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Histology - Respiratory System
Respiratory Histology
Key structures of the respiratory system
2 Functional Divisions
The conducting portion
- Conducts air, and comprises: the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles.
- No gas exchange occurs in these structures.
- Terminal bronchiole terminates the conducting portion of the respiratory tract.
- Site of gas exchange, and comprises: the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
Nose
- Opens the respiratory system to the outside environment.
Nasal cavity
- Its mucosal lining moistens, warms, and cleans the inhaled air.
- Muscular tube that lies behind the nasal cavity, oral cavity, and larynx; it is open to them, and acts a conduit for air and food/liquid. Thus, it serves both the respiratory and digestive systems.
Esophagus
- Continues posteriorly to carry food to the stomach, included for context.
- The cartilaginous structure that prevents food and liquid from entering the lower respiratory tract, and produces and modifies sounds (and is often referred to as the "voice box").
Tracheobronchial tree
- Collective term for trachea and its bronchial branches.
Trachea (the "windpipe")
- Descends through the neck to the thorax, and comprises C-shaped (vertically-stacked) cartilaginous rings.
Primary Bronchi
- First divisions of tracheobronchial tree
- Aka, main bronchi
Secondary bronchi
- Serve lobes of the lungs
- Aka, lobar bronchi
Tertiary bronchi
- Serve lung segments, called bronchopulmonary segments.
- Aka, segmental bronchi
With each successive division, the branches get narrower and the walls of the branches get thinner.
Bronchioles
- Terminal bronchioles are last portion of conducting division.
Respiratory bronchioles
- Beginning of respiratory division
Alveolar sacs
- Comprise small out pockets called alveoli, which have specialized walls to facilitate gas exchange with surrounding pulmonary capillaries.
- The hundreds of millions of alveoli within give the lungs a light, spongy texture.
- Right lung comprises three lobes (divisions) and the left lung has only two lobes.
- The heart nestles into the medial aspect of the left lung, which makes it slightly smaller than the right.
Histological Features
Tracheal ring
- Comprises thick layer of purple-staining hyaline cartilage, which is covered by perichondrium on both sides.
- Lamina propria and submucosa; though not visible in our sample, the submucosa contains seromucous glands and blood vessels.
- Mucosal folds, which are lined with pseudostratified epithelia.
Bronchi
- Lumen is surrounded by mucosal pseudostratified epithelium.
- Submucosal glands
- Bundles of smooth muscle
- Large plates of hyaline cartilage distinguish the larger bronchi.
Smaller tertiary bronchi
- Characterized by highly fractured and thin pieces of hyaline cartilage.
Respiratory bronchioles
- Thinner walls that lack cartilage and comprise simple cuboidal epithelial cells.
- Alveolar outpockets arises directly from the respiratory bronchioles.
- Club cells (formerly known as Clara cells), are cuboidal, non-ciliated cells in the bronchioles that secrete proteins.
- Respiratory bronchiole gives rise to the alveolar ducts, which open to alveolar sacs.
Alveoli
- Type I pneumocytes (aka, alveolar cells), which are squamous epithelial cells.
- Type I cells provide a thin surface for easy gas exchange with nearby pulmonary capillaries, which we can identify by the presence of red blood cells in their lumens.
- Type II pneumocytes, which are rounder and bulge into the alveolus.
- Type II cells produce and secrete surfactant, which reduces surface tension and prevents alveolar collapse in exhalation; they also maintain and repair the alveolar wall.
- Alveolar macrophages, aka, dust cells, fibroblasts, and mast cells are also present.
Clinical Correlation
Asthmatic airway:
- Prolific goblet cells, lumen-obstructing mucus, and thickened basement membrane.
- Allergic asthma is caused by hypersensitivity to allergens that trigger inflammatory responses, including mucus over-production, in the lungs and obstruct air flow.
- Non-allergic asthma, on the other hand, is caused by pathological neural regulation of bronchiole diameter, and, therefore, air flow.
Images:
Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)c602web/602/c602web/toc.htm)