L14 & L15. Histology - Microanatomy of the Respiratory System

Respiratory System

• The respiratory system is responsible for gas exchange between the body and the environment
• The system consists of the nose, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli
• Primary functions of the respiratory system are:
  • Gas exchange
  • Filter air from environmental pathogens
  • Warm and humidify air
  • Smell
  • Vocalization
• The respiratory system is lined with a special type of epithelium, called respiratory epithelium, that is a pseudostratified ciliated columnar epithelium with goblet cells
• Respiratory epithelium is located in the nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles
• The respiratory epithelium changes to stratified squamous epithelium in the upper portion of the larynx

Olfactory Mucosa

• Located in the roof of the nasal cavity
• Responsible for smell
• Contains four types of cells that contribute to smell:
  • Supporting (sustentacular) cells: Provide metabolic and physical support for olfactory receptors. They are tall, cylindrical cells with apical microvilli.
  • Basal cells: They are small, conical cells with dark ovoid nuclei. They act as a stem cell population, regenerating other cells.
  • Sensory (olfactory) cells: Bipolar neurons. They are spindle-shaped cells with an apical dendrite that extends to the surface. The apical process ends in a bulbous expansion called the olfactory vesicle or knob, which contains 10-20 modified cilia. The proximal part of the cell extends into the lamina propria as an axon, which continues to the olfactory bulb of the brain.
  • Brush cells: possess apical microvilli. They are thought to function in general sensation
• The olfactory neurons are unusual in that they are continuously replaced and have only a 30-60 day life span
• The olfactory transduction pathway begins with odorant binding to G-protein-linked receptors on the olfactory cilia which triggers a cascade of events ultimately leading to the generation of an action potential that is transmitted to the brain

Lamina Propria

• Located beneath the epithelium
• Contains a mix of connective tissue, blood vessels and glands
• Important functions:
  • Support for respiratory epithelium: Provides structural support for the epithelium.
  • Defense: It houses immune cells that defend against pathogens.
  • Warming and humidifying air: Capillary loops in the lamina propria warm the incoming air.

Submucosa

• Located beneath the lamina propria
• Contains loose connective tissue, glands, and lymphatic tissue
• Provides additional support for the epithelium and provides a pathway for blood vessels and nerves.
• Contains glands that secrete mucus which helps to trap inhaled particles

Trachea

• Connects the larynx to the bronchi
• It is a tube lined with respiratory epithelium and goblet cells
• Contains C-shaped hyaline cartilage rings that provide structural support
• The rings of cartilage allow the trachea to expand and contract during breathing.

Bronchi

• Division of the trachea that leads into the lungs
• Histologically similar to the trachea, lining is composed of respiratory epithelium, goblet cells, and hyaline cartilage rings

Bronchioles

• Smaller branches of the bronchi
• Lining epithelium gradually changes from pseudostratified columnar to simple cuboidal in the bronchioles
• Contain less cartilage and more smooth muscle in their walls
• Smooth muscle aids in controlling air flow into and out of the lungs

Alveoli

• The functional unit of the lungs
• Site of gas exchange between the lungs and blood
• Thin-walled air sacs surrounded by capillaries
• The walls of the alveoli are composed of two types of cells:
  • Type I alveolar cells: Flattened cells that make up the majority of the alveolar surface. They are involved in gas exchange.
  • Type II alveolar cells: Smaller, cuboidal cells that produce surfactant. Surfactant acts as a detergent, reducing the surface tension of the alveoli and preventing them from collapsing.

Alveolar Macrophages

• Also called dust cells
• Found within the interstitium of the alveolar walls and in the alveolar lumen.
• Highly phagocytic cells
• Remove inhaled pollutants, bacteria, and other debris from the alveoli.

Respiratory Epithelium

• The lining of the trachea and bronchi is a pseudostratified ciliated columnar epithelium with goblet cells.
• Ciliated cells: Move mucus, trapped particles, and debris upward towards the pharynx for removal (mucociliary escalator)
• Goblet cells: Secrete mucus, which traps inhaled particles and pathogens.

Summary: Respiratory Epithelium

• Ciliated cells move mucus and debris towards the pharynx
• Goblet cells secrete mucus to trap pollutants
• Basement membrane connects the epithelium to the underlying connective tissue

Pulmonary Blood Supply

• The lungs receive blood from two sources:
• Pulmonary arteries: Carry deoxygenated blood from the right ventricle of the heart to the lungs.
• Bronchial arteries: Carry oxygenated blood from the aorta to the lungs to supply their tissues.

Other Key Concepts

• APUD system: The amine precursor uptake and decarboxylation (APUD) system is a group of cells that produce and secrete hormones. Small granule cells in the respiratory system are part of this system.