Untitled Quiz

Respiratory System Histology

• The respiratory system is divided into two functional portions: a conducting portion and a respiratory portion.
• The conducting portion connects the external environment with the respiratory portion and performs functions like air conduction, conditioning (cleaning, moistening, and warming inspired air), and warming.
• The respiratory portion, including respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, facilitates gas exchange.
• The conducting portion includes the nasopharynx, larynx, trachea, bronchi, and terminal bronchioles.
• The walls of the respiratory portion enable gas exchange.

Respiratory Epithelium

• Most of the conducting portion of the respiratory tract has pseudostratified columnar ciliated epithelium with goblet cells.
• Five cell types are recognized in this epithelium:
  • Ciliated columnar cells (most abundant)
  • Mucous goblet cells (next most abundant)
  • Brush cells (sensory receptors)

Trachea

• A flexible tube, approximately 10 cm long, extending from the larynx and bifurcating into the primary bronchi.
• It's maintained open by 16-20 C-shaped hyaline cartilage rings.
• Its wall has three layers:
  • Mucosa: Contains respiratory epithelium and lamina propria (loose connective tissue with elastic fibers).
  • Submucosa: Contains seromucous glands and lymphoid nodules.
  • Adventitia: Contains fibroelastic connective tissue and 20 C-shaped hyaline cartilages.

Bronchial Tree

• The conducting portion of the bronchial tree includes extrapulmonary (primary) bronchi, intrapulmonary (secondary) bronchi, bronchioles, and terminal bronchioles.
• The respiratory portion of the bronchial tree includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
• Extra-pulmonary bronchi have the same structure as the trachea.
• Intrapulmonary bronchi have;
  • A pseudostratified columnar ciliated epithelium with goblet cells
  • Connective tissue with elastic fibers
  • No submucosa
• The walls of the bronchioles have spiral smooth muscle, and the outer layer is composed of connective tissue.
  • No cartilage, glands, or lymphatic nodules are present in bronchioles.

Clara Cells

• Tall, dome-shaped, non-ciliated cells found in the bronchiolar wall.
• Functions include protecting against inhaled harmful substances and acting as stem cells for other types of bronchiolar epithelium.

Alveoli

• The structural and functional units of gas exchange in the lungs.
• Separated by inter-alveolar septa.
• Lined with alveolar epithelium, consisting of type I and type II pneumocytes.
  • Type I pneumocytes are the most numerous, consisting of flat, simple squamous cells with connections via tight junctions. They form a very thin membrane for gas exchange.
  • Type II pneumocytes are fewer in number, cuboidal cells with round nuclei. They secrete pulmonary surfactant, reducing surface tension and preventing alveolar collapse during expiration.
• The inter-alveolar septa have abundant blood capillaries and a network of reticular fibers.

Blood-Air Barrier

• The blood-air barrier is a layer of cells allowing gas exchange.
• Consist of four layers:
  • Pulmonary surfactant lining
  • Cytoplasm of type I pneumocytes
  • Fused basal lamina of type I alveolar cells and capillary endothelial cells
  • Endothelial cells of blood capillaries.