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Figure

Cross section of the trachea. Note the C-shaped rings of hyaline cartilage in the wall.

Light micrograph of a section of the tracheal wall (63x).

Bronchial Tree

The bronchial tree (brong'ke-al tre) consists of branched airways leading from the trachea to the microscopic air sacs in the lungs. Its branches begin with the right and left primary bronchi, which arise from the trachea at the level of the fifth thoracic vertebrae. The openings of the primary bronchi are separated by a ridge of cartilage called the carina (see fig. 19.8). Each bronchus, accompanied by large blood vessels, enters its respective lung.

Branches of the Bronchial Tree A short distance from its origin, each primary bronchus divides into secondary, or lobar, bronchi (two on the left and three on the right) that, in turn, branch repeatedly into finer and finer tubes (figs. 19.12 and 19.13). When stripped of their associated blood vessels and tissues, the airways appear as an upside down tree. The successive divisions of these branches from the trachea to the microscopic air sacs follow:

1. Right and left primary bronchi.

2. Secondary, or lobar, bronchi. Three branch from the right primary bronchus, and two branch from the left.

3. Tertiary, or segmental, bronchi. Each of these branches supplies a portion of the lung called a bronchopulmonary segment. Usually there are ten such segments in the right lung and eight in the left lung.

4. Intralobular bronchioles. These small branches of the segmental bronchi enter the basic units of the lung—the lobules.

Thyroid gland Incision Trachea

Hyoid bone Thyroid cartilage Cricoid cartilage Sternal notch

Figure 19.11

A tracheostomy may be performed to allow air to bypass an obstruction within the larynx.

Thyroid gland Incision Trachea

Figure 19.11

A tracheostomy may be performed to allow air to bypass an obstruction within the larynx.

Hyoid bone Thyroid cartilage Cricoid cartilage Sternal notch

Right superior lobe

Right primary bronchus

Secondary bronchus Tertiary bronchus-

Terminal bronchiole Right inferior lobe

Respiratory bronchiole Alveolar duct

Alveolus

Respiratory bronchiole Alveolar duct

Larynx Trachea

Left superior lobe

Right superior lobe

Right primary bronchus

Larynx Trachea

Left superior lobe

Left inferior lobe

Alveolus

Figure 19.12

The bronchial tree consists of the passageways that connect the trachea and the alveoli. The alveolar duct and alveoli are enlarged to show their location.

Figure 19.13

A plastic cast of the bronchial tree.

Figure 19.13

A plastic cast of the bronchial tree.

5. Terminal bronchioles. These tubes branch from a bronchiole. Fifty to eighty terminal bronchioles occupy a lobule of the lung.

6. Respiratory bronchioles. Two or more respiratory bronchioles branch from each terminal bronchiole. Short and about 0.5 millimeter in diameter, these structures are called "respiratory" because a few air sacs bud from their sides, making them able to take part in gas exchange.

7. Alveolar ducts. Two to ten long, branching alveolar ducts extend from each respiratory bronchiole

8. Alveolar sacs. Alveolar sacs are thin-walled, closely packed outpouchings of the alveolar ducts.

9. Alveoli. Alveoli are thin-walled, microscopic air sacs that open to an alveolar sac. Thus, air can diffuse freely from the alveolar ducts, through the alveolar sacs, and into the alveoli (fig. 19.15).

Dust particles, asbestos fibers, and other pollutants travel at speeds of 200 centimeters per second in the trachea but slow to 1 centimeter per second when deep in the lungs. Gravity deposits such particles, particularly at branchpoints in the respiratory tree. It is a little like traffic backing up at an exit to a highway.

Structure of the Respiratory Tubes

The structure of a bronchus is similar to that of the trachea, but the C-shaped cartilaginous rings are replaced with cartilaginous plates where the bronchus enters the lung. These plates are irregularly shaped and completely surround the tube. However, as finer and finer branch tubes appear, the amount of cartilage decreases, and it finally disappears in the bronchioles, which have diameters of about 1 millimeter. Meanwhile, a layer of smooth muscle that surrounds the tube just beneath the mucosa becomes more prominent. This muscular layer remains in the wall to the ends of the respiratory bronchioles,

Capillary

Simple squamous epithelial cells

Alveolus

Capillary

Simple squamous epithelial cells

Alveolus

Figure 19.15

Light micrograph of alveoli (250x).

Figure 19.15

Light micrograph of alveoli (250x).

and only a few muscle fibers are in the walls of the alveolar ducts.

Elastic fibers are scattered among the smooth muscle cells and are abundant in the connective tissue that surrounds the respiratory tubes. These fibers play an important role in breathing, as is explained later in this chapter.

As the tubes become smaller in diameter, the type of cells that line them changes. The lining of the larger tubes consists of pseudostratified, ciliated columnar epithelium and mucus-secreting goblet cells. However, along the way, the number of goblet cells and the height of the other epithelial cells decline, and cilia become scarcer. In the finer tubes, beginning with the respiratory bronchioles, the lining is cuboidal epithelium; in the alveoli, it is simple squamous epithelium closely associated with a dense network of capillaries. The mucous lining gradually thins, until none appears in the alveoli.

A flexible optical instrument called a fiberoptic bronchoscope is used to examine the trachea and bronchial tree. This procedure (bronchoscopy) is used in diagnosing tumors or other pulmonary diseases, and to locate and remove aspirated foreign bodies in the air passages.

Functions of the Respiratory Tubes and Alveoli

The branches of the bronchial tree are air passages, which continue to filter incoming air and distribute it to the alveoli in all parts of the lungs. The alveoli, in turn, provide a large surface area of thin epithelial cells through which gas exchanges can occur (fig. 19.16). If the

Figure

Oxygen diffuses from the air within the alveolus into the capillary, whereas carbon dioxide diffuses from the blood within the capillary into the alveolus.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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