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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.
The respiratory portion
  • 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.
Pharynx
  • 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.
Larynx
  • 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
  • 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.
Lungs
  • 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)