Objectives
- Identify types of epithelium starting with nasal passages and moving distally toward alveoli.
- Describe the general structure and function of organs in the respiratory system: Nasal Cavity, Epiglottis, Larynx, Trachea, Bronchi, Bronchioles, Terminal Bronchioles, Respiratory Bronchioles, Alevolar Ducts and Sacs, and Alveoli.
- Describe function and structure of cells specific to the respiratory system (type I, type II, and pulmonary macrophages)
Overview
The respiratory system can be divided into an upper and lower respiratory system. In the upper respiratory system, we will cover the nasal cavity, nasopharynx, and larynx. The lower respiratory system section will cover the trachea, lungs and tubule passages.
The functions of the respiratory system include air conduction, air filtration, and gas exchange (through respiration). A secondary function includes production of sound used to create speech as air passes through the larynx. Another function includes smell as air passes over olfactory mucosa in the nasal cavity. The respiratory system also plays a role in the immune system as it reacts to inhaled antigens.
The conduction portion of the respiratory system consists of air passages that lead to sites of respiration (gas exchange).
Conduction portion:
- Nasal cavity– Air enters the 2 large spaces separated by the nasal septum where it is cleansed, warmed or cooled, and moistened. Contains bony turbinates known as conchae and the superior portion is lined with olfactory mucosa (pseudostratified with specialized cell types) while the rest of the cavity s lined by respiratory mucosa (ciliated, pseudostratified columnar epithelium). Openings known as sinuses lie on the lateral sides, superiorly, and superoposterior to the nasal cavity which all drain into the cavity. The nasolacrimal duct also drains into the nasal cavity from the eye.
- Respiratory mucosa– pseudostratified columnar ciliated epithelium containing ciliated cells, goblet cells, brush cells (chemosensory receptors with microvilli), small granule (Kulchitsky) cells (endocrine cells), and basal cells.
- Olfactory mucosa– pseudostratified columnar epithelium containing olfactory receptor cells (bipolar neurons), supporting (sustentacular) cells (columnar cells that synthesize and secrete odorant-binding proteins), basal cells, brush cells, olfactory (Bowman’s) glands in the lamina propria.
- Nasopharynx– lies posterior to the nasal cavity and leads to the oropharynx. The auditory (Eustacian) tubes open into the nasopharynx and the space contains the pharyngeal tonsils. The nasopharynx is lined with respiratory mucosa.
- Larynx– air enters the larynx while food enters the esophagus (posterior to the larynx). The tube is shaped with irregular plates of hyaline (rings) and elastic cartilage (epiglottis and arytenoid cartilages). The larynx cartilages are controlled by laryngeal muscles (intrinsic and extrinsic) which aid in phonation and swallowing). The mucosa is folded in a way that produces two pairs of shelf-like folds- the vestibular folds (false vocal cords) and the vocal folds (true vocal cords) separated by an invagination called the ventricle. Most of the larynx is lined with respiratory epithelium while the vocal cords and most of the epiglottis are covered by stratified squamous epithelium.
- Trachea– 4 definable layers including the mucosa (respiratory epithelium resting on a thick basement membrane + elastic-rich lamina propria), submucosa (CT is more dense than lamina propria), cartilaginous layer (C-shaped hyaline cartilage) and adventitia (loose CT).
- Primary bronchi (paired)- right bronchus is wider and shorter compared to left. Divisions: Bronchi > lobar (secondary) bronchi > segmental (tertiary) bronchi > terminal bronchioles > respiratory bronchioles
- Right lung 3 lobes and therefore, 3 lobar bronchi; 10 bronchopulmonary segments and therefore, 10 segmental bronchi
- Left lung 2 lobes and therefore, 2 lobar bronchi; 8 bronchopulmonary segments and therefore, 8 segmental bronchi
- As bronchi decrease in size, cartilage plates become smaller and less numerous, cartilage disappears at the level of bronchiole.
- Mucosa (same as trachea but with height of cells decreasing with diameter and the basement membrane diminishes); Muscularis (continuous layer of smooth muscle that becomes less organized as size decreases); Submucosa (relatively loose CT with glands larger in bronchi); Adventitia (continuous with adjacent structures)
- Terminal bronchioles– ≤1mm in diameter and lead to respiratory bronchioles, no cartilage plates or glands present; ciliated pseudostratified columnar epithelium becomes simple cuboidal (some ciliated, may have a few columnar cells) with increasing number of club cells as duct narrows. Amounts of smooth muscle decrease distally.
Respiratory portion:
- Respiratory bronchioles– simple cuboidal (some ciliated) to simple squamous with increasing number of club cells. Club cells dominating distally; no cartilage or glands present
- Alveolar ducts– elongated, thin airways, simple squamous
- Alveolar sacs- spaces surrounded by clusters of alveioli
- Alveoli– site of gas exchange, surrounded by capillaries
- Type I alveolar cells- squamous cells joined by occluding junctions
- Type II alveolar cells (septal cells)- cuboidal secretory cells which bulge into the air space, produce surfactant
- Brush cells- few in number, contain microvilli, serve as chemoreceptors
Air-Blood Barrier
The interalveolar septum creates an air-blood barrier where gasses diffuse between the alveolar and capillary compartments. This consists of a thin layer of surfactant, a type I epithelial cell and its basal lamina, and a capillary endothelial cell and its basal lamina
Upper Respiratory Slides
This specimen is a frontal section showing 1 side of the nasal cavity. The nasal septum indicates the middle of the 2 halves of the nasal cavity, so this specimen is missing the other half. This is not a human specimen but the general structure is similar. In this specimen you will find hyaline cartilage of the nasal septum, compact bone from bones of the skull and folded bone that make up the chonchae, or bony turbinates. Conchae extend from the ethmoid bone and nasal bone in humans and are less complex than in this example. Look for the respiratory mucosa that lines the majority of the nasal cavity and the olfactory mucosa found in the more superior aspect (as labeled). What cells and structures distinguish the two types of mucosa?
The epiglottis is a cartilaginous flap that closes off the trachea when swallowing so food enters the esophagus and not the airway. It is attached to the thyroid cartilage anteriorly and extends posterior to the root of the tongue. The anterior-superior surface (lined in yellow in image to right) that is facing the tongue is covered by epithelium seen in the oral cavity- stratified squamous epithelium. This epithelium extends to distal portion facing the laryngopharynx. The inferior portion (lined in blue in image to right) closest to the trachea, is covered by respiratory epithelium- ciliated, pseudostratified epithelium. You will find mixed glands in the lamina propria and elastic cartilage.
This specimen is a section through part of the wall of the larynx. The large section of cartilage is the thyroid cartilage (hyaline cartilage). The vestibular fold (false cord) and vocal fold (true cord) are present with the ventricle in between the folds. Follow the epithelium along the mucosa of the larynx and you will notice the epithelium change from pseudostratified columnar, ciliated (PCC) to stratified squamous, non-keratinized over the vocal fold and back to PCC. The change in epithelium is due to the constant abrasion the vocal fold undergoes. The lamina propria of the larynx contains mixed glands which empty into the ventricle. There are also lymphoid aggregates and lymph nodules and you may see skeletal muscle which helps control the vocal folds.
Lower Respiratory Slides
The trachea extends from the larynx and sits anteriorly to the esophagus. The flexibility of this tube is due to the cartilage plates that form a c-shaped rings while also keeping the air way patent, or open. In this specimen slide, there is both a longitudinal section (left) of a portion of the trachea and and a cross-section of a portion of the trachea (right). The luminal surface of both sections can be found between each other, look for the respiratory epithelium. Within the epithelium, you should be able to identify ciliated cells, basal cells, and goblet cells. The brush cells are not very distinct in this specimen. The basement membrane is more visible in this tissue which you can find just superficial to the lamina propria. Look for the mixed glands in the connective tissue, these are compound tubuloalveolar glands. Review the histologic structure of hyaline cartilage while examining the trachea.
This specimen slide contains several structures in the neck that surround the trachea. You will find the esophagus (posterior to trachea), thyroid gland (lateral to trachea), and skeletal muscles (anterior to trachea, also in proximal esophagus). You may notice some large neurovascular bundles supplying these structures as you move through the slide. In the trachea itself, you’ll see the mucosa made up of respiratory epithelium with a thick basement membrane and thin lamina propria. You see some glands but they are not numerous in this section. You’ll see the c-shaped hyaline cartilage ring and you’ll notice a band of smooth muscle that surrounds the trachea known as the trachealis muscle that was not visible in the previous slide. Please note that some parts of the slide are a bit fuzzy.
When examining the lung tissue, look for different structures of airway. Use your knowledge of changes in epithelium and changes in presence of cartilage and smooth muscle to determine what size airway you are viewing. In this specimen, you will find branches of primary bronchi (pseudostratified, ciliated columnar epithelium with hyaline cartilage plates and smooth muscle); primary muscular bronchioles (simple, ciliated columnar epithelium, bands of smooth muscle, no cartilage); terminal bronchioles (simple, cuboidal epithelium, incomplete bands of smooth muscle, no cartilage); respiratory bronchioles (mixture of simple columnar and simple cuboidal epithelium, little smooth muscle, no cartilage). You will also find alveolar ducts leading to alveolar sacs. The cells of the alveoli are not clearly identifiable in this specimen.
In this specimen of the lung, you can find different size airways. Examine the cell types found in the smaller airways. Look for club cells in terminal and respiratory bronchioles. Look for type I and type II pneumocytes in the alveolar sacs. Also examine the lining of the lung and look for mesothelial cells. You will also notice different size vessels throughout the specimen.
More Practice…Coming Soon
