Respiratory System: Conducting Portion & Nasal Cavity

Overview of the Respiratory System

• Consists of lungs and a series of airways connecting them to the external environment.
• Has a functional division with two key parts: conducting and respiratory.

Conducting Portion

• Part of the respiratory system responsible for delivering air to the lungs.
• Includes the nose, the nasopharynx, the larynx, the trachea, and the bronchi.
• Functions include warming, moistening, filtering air, and phonation.

Nasal Cavity

• Divided into left and right cavities by the nasal septum.
• Associated with paranasal sinuses.
• Nares (nostrils) are the outer portions, lined by thin skin, opening into the vestibule.
• The vestibule has a nonkeratinized epithelial lining.
• The lining transitions to respiratory epithelium posteriorly.
• Vibrissae (thick, short hairs) are located in the vestibule and filter large particles from inspired air.
• Contains a richly vascularized lamina propria and seromucous glands.
• Houses three bony shelves: superior, middle, and inferior conchae, which function as turbinates.

Respiratory Mucosa Cell Types

• Ciliated cells are tall columnar cells with cilia projecting into the mucus.
• Goblet cells synthesize and secrete mucus.
• Brush cells are cells with short, blunt microvilli.
• Small granule cells (Kulchitsky cells) resemble basal cells and contain secretory granules; enteroendocrine cells of the APUD system.
• Basal cells are stem cells that give rise to the other cell types.

Olfactory Epithelium

• Composed of olfactory receptor cells, supporting cells (sustentacular cells), basal cells, and brush cells.
• Olfactory receptor cells are bipolar neurons spanning the epithelium and entering the central nerve system.
• Supporting (sustentacular) cells are columnar, similar to neuroglia, providing mechanical and metabolic support, synthesizing odorant-binding proteins.
• Basal cells give rise to olfactory receptor cells and supporting cells.
• Bowman glands (serous glands) release watery secretions via narrow ducts onto the olfactory epithelial surface.
• Odorous substances are detected by the olfactory cilia when dissolved in this material.
• Secretions flush the surface, preparing receptors for new odorous stimuli.

Pharynx

• Connects the nasal and oral cavities to the larynx and the esophagus.
• Acts as a passageway for both air and food.
• Functions as a resonating chamber during speech.
• Located posterior to the nasal and oral cavities.
• Divided into the nasopharynx and the oropharynx.
• Auditory (Eustachian) tubes connect the nasopharynx to the middle ear.
• Diffuse lymphatic tissue and lymphatic nodules in the nasopharynx wall.
• Lymphatic nodules (pharyngeal tonsil) are present at the junction between the superior and posterior walls.

Larynx

• Serves as an air passageway between the oropharynx and trachea.
• Acts as a conduit for air and sound production.
• Vocal folds regulate airflow and vibrate to create sound.
• Vocal folds form the lateral boundaries of the rima glottidis (laryngeal opening).
• Vocal folds contain a supporting ligament and skeletal muscle (vocalis muscle).
• Expelled air passing through the rima glottidis causes vocal folds to vibrate, producing sound.
• Ventricular folds located above the vocal folds are the false vocal cords.
• False vocal folds do not modulate sound due to a lack of skeletal muscle.
• Larynx is lined with stratified squamous and ciliated pseudostratified columnar epithelium.

Trachea

• Is a short, flexible air tube approximately 2.5 cm in diameter and 10 cm long.
• Functions as a conduit for air and air conditioning.
• Held open by cartilaginous rings.
• There are around 16 to 20 cartilages.
• Has four coats.
• Tracheal cartilages and the trachealis muscle separate the submucosa from the adventitia.
• Hyaline cartilage may be replaced by bone tissue with age, reducing flexibility.
• Tracheal epithelium cell types include ciliated cells, mucous cells, brush cells, small granule cells (Kulchitsky cells), enteroendocrine cells like those producing catecholamine, serotonin, calcitonin, gastrin-releasing peptide (bombesin) and basal cells

Bronchi

• Primary bronchi include 2, left and right.
• Secondary bronchi include 2 on the left and 3 on the right.
• Tertiary (segmental) bronchi include 8 on the left and 10 on the right.
• A segmental bronchus and the lung parenchyma it supplies constitute a bronchopulmonary segment.
• Bronchi have five coats.
• Are identified by cartilage plates and a circular layer of smooth muscle.
• Smooth muscle layer increases, as cartilage diminishes.
• Smooth muscle may appear discontinuous in smaller bronchi.
• The mucosa is respiratory epithelium.
• Cell height decreases with decreasing diameter.
• Lamina propria is similar to the trachea but reduced in amount.
• Muscularis is continuous, a layer of smooth muscle.
• Muscle contraction regulates airway diameter.
• Submucosa is relatively loose connective tissue.
• Larger bronchi contain glands and adipose tissue in the submucosa.
• Cartilage layer has discontinuous cartilage plates.
• Cartilage plates become smaller as bronchial diameter diminishes.
• Adventitia is moderately dense connective tissue.
• Connective tissue is continuous with adjacent structures.

Bronchioles

• Bronchopulmonary segments subdivide further into pulmonary lobules.
• Each lobule is supplied by a bronchiole.
• Delicate connective tissue septa partially separate adjacent lobules and are visible on the lung surface as outlined polygonal areas.
• Pulmonary acini are smaller structural units within the lobules.
• Each acinus consists of a terminal bronchiole, and the respiratory bronchioles and alveoli it aerates.
• The respiratory bronchiolar unit is the smallest functional unit and consists of a single respiratory bronchiole plus associated alveoli.
• Bronchioles lack both mucosal glands and cartilage.
• Epithelium is ciliated pseudostratified columnar in larger bronchioles.
• Epithelium decreases in height and complexity; simple columnar ciliated or simple cuboidal epithelium in terminal bronchioles.
• Terminal bronchioles are the last parts of the air conducting system.
• Ciliated cells in the bronchiole facilitate the mucociliary apparatus or escalator.
• Cuboidal cells include Clara cells, which are nonciliated cuboidal cells.
• Clara cells have secretory granules.
• Clara cells functions include secretion of surfactant; detoxification of inhaled compounds via SER enzymes; secretion of antimicrobial peptides and cytokines, and injury-induced mitosis.

Respiratory Bronchioles

• Each terminal bronchiole divides into two or more respiratory bronchioles with saclike alveoli.
• Mucosa lining consists of Clara cells and ciliated cuboidal cells.
• Simple squamous cells line the alveolar openings.

Alveolar Ducts

• The distal ends of respiratory bronchioles branch into alveolar ducts.
• Alveolar ducts and alveoli are lined by attenuated squamous cells.
• The smooth muscle cells surround each alveolar opening.
• Clusters of alveoli, known as alveolar sacs, form the ends of the alveolar ducts distally.

Alveoli

• Saclike evaginations that are approximately 200 μm in diameter.
• Responsible for the spongy nature of the lungs.
• Each adult lung has approximately 200 million alveoli.
• Total internal surface area is 75 m2.
• Air in these structures exchanges O2 and CO2 with blood in surrounding capillaries.
• Alveolar walls enhance diffusion.
• Thin interalveolar septa lie between alveoli.
• Interalveolar septa consist of scattered fibroblasts and sparse extracellular matrix (ECM), including elastic and reticular fibers.

Blood-Air Barrier

• Air in the alveoli separated from from capillary blood.
• Interalveolar septa are vascularized with capillary networks.
• Made up of highly-attenuated thin cells lining the alveolus, fused basal laminae of their epithelium and the capillary endothelial cells, and thin endothelial cells of the capillary.

Alveolar Epithelium Cell Types

• Composed of type I and II alveolar cells and occasional brush cells.
• Type I alveolar cells (type I pneumocytes) comprise 40% of the alveolar lining cells and cover 95% of the surface.
• Type I cells are incapable of cell division.
• Joined with occluding junctions.
• Type II alveolar cells (type II pneumocytes or septal cells) are secretory cuboidal cells interspersed among type I cells.
• Type II cells account for 60% of the alveolar lining cells but cover only 5% of the air surface.
• Brush cells are present in the alveolar wall, are few in number and may serve as receptors for monitoring air quality.