RESPIRATORY SYSTEM

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13 RESPIRATORY SYSTEM

General outline of the respiratory system

The respiratory system consists of three main portions with distinct functions:

1. An air–conducting portion.

2. A respiratory portion for gas exchange between blood and air.

3. A mechanism for ventilation, driven by the inspiratory and expiratory movements of the thoracic cage.

The air-conducting portion consists, sequentially, of the nasal cavities and associated sinuses, the nasopharynx, the oropharynx, the larynx, the trachea, the bronchi, and the bronchioles. The oropharynx also participates in food transport. The conducting portion provides a passage for inhaled and exhaled air in and out of the respiratory portion.

The respiratory portion is composed, in sequence, of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The main function is the exchange of gases between air and blood.

Respiration involves the participation of a ventilation mechanism. The inflow (inspiration) and outflow (expiration) of air occur with the aid of four elements:

1. The thoracic or rib cage.

2. Associated intercostal muscles.

3. The diaphragm muscle.

4. The elastic connective tissue of the lungs.

NASAL CAVITIES AND PARANASAL SINUSES

The nasal cavities and paranasal sinuses provide an extensive surface area for (1) warming and moistening air and (2) filtering dust particles present in the inspired air. In addition, the roof of each nasal cavity and part of the superior concha contain the specialized olfactory mucosa.

Each nasal cavity, separated from the other by the septum, consists of the vestibule, the respiratory portion, and the olfactory area (Figure 13-1).

Figure 13-1 Nasal cavities

Air enters through the nostril, or naris, whose external surface is lined by keratinized squamous epithelium. At the vestibule, the epithelium becomes nonkeratinized.

The respiratory portion is lined by a pseudostratified ciliated epithelium with goblet cells supported by the lamina propria, which consists of connective tissue with seromucous glands. The lamina propria has a rich superficial venous plexus, known as cavernous or erectile tissue. The lamina propria is continuous with the periosteum or perichondrium of bone or cartilage, respectively, forming the wall of the nasal cavities.

Projecting into each nasal cavity from the lateral wall are three curved plates of bone covered by a mucosa: the superior, middle, and inferior turbinate bones, or conchae (Latin concha, shell).

Secretions from goblet cells and seromucous glands maintain the mucosal surface moist and humidify the inspired air. Incoming air is warmed by blood in the venous plexus, which flows in a direction opposite to that of the inspired air (countercurrent flow). The highly vascular nature of the nasal mucosa, in particular of the anterior septum, accounts for common bleeding (epistaxis) after trauma or acute inflammation (rhinitis).

Conchae cause airflow turbulence, thus facilitating contact between the air and the mucus blanket covering the respiratory region of each nasal cavity. The mucus blanket traps particulates in the air that are transported posteriorly by ciliary action to the nasopharynx, where they are swallowed with the saliva.

Paranasal sinuses are air-containing cavities within the bones of the skull. They are the maxillary, frontal, ethmoidal, and sphenoid sinuses. The sinuses are lined by a thin pseudostratified columnar ciliated epithelium, with fewer goblet cells and glands in the lamina propria. No erectile tissue is present in the paranasal sinuses. Sinuses communicate with the nasal cavity by openings lined by an epithelium similar to that of the main nasal cavity. The ethmoidal sinuses open beneath the superior conchae and the maxillary sinus opens under the middle concha.

Nasopharynx

The posterior portion of the nasal cavities is the nasopharynx, which at the level of the soft palate becomes the oropharynx.

The auditory tubes (eustachian tubes), extending from the middle ear, open into the lateral walls of the nasopharynx.

The nasopharynx is lined by a pseudostratified columnar epithelium like the nasal cavities, and changes into nonkeratinizing squamous epithelium at the oropharynx. Abundant mucosa-associated lymphoid tissue is present beneath the nasopharyngeal epithelium, forming Waldeyer’s ring. The nasopharyngeal tonsils (adenoids) are present at the posterior and upper regions of the nasopharynx.

Olfactory epithelium

The olfactory epithelium contains three major types of cells (Figures 13-2 and 13-3): (1) basal cells, (2) olfactory cells (bipolar neurons), and (3) supporting or sustentacular cells.

Figure 13-2 Olfactory mucosa

Figure 13-3 Olfactory epithelium

The basal cells are mitotically active stem cells, producing daughter cells that differentiate first into immature olfactory cells and then into mature olfactory cells. Olfactory cells proliferate during adult life. The life span of an olfactory cell is about 30 to 60 days.

The olfactory cell is highly polarized (see Figure 13-3). The apical region, facing the surface of the mucosa, forms a knoblike ending (called olfactory vesicle or olfactory knob) with 10 to 20 modified cilia. The basal region gives rise to an axon. Several axons, projecting from the olfactory cells, form small unmyelinated bundles (called olfactory fila; from Latin filum, thread) surounded by glial-like cells. Nerve bundles cross the cribriform plate of the ethmoid bone and contact in the glomerulus dendrites of mitral cells, neurons of the olfactory bulb, to establish appropriate synaptic connections (see Box 13-A).

Box 13-A Olfactory epithelium

• The olfactory epithelium consists of olfactory cells (bipolar neurons), basal cells (a stem cell that differentiates into olfactory cells), and sustentacular or supporting cells. These cells can be identifed on the basis of the position and shape of their nuclei (see Figure 13-3).

• An olfactory cell has two portions: an apical dendrite with a knob bearing about 10 to 20 olfactory nonmotile modified cilia and a basal axon, forming bundles that will pass through the cribriform plate of the ethmoid bone.

• Cilia contain the odorant receptor (OR). There are about 1000 genes expressing ORs, but each olfactory receptor cell expresses only one OR gene.

• Secretions of the serous glands of Bowman contain odorant-binding protein.

• Axons from olfactory cells with the same OR terminate in one to three glomeruli present in the olfactory bulb. Dendritic endings of predominantly mitral cells extend into the glomeruli. Axons of mitral cells form the olfactory tract.

• Olfactory receptor cells have a life span of 30 to 60 days and can regenerate from basal cells.

• Temporary or permanent damage to the olfactory epithelium causes anosmia (Greek an, not; osme, sense of smell).

Olfactory serous glands (called glands of Bowman), which are present under the epithelium, secrete a serous fluid in which odoriferous substances are dissolved. The secretory fluid contains the odorant-binding protein (OBP) with high binding affinity for a large number of odorant molecules. OBP carries odorants to receptors present on the surface of the modified cilia and removes them after they have been sensed. In addition, the secretory product of the glands of Bowman contains protective substances such as lysozyme and immunoglobulin A (IgA) secreted by plasma cells.

LARYNX

The two main functions of the larynx are (1) to produce sound and (2) to close the trachea during swallowing to prevent food and saliva from entering the airway.

The wall of the larynx is made up of the thyroid and cricoid hyaline cartilage and the elastic cartilage core of the epiglottis extending over the lumen (Figure 13-4).

Figure 13-4 Structure of the larynx

Extrinsic laryngeal muscles attach the larynx to the hyoid bone to raise the larynx during swallowing.

Intrinsic laryngeal muscles (abductor, adductors, and tensors), innervated by the recurrent laryngeal nerve, link the thyroid and cricoid cartilages. When intrinsic muscles contract, the tension on the vocal cords changes to modulate phonation. The middle and lower laryngeal arteries (derived from the superior and inferior thyroid artery) supply the larynx. Lymphatic plexuses drain to the upper cervical lymph nodes and to the nodes along the trachea.

The larynx can be subdivided into three regions:

1. The supraglottis, which includes the epiglottis, false vocal cords (or folds), and laryngeal ventricles.

2. The glottis, consisting of the true vocal cords (or folds) and the anterior and posterior commissures.

3. The subglottis, the region below the true vocal cords, extending down to the lower border of the cricoid cartilage.

During forced inspiration, vocal cords are abducted, and the space between the vocal cords widens.

During phonation, the vocal cords are adducted and the space between the vocal cords changes into a linear slit. The vibration of the free edges of the cords (a cover consisting of both the stratified squamous epithelial covering and the superficial layer of the lamina propria, known as Reinke’s space) during passage of air between them produces sound. The contraction of the intrinsic muscles of the larynx, forming the body of the cords, increases tension on the vocal cords, changing the pitch of the produced sound (see Box 13-B).

Box 13-B True vocal cords or folds

• The true vocal cords or folds consist of two regions—the cover and the core—with different structural properties.

• The cover consists of stratified squamous epithelium and the superficial layer of the lamina propria (the space of Reinke). The core is composed of the intermediate and deep layers of the lamina propria (representing the vocal ligament) and the vocalis or thyroarytenoid muscle.

The cover is flexible, whereas the core is stiff and has contractile properties allowing adjustment of stiffness.

• During phonation, the cover of the vocal cords displays horizontal movements and vertical undulation (known as mucosal wave). Changes in the stiffness in the core of the cords modify the mucosal wave. As the stiffness of the cord increases, the velocity of the mucosal wave increases and the pitch rises.

The mucosa of the larynx is continuous with that of the pharynx and the trachea. A stratified squamous epithelium covers the lingual surface and a small extension of the pharyngeal surface of the epiglottis and the true vocal cords. Elsewhere, the epithelium is pseudostratified ciliated, with goblet cells.

Laryngeal seromucous glands are found throughout the lamina propria, except at the level of the true vocal cords. The lamina propria of the true vocal cords consists of three layers (see Figure 13-4): (1) a superficial layer containing extracellular matrix and few elastic fibers. This layer is known as Reinke’s space; (2) an intermediate layer with an increased content of elastic fibers; and (3) a deep layer with abundant elastic and collagen fibers.

Reinke’s space and the epithelial covering are responsible for vocal cord vibration. Reinke’s edema results when viral infection, trauma (laryngeal endoscopy), or severe coughing spells cause fluid to accumulate in the superficial layer of the lamina propria. Both the intermediate and deep layer of the lamina propria constitute the vocal ligament.

The lamina propria is usually rich in mast cells. Mast cells participate in hypersensitivity reactions leading to edema and laryngeal obstruction, a potential medical emergency. Croup designates a laryngotracheobronchitis in children, in which an inflammatory process narrows the airway and produces inspiratory stridor.

TRACHEA

The trachea, the major segment of the conducting region of the respiratory system, is the continuation of the larynx.

The trachea branches to form the right and left primary bronchi entering the hilum of each lung. The hilum is the region where the primary bronchus, pulmonary artery, pulmonary vein, nerves, and lymphatics enter and leave the lung. Secondary divisions of the bronchi and accompanying connective tissue septa divide each lung into lobes.

The right lung has three lobes, whereas the left lung has two lobes.

Subsequent bronchial divisions further subdivide each lobe into bronchopulmonary segments. The bronchopulmonary segment is the gross anatomic unit of the lung that can be removed surgically. Successive bronchial branching gives rise to several generations of bronchopulmonary subsegments.

The trachea and main bronchi are lined by pseudostratified columnar ciliated epithelium resting on a distinct basal lamina. Several types of cells can be identified (Figure 13-5):

1. Columnar ciliated cells are the predominant cell population, extending from the lumen to the basal lamina.

2. Goblet cells are abundant nonciliated cells, also in contact with the lumen and the basal lamina. They produce mucin polymers MUC5AC and MUC5B (see Figure 13-5).

3. Basal cells rest on the basal lamina but do not extend to the lumen.

4. Cells of Kulchitsky are neuroendocrine cells also resting on the basal lamina and are predominantly found at the bifurcation of lobar bronchi. They give rise to bronchial carcinoid tumors within the bronchial mucosa. These cells secrete peptide hormones such as serotonin, calcitonin, antidiuretic hormone (ADH), and adrenocorticotropic hormone (ACTH).

Figure 13-5 Structure of the trachea

The lamina propria contains elastic fibers. The submucosa displays mucous and serous glands that, together with goblet cells, produce components of the airways mucus (see Box 13-C).

Box 13-C Airway mucus

• Airway mucus traps inhaled particles and transports them out of the lungs by ciliary beating and cough. Excessive mucus or deficient clearance are characteristics of all common airway diseases.

• Airway mucus is produced by three secretory cell types: (1) Goblet cells; (2) Clara cells of the terminal bronchioles; and (3) serous cells of the submucosal glands.

• Mucus contains: (1) Mucins MUC5AC and MUC5B; (2) antimicrobial molecules (defensins, lysozyme, and immunoglobulin A); (3) immunomodulatory molecules (secretoglobin and cytokines); and (4) protective molecules (trefoil proteins and heregulin.

• Normal airway mucus is 97% water and 3% solids (mucins, non-mucin proteins, salts, lipids, and cellular debris). The hydration of the mucus determines its viscosity and elastic properties, two essential characteristics for normal clearance of mucus by ciliary action and cough.

• Airway mucus consists of two layers: (1) A periciliary layer; and (2) a mucus gel layer atop the periciliary layer. Polymeric MUC5AC and MUC5B are continuously synthesized and secreted to replenish the mucus gel layer cleared by ciliary beating to eliminate inhaled particles, pathogens, and dissolved chemicals that might damage the lungs.

The framework of the trachea and extrapulmonary bronchi consists of a stack of C-shaped hyaline cartilages, each surrounded by a fibroelastic layer blending with the perichondrium. In the trachea and primary bronchi, the open ends of the cartilage rings point posteriorly to the esophagus. The lowest tracheal cartilage is the carinal cartilage. Transverse fibers of the trachealis muscle attach to the inner ends of the cartilage. In branching bronchi, cartilage rings (see Figure 13-5) are replaced by irregularly shaped cartilage plates (Figure 13-6), surrounded by smooth muscle bundles in a spiral arrangement.

Figure 13-6 Segmentation of the intrapulmonary bronchial tree

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