Objectives
- 1.
Introduce the major functions of respiration.
- 2.
Describe the components of the upper and lower airways.
- 3.
Outline and briefly describe the components of the respiratory system including:
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The conducting airways
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The alveolar–capillary unit
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The alveolar surface
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The pulmonary circulation
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The cells of the airway
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The muscles of respiration
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The central nervous system and neural pathways regulating respiration
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- 4.
Relate lung structure to lung function.
The principal function of the respiratory system is to bring oxygen from the external environment to the tissues in the body and to remove from the body the carbon dioxide produced by cell metabolism. In addition, respiration functions in acid–base balance (see Chapter 9 ), in host defense, in metabolism, and in the handling of bioactive materials (see Chapter 11 ).
The respiratory system is composed of the lungs; the upper and lower airways, including the nose; the chest wall, including the muscles of respiration (diaphragm, intercostal muscles, and abdominal muscles) and the rib cage; the pulmonary circulation; and those parts of the central nervous system that regulate respiration ( Fig. 1.1 ).
Basic Structure of The Respiratory System
The airways are divided into upper and lower airways. The upper airway consists of all structures from the nose to the vocal cords, whereas the lower airway consists of the trachea and the bronchial structures to the alveolus.
Air flows to the lower airways through either the mouth or the nose. Nasal breathing is the preferred route for two reasons: first, the nose filters particulate matter and plays a major role in lung defense (see Chapter 11 ); second, the nose humidifies inspired air as a result of the large surface area created by the nasal septum and the nasal turbinates. The nose also offers a higher resistance to airflow than the mouth, however, and this resistance is increased in the presence of nasal congestion, large adenoids, or nasal polyps. Increasing airflow as occurs during exercise results in increasing resistance in the nose, with a switch from nasal to mouth breathing during exercise around inspiratory flow rates of 35 L/min.
The tracheobronchial tree is an arrangement of branching tubes that begins at the larynx and ends in the alveoli. The trachea begins at the larynx and in the tracheobronchial tree nomenclature has been designated Generation 0. The trachea divides at the carina, or “keel” (so named because it looks like the keel of a boat), into the right and left main-stem bronchi (Generation 1) that penetrate the lung parenchyma (tissue of the lung). The right main-stem bronchus is larger than the left, and the angle of the takeoff is less acute. This has implications for aspiration of foreign bodies, which most often enter the right rather than the left main-stem bronchus. Main-stem bronchi branch into lobar bronchi (three on the right and two on the left) (Generation 2) that in turn branch into segmental bronchi (Generation 3) and an extensive system of subsegmental and smaller bronchi. As a rough rule, in the first six airway generations, the number of airways in each generation is double that in the previous generation and the number of airways in each generation is equal to the number 2 raised to the generation number. Airway branching beyond the sixth generation is asymmetric in branching angle, size, number of branches, and number of subsequent generations. As a result, although in general there are between 15 and 20 generations of airways from the trachea to the level of the terminal bronchioles, there can be as few as 10 or as many as 20 generations ( Fig. 1.2 ). With each airway generation, the airways become smaller and more numerous ( Fig. 1.3 ) as they penetrate deeper into the lung parenchyma.
Both the right and the left lung are encased by two membranes—the visceral pleura and the parietal pleura . The visceral pleural membrane completely envelops the lung except at the hilum where the bronchus, pulmonary vessels, and nerves enter the lung parenchyma. The parietal pleural membrane lines the inner surface of the chest wall, mediastinum, and diaphragm and becomes continuous with the visceral pleura at the hilum. Under normal conditions, the space between the two pleuras contains a small amount of clear, serous fluid that is produced by filtration from the parietal pleural capillaries and is resorbed by the visceral pleural capillaries. This fluid facilitates the smooth gliding of the lung as it expands in the chest and creates a potential space that can be involved in disease. Air can enter this potential space between the visceral and parietal pleuras because of trauma, rupture of a weakened area at the surface of the lung, or surgery producing a pneumothorax . Fluid can also enter this space, creating a pleural effusion . Because the pleuras of the right and left lung are separate, a pneumothorax involves only the right or the left hemithorax.
Structurally, the trachea is supported by C-shaped (sometimes referred to as U-shaped) cartilage anteriorly and laterally that prevents tracheal collapse and by smooth muscle posteriorly, which can invaginate and markedly decrease the cross-sectional area of the trachea. Like the trachea, cartilage in large bronchi is also semicircular, but as the bronchi enter the lung parenchyma, the cartilage rings disappear and are replaced by plates of cartilage. As the airways further divide, these plates of cartilage decrease in size and eventually disappear around the 11th airway generation. Airways beyond the 11th generation are imbedded in the lung parenchyma, and the caliber of their lumen is regulated by the elastic recoil of the lung and lung volume. In addition, the number of bronchioles increases beyond the 11th generation more rapidly than the diameter decreases. As a result the cross-sectional area increases rapidly at this point and is 30 times the cross-sectional area of the main-stem bronchi. This results in a marked decrease in airway resistance to approximately one-tenth of the resistance of the entire respiratory system (see Chapter 3 ).
The airways can thus be divided into two types: cartilaginous airways, or bronchi; and noncartilaginous airways, or bronchioles ( Table 1.1 ). Bronchi contain cartilage and are the conductors of air between the external environment and the distal sites of gas exchange. They do not participate in gas exchange. Bronchioles do not contain cartilage and are subdivided into terminal bronchioles, which do not participate in gas exchange; and respiratory bronchioles, which contain alveoli and alveolar ducts and function as sites of gas exchange.
| Cartilage Present | Size | Epithelium | Blood Supply | Alveoli | Volume | |
|---|---|---|---|---|---|---|
| Bronchi | Yes | >1 mm | Pseudostratified columnar | Bronchial | No | ∼675 mL |
| Terminal bronchioles | No | <1 mm | Cuboidal | Bronchial | No | >150 mL |
| Respiratory bronchioles | No | <1 mm | Cuboidal/alveolar | Pulmonary | Yes | 2500 mL |
The airways from the nose to and including the terminal bronchioles are known as the conducting airways because they bring (conduct) gas to the gas-exchanging units but do not actually participate in gas exchange. The conducting airways (primarily the nose) also function to warm and humidify inspired air. Because the conducting airways contain no alveoli and therefore take no part in gas exchange, they constitute the anatomic dead space (see Chapter 5 ). In normal individuals, the first 16 generations of airway branchings, with a volume of 150 mL, constitute the anatomic dead space, whereas the next 7 generations contain an increasing number of alveoli and constitute the gas exchange unit.
Alveolar–Capillary Unit
The terminal bronchioles divide into respiratory bronchioles, which contain alveolar ducts and alveoli and constitute the last three to five generations of the respiratory system. Gas exchange occurs in the alveoli through a dense meshlike network of capillaries and alveoli called the alveolar–capillary network ( Fig. 1.4 ). The alveolar–capillary unit consists of the respiratory bronchioles, the alveolar ducts, the alveoli, and the pulmonary capillary bed. It is the basic physiologic unit of the lung and is characterized by a large surface area and a blood supply that originates from the pulmonary arteries. In the adult, there are approximately 300 million alveoli, which are 250 μm in size and are entirely surrounded by capillaries. In addition, there are 280 billion capillaries in the lung or almost 1000 capillaries for each alveolus. The result is a large surface area for gas exchange—approximately 50 to 100 m 2 , which occurs in a space that is only 5 mm in length. It is one of the most remarkable engineering feats in the body. The portion of the lung supplied by respiratory bronchioles is called an acinus . Each acinus contains in excess of 10,000 alveoli; gas movement in the acinus is by diffusion rather than tidal ventilation.
The barrier between the gas in the alveoli and the red blood cells is only 1 to 2 μm in thickness and consists of type I alveolar epithelial cells, capillary endothelial cells, and their respective basement membranes ( Fig. 1.5 ). O 2 diffuses across this barrier into plasma and red blood cells, whereas the reverse occurs for CO 2 (see Chapter 8 ). Red blood cells pass through the pulmonary network in less than 1 second, which is sufficient time for CO 2 and O 2 gas exchange to occur.
In some regions of the alveolar wall there is nothing between the airway epithelial cells and the capillary endothelial cells other than their fused basement membranes. In other regions there is a space between the epithelial and endothelial cells called the interstitial space or interstitium (see Fig. 1.5 ). The interstitium is composed of collagen, elastin, proteoglycans, a variety of macromolecules involved with cell–cell and cell–matrix interactions, some nerve endings, and some fibroblast-like cells. The alveolar septum creates a fiber scaffold through which pulmonary capillaries are threaded and is supported by the basement membrane. There are also small numbers of lymphocytes that have migrated out of the circulation in the interstitium and capillary endothelial cells. The basement membrane is capable of withstanding high transmural pressures and sometimes is the only remaining separation between blood and gas.
Alveolar Surface
The alveolar epithelium is a continuous layer of tissue composed primarily of type I cells or squamous pneumocytes. These cells have broad, thin extensions that cover approximately 93% of the alveolar surface ( Fig. 1.6 ). They are highly differentiated cells that do not divide, which makes them particularly susceptible to injury from inhaled or aspirated toxins and from high concentrations of oxygen (see Chapter 11 ). They are joined into a continuous sheet by tight junctions that prevent large molecules such as albumin from entering the alveoli, resulting in pulmonary edema. The thin cytoplasm of the type I cell is ideal for optimal gas diffusion.
Type II cells , or granular pneumocytes, are more numerous than type I cells; however, because of their cuboidal shape, they occupy only approximately 7% of the alveolar surface and are located in the corners of the alveolus (see Fig. 1.6 ). The hallmarks of the type II cell are their microvilli and their osmiophilic lamellar inclusion bodies that contain surfactant , a compound with a high lipid content that acts as a detergent to reduce the surface tension of the alveoli ( Fig. 1.7 ; also see Chapter 2 ). The type II cell is the progenitor cell of the alveolar epithelium. When there is injury to the type I cell, the type II cell multiplies and eventually differentiates into a type I cell. In a group of diseases that result in pulmonary fibrosis, the type I cell is injured and the alveolar epithelium is now lined entirely by type II cells, a condition that is not conducive to optimal gas exchange. This repair system is an example of phylogeny recapitulating ontogeny , because the epithelium of the alveolus is composed entirely of type II cells until late in gestation.
The lumen of the alveolus is covered by a thin layer of fluid composed of a water phase immediately adjacent to the alveolar epithelial cell and covered by surfactant. Within the alveolar epithelium there are also a small number of macrophages , a type of phagocytic cell that patrols the alveolar surface and ingests (phagocytizes) bacteria and inhaled particles (see Chapter 11 ).
Pulmonary Circulation
The lung has two separate blood supplies (see Chapter 6 ). The pulmonary circulation brings deoxygenated blood from the right ventricle to the gas-exchanging units (alveoli). Pulmonary perfusion (Q̇) refers to pulmonary blood flow, which equals the heart rate multiplied by the right ventricular stroke volume. The lungs receive the entire right ventricular cardiac output and are the only organ in the body that functions in this manner. The bronchial (or lesser) circulation arises from the aorta and provides nourishment to the lung parenchyma. The dual circulation to the lung is another of the unique features of the lung.
The pulmonary capillary bed is the largest vascular bed in the body, with a surface area of 70 to 80 m 2 . It is best viewed as a sheet of blood interrupted by small vertical supporting posts ( Fig. 1.8 ). When the capillaries are filled with blood, about 75% of the surface area of the alveoli overlies the red blood cells. The capillaries allow red blood cells to flow through in single file only; this greatly facilitates gas exchange between the alveoli and the red blood cells. Once gas exchange is complete, the oxygenated blood returns to the left side of the heart through pulmonary venules and veins and is ready for pumping to the systemic circulation. In contrast to the systemic circulation, the pulmonary circulation is a highly distensible, low-pressure system capable of accommodating large volumes of blood at low pressure. This is another unique feature of the lung.
Pulmonary arteries that contain deoxygenated blood follow the bronchi in connective tissue sheaths, whereas pulmonary veins cross segments on their way to the left atrium ( Fig. 1.9 ). Bronchial arteries also follow the bronchi and divide with them. In contrast, one-third of the blood from the bronchial veins (deoxygenated blood) drains into the right atrium, and the remainder drains into pulmonary veins that drain into the left atrium. Thus a small amount of deoxygenated blood that has nourished the lung parenchyma mixes with oxygenated blood in the left atrium. Pulmonary capillaries, on the other hand, are not confined to a single alveolus but pass from one to another as well as to adjacent alveolar septae before emptying into a venule. This improves the efficiency of gas exchange and minimizes the effect of alveolar disease on gas exchange.
