23.1: Structure and Function
By the end of this section, you will be able to:
- Identify the structures of the thorax
- Recognize the mechanism of breathing
- Describe the differences in lung volumes or capacities
The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove carbon dioxide as a waste product, and help maintain acid-base balance (Figure 23.2). Portions of the respiratory system are also used for nonvital functions, such as sensing odors, speech production, and for straining, such as during childbirth or coughing.
Structures of the Thorax
The thorax , commonly known as the chest, begins at the neck and extends to the top of the abdomen. The two major parts of the thorax include the thoracic cage and the thoracic cavity.
Thoracic Cage
The thoracic cage , also called the rib cage, forms the thorax (chest) portion of the body (Figure 23.3). It consists of the twelve pairs of ribs with their costal cartilages and the sternum . The ribs are anchored posteriorly to the twelve thoracic vertebrae (T1–T12). The main function of the thoracic cage is to protect the heart and lungs from injury.
Sternum and Clavicles
The sternum is the elongated bony structure that anchors the anterior thoracic cage. It consists of three parts: the manubrium, the body, and the xiphoid process (Figure 23.3). The manubrium is the wider, superior portion of the sternum. The top of the manubrium has a shallow, U-shaped border called the jugular (suprasternal) notch . This can be easily felt at the anterior base of the neck, between the medial ends of the clavicles. The clavicular notch is the shallow depression located on either side at the superior-lateral margins of the manubrium. This is the site of the sternoclavicular joint , between the sternum and clavicle. The first ribs also attach to the manubrium.
The elongated, central portion of the sternum is the body. The manubrium and body join at the sternal angle, so named because the junction between these two components forms a slight bend. The second rib attaches to the sternum at the sternal angle. Because the first rib is hidden behind the clavicle, the second rib is the highest rib that can be identified by palpation. Thus, the sternal angle and second rib are important landmarks for the identification and counting of the lower ribs. Ribs 3 to 7 are attached to the sternal body. The inferior tip of the sternum is the xiphoid process . This small structure is cartilaginous early in life, but gradually becomes ossified (turns into bone) starting during middle age.
Ribs and Thoracic Vertebrae
Each rib is a curved, flattened bone that contributes to the wall of the thorax . The ribs articulate posteriorly with the T1 to T12 thoracic vertebrae, and most attach anteriorly via their costal cartilages to the sternum. There are twelve pairs of ribs that are numbered 1 to 12 in accordance with the thoracic vertebrae. Thoracic vertebrae have articulation sites, called facets, where the rib is attached. Most thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet ( costal means “rib”). These are for articulation with the head (end) of a rib. An additional facet is located on the transverse process for articulation with the tubercle of a rib.
Reference Lines
Imaginary “reference” lines can be visualized on the thorax to assist with performing the physical examination and determining the anatomical layout of the chest. The three reference lines are the midclavicular line, the anterior axillary line, and the midaxillary line (Figure 23.4). The midclavicular line is used most often and runs from the middle of the clavicle down the chest. This line can be used to help find the apex of the heart and various intercostal spaces during the physical assessment.
Thoracic Cavity
The thoracic cavity is the large compartment in the chest that contains many vital organs and tissues including the heart and lungs (Figure 23.5). The organs and tissues in the thoracic cavity are protected from injury by the bony structures (ribs and sternum) of the thoracic cage.
Larynx and Trachea
The larynx is a cartilaginous structure that connects the pharynx (part of throat) to the trachea and helps regulate the volume of air that enters and leaves the lungs (Figure 23.6). Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.
The trachea (windpipe) extends from the larynx toward the lungs. The trachea is formed by sixteen to twenty stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane , a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation.
Bronchi and Bronchioles
The trachea branches into the right and left primary bronchi at the carina (Figure 23.7). The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. Rings of cartilage, like those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum , a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs. The main function of the bronchi is to provide a passageway for air to move into and out of each lung. The primary bronchi branch into secondary and tertiary bronchi, which then form bronchioles. Bronchioles, which are about 0.04 in (1 mm) in diameter, further branch until they become the tiny terminal bronchioles and lead to the structures of gas exchange , called alveoli (Figure 23.8).
Lungs and Pleural Membranes
The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi and bordered inferiorly by the diaphragm. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right. The cardiac notch is an indentation on the surface of the left lung, and it allows space for the heart (Figure 23.9). The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm. Each lung is composed of smaller units called lobes. The right lung consists of three lobes: the superior, middle, and inferior lobes. The left lung consists of two lobes: the superior and inferior lobes. Each segment of the lung receives air from its own tertiary bronchus and is supplied with blood by its own artery.
Each lung is enclosed within a cavity that is surrounded by the pleura , a double-layered serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, consist of two layers. The visceral pleura is the innermost layer, which lies superficial to the lungs (Figure 23.10). In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, mediastinum, and diaphragm. The pleural cavity is the space located between the visceral and parietal layers. The pleural cavity contains a small amount of pleural fluid that is used as lubrication to reduce friction between the layers to prevent trauma during breathing and create surface tension that helps maintain the position of the lungs against the thoracic wall.
Mechanism of Breathing
Breathing usually occurs without thought, although at times you can consciously control it, such as when you swim under water, sing a song, or blow bubbles. The respiratory rate is the total number of breaths, or respiratory cycles, that occur each minute. Respiratory rate can be an important indicator of disease, as the rate may increase or decrease during an illness or with certain medical conditions. The respiratory rate is controlled by the respiratory center located in the brain, which responds to changes in carbon dioxide, oxygen, and pH levels in the blood.
Pressure Relationships
The intra-alveolar and intrapleural pressures are dependent on certain physical features of the lung. However, the ability to breathe—to have air enter the lungs during inspiration and air leave the lungs during expiration —is dependent on the air pressure of the atmosphere and the air pressure within the lungs. Pulmonary ventilation (breathing) is dependent on three types of pressure: atmospheric, intra-alveolar, and intrapleural (Table 23.1).
| Type of Pressure | Description |
|---|---|
| Atmospheric pressure |
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| Intra-alveolar (intrapulmonary) pressure |
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| Intrapleural pressure |
Pressure of air within the pleural cavity (between visceral and parietal pleurae)
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The difference in pressures drives pulmonary ventilation because air flows down a pressure gradient; that is, air flows from an area of higher pressure to an area of lower pressure. Air flows into the lungs largely due to a difference in pressure; atmospheric pressure is greater than intra-alveolar pressure, and intra-alveolar pressure is greater than intrapleural pressure. Air flows out of the lungs during expiration based on the same principle; pressure within the lungs becomes greater than the atmospheric pressure. For example, in geographical areas of higher altitude, there is less atmospheric pressure, which affects breathing.
Pulmonary Ventilation (Inspiration and Expiration)
Pulmonary ventilation is comprised of two major steps: inspiration and expiration. Inspiration is the process that causes air to enter the lungs, and expiration is the process that causes air to leave the lungs (Figure 23.11). A respiratory cycle is one sequence of inspiration and expiration . In general, two muscle groups are used during normal inspiration: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity and more space for the lungs. Contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.
Oxygen Transport
All organs and tissues require oxygen for optimal functioning, so it is vital that oxygen-transport mechanisms work properly to maintain an adequate supply for the body. Deoxygenated, or venous, blood is brought to the right side of the heart from the veins. Once in the heart, the blood is sent to the lungs via the pulmonary artery so that gas exchange may occur within the alveoli . During gas exchange, oxygen is added to the blood and carbon dioxide is removed from the blood as a waste product (Figure 23.12). Oxygen is then transported by the blood to organs and tissues in two different forms: bound to hemoglobin in red blood cells or directly dissolved in plasma.
Respiratory Volumes and Capacities
The term used for various volumes of air moved by or associated with the lungs at a given point in the respiratory cycle is called respiratory volume . There are four major types of respiratory volumes: tidal, residual, inspiratory reserve, and expiratory reserve (Table 23.2). Respiratory volumes are dependent on a variety of factors, and measuring the different types can provide important clues about a patient’s respiratory health.
| Type of Respiratory Volume | Description |
|---|---|
| Tidal volume (TV) |
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| Expiratory reserve volume (ERV) |
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| Inspiratory reserve volume (IRV) |
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| Residual volume (RV) |
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The combination of two or more selected respiratory volumes is called respiratory capacity , which further describes the amount of air in the lungs during a given time. For example, total lung capacity (TLC) is the sum of all the lung volumes (TV, ERV, IRV, and RV), which represents the total amount of air a person can hold in the lungs after a forceful inhalation. The other types of respiratory capacities are described in Table 23.3.
| Type of Respiratory Capacity | Description |
|---|---|
| Vital capacity (VC) |
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| Inspiratory capacity (IC) |
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| Functional residual capacity (FRC) |
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