pressure is 10 mmHg and the oncotic pressure is 25 mmHg. The osmotic uptake of water thus overrides filtration and keeps the alveoli free of fluid. The lungs also have a more extensive lymphatic drainage than any other organ in the body. The low capillary blood pressure also prevents the rupture of the delicate respiratory membrane.

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  • maya
    How far deep mucociliary escalator?
    9 years ago
    Why do inhaled objects go to right lung not left?
    8 years ago
  • oona
    Why do inhaled objects go to the right main bronchus instead of the left?
    8 years ago

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