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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.

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").

1 / 4 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.

2 / 4 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.

3 / 4 - 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)