Human Respiratory System Overview

Respiratory System Division

• The respiratory system is divided into conducting and respiratory portions
• The conducting portion includes the nasal cavities, nasopharynx, oropharynx, larynx, trachea, bronchi, and bronchioles
• The respiratory portion includes the respiratory bronchioles and lung alveoli
• The conducting portion does not allow for gas exchange
• Gas exchange occurs in the respiratory portion

Nasal Cavities

• The nasal cavities are divided into the anterior portion (vestibule) and the posterior portion
• The anterior portion is lined with modified skin, comprised of stratified squamous keratinized epithelium and dermis with sweat and sebaceous glands
• The posterior portion contains the respiratory and olfactory areas
• The anterior portion filters inspired air from dust particles
• The posterior portion is lined by a mucous membrane, consisting of pseudostratified columnar ciliated epithelium with goblet cells resting on a basement membrane
• The corium is a dense fibro-elastic connective tissue

Olfactory Area

• The olfactory area features a pseudostratified neuroepithelium
• The olfactory epithelium consists of three cell types: sustentacular (supporting) cells, olfactory cells, and basal cells
• Sustentacular cells are columnar, possess microvilli, and contain a light-yellow pigment that gives olfactory mucosa its color
• Olfactory cells are bipolar neurons with dendrites extending towards the surface and forming olfactory hair vesicles containing cilia
• Basal cells are small, pyramidal cells that act as stem cells
• The corium is dense fibro-elastic tissue, highly vascular, and contains tubulo-alveolar glands (Bowman's glands) that secrete serous fluid

Larynx

• The larynx is kept open by hyaline cartilages (thyroid and cricoid) and elastic cartilages (epiglottis, corniculate, and cuneiform)
• The larynx is lined by respiratory epithelium, except for the superior surfaces of the epiglottis and vocal cords, which are lined by stratified squamous nonkeratinized epithelium

Trachea

• The trachea starts at the end of the larynx and ends by dividing into two bronchi
• The tracheal wall consists of four layers: mucosa, submucosa, fibro-cartilaginous coat, and adventitia
• The mucosa features pseudostratified columnar ciliated epithelium with goblet cells resting on a basement membrane, a thin corium rich in elastic fibers, and a submucosa with tracheal glands
• The fibro-cartilaginous coat is composed of 16-20 C-shaped rings of hyaline cartilage embedded in connective tissue
• The adventitia is formed of connective tissue

Respiratory Epithelium Cells

• The respiratory epithelium is comprised of several cell types: ciliated cells, goblet cells, basal cells, serous cells, brush cells, neuroendocrine cells, and migratory cells
• Ciliated cells are columnar ciliated cells that beat towards the larynx
• Goblet cells contain mucinogen granules and produce mucus
• Basal cells act as reserve stem cells for ciliated and goblet cells
• Serous cells produce a secretion of lower viscosity than mucus
• Brush cells are columnar cells with microvilli and are considered sensory receptors
• Neuroendocrine cells possess neuroendocrine function and secrete serotonin and calcitonin
• Migratory cells are lymphocytes

Practical Considerations

• The nasal cavity is lined by pseudostratified columnar ciliated epithelium with goblet cells and features large venous plexuses and glands
• The olfactory epithelium includes olfactory hairs, pigment, microtubules, supporting cells, olfactory bipolar neurons, basal cells, mucus, olfactory receptor neurons, Bowman's gland, lamina propria, and cribriform plate
• The respiratory epithelium includes cilia, goblet cells, and lamina propria
• The trachea features distinct mucosa, submucosa, cartilage, and adventitia