Respiratory System: Larynx, Trachea, Bronchi, and Lungs

Larynx (Voice Box)

  • Connects laryngopharynx to trachea.
  • Located in midline of neck, anterior to esophagus, between cervical vertebrae 4-6.
  • Composed of nine pieces of cartilage:
    • Three single:
      • Thyroid cartilage (Adam's apple).
      • Epiglottis.
      • Cricoid cartilage.
    • Three paired:
      • Arytenoid cartilage.
      • Corniculate cartilage.
      • Acuniform cartilage.
  • Arytenoid cartilage: most important of the paired cartilages, influences vocal cord position and tension.

Epiglottis

  • Leaf-shaped elastic cartilage covered with epithelium.
  • Stem attached to inferior portion of the anterior rim of the thyroid cartilage.
  • Leaf portion is unattached and moves like a trap door.
  • During swallowing:
    • Pharynx and larynx rise.
    • Epiglottis moves down to cover the glottis, prevents food/drink from entering the trachea.

Vocal Cords

  • Mucous membrane of larynx forms two pairs of folds:
    • Superior pair: vestibular folds (false vocal cords).
      • Space between: rima vestibuli.
      • Function: holding breath against thoracic cavity pressure.
    • Inferior pair: vocal cords (true vocal cords).
      • Principal structures for voice production.
      • Elastic ligaments stretched between laryngeal cartilages, like guitar strings.
      • Intrinsic laryngeal muscles move cartilages to adjust tension.
      • Air passing through vibrates folds, producing sound (phonation).
      • Sound waves set up in pharynx, nose, and mouth.
      • Vibration and pitch related to vocal cord tension.
      • Greater air pressure = louder sound.

Trachea (Windpipe)

  • Passageway for air, part of the conducting zone.
  • About 12 cm long and 2.5 cm in diameter.
  • Anterior to the esophagus.
  • Extends from larynx to superior border of the fifth thoracic vertebra where it branches into left and right primary bronchi.
  • Reinforced by 16-20 C-shaped rings of hyaline cartilage connected by dense connective tissue.
    • Keeps trachea open, preventing collapse during breathing.
    • C-shaped to accommodate the esophagus posteriorly.

Bronchi

  • Trachea divides into:
    • Right main bronchus: more vertical, shorter, and wider than the left.
      • Aspirated objects more likely to lodge here.
    • Left main bronchus.
  • Main bronchi also have incomplete C-shaped cartilage rings.
  • Main bronchi branch into lobar (secondary) bronchi upon entering the lungs.
    • Three in right lung, two in left lung.
  • Lobar bronchi divide into segmental (tertiary) bronchi.
    • Supply specific bronchopulmonary segments within each lobe.
  • Segmental bronchi branch into bronchioles, then terminal bronchioles.
  • Bronchioles contain Clara cells:
    • Protect against inflammation and toxins.
    • Produce surfactant.
    • Function as stem cells for epithelial repair.
  • Terminal bronchioles mark the end of the conducting zone.
  • Branching pattern: bronchial tree.

Lungs

  • Two spongy, cone-shaped organs in the thoracic cavity.
  • Responsible for gas exchange (O2 in, CO2 out).
  • Divided into lobes:
    • Three in right lung.
    • Two in left lung.
  • Left lung has a cardiac notch.
    • A concave groove to accommodate the heart.
    • Left lung ~10% smaller than right lung.
  • Lungs separated by heart and mediastinum.
    • Trauma to one lung doesn't necessarily affect the other.
  • Enclosed by pleural membrane (double-layered serous membrane):
    • Parietal pleura: lines the thoracic cavity wall.
    • Visceral pleura: covers the lungs.
    • Pleural cavity: space between layers, contains lubricating fluid.
      • Reduces friction during breathing.
      • Surface tension causes membranes to adhere to each other.
      • Separate pleural cavities for each lung.
  • Inflammation of pleural membrane (pleuritis) can cause pain.
    • Excess fluid accumulation = pleural effusion.
  • Lungs extend from diaphragm to slightly above clavicle, lie against ribs.
    • Base: broad, inferior, concave (fits over diaphragm).
    • Apex: narrow, superior portion.
    • Surface against ribs matches rib curvature.
    • Hilum: region on medial surface where blood vessels, nerves, and bronchi enter/exit (root of the lung).
    • Cardiac notch on left lung.
  • Right lung thicker and broader but shorter due to the liver.
  • Apex of lungs lies above the medial third of the clavicle.
  • Base of lungs extends from 6th costal cartilage anteriorly to the 10th thoracic vertebra posteriorly.
  • Pleura extends about 5 cm below the base of the lungs.
  • Removal of excess pleural fluid via needle insertion in the 7th intercostal space.
    • Needle passed through the superior border of the lower rib to avoid nerve/vessel damage.
    • Avoid penetrating the diaphragm.

Microscopic Airways and Alveoli

  • Terminal bronchioles subdivide into respiratory bronchioles.
    • Alveoli bud from walls.
    • Respiratory zone begins here.
  • Respiratory bronchioles subdivide into alveolar ducts.
  • Alveoli: Cup-shaped outpouchings supported by elastic basement membrane.
  • Alveolar sacs: Two or more alveoli sharing a common opening.
  • Alveolar macrophages (dust cells): phagocytose fine dust particles and debris.
  • Fibroblasts produce reticular and elastic fibers.

Respiratory Membrane

  • Gas exchange (O2 and CO2) occurs by diffusion across the respiratory membrane.
    • Movement of gases from high concentration to low concentration.
  • Four layers:
    1. Type I alveolar cells (alveolar walls): Type I and Type II alveolar cells, alveolar macrophages.
    2. Epithelial basement membrane:
      • Provides structural support to alveolar cells.
    3. Capillary basement membrane:
      • May be fused w/ epithelial basement membrane.
      • Helps reduce distance for gas diffusion.
    4. Capillary endothelium:
      • Innermost layer, allows rapid gas exchange.
  • Membrane is very thin: ~0.5μm0.5 \mu m thick (~1/16th the diameter of a red blood cell).
  • Lungs contain ~300,000,000300,000,000 alveoli.
  • Surface area of ~70m270 m^2 (size of a racquetball court) for gas exchange.