Bronchial muscle

• It is involuntary smooth muscle, not skeletal muscle.
• In larger bronchi it forms a near-continuous circular layer; in smaller bronchi it has a more spiral arrangement.
• Its contraction narrows the airway, called bronchoconstriction. Relaxation widens the airway, called bronchodilation.
• Unlike bronchi, bronchioles have no cartilage plates, but they still have prominent smooth muscle that controls their diameter.

Bronchial tree diagram

Bronchial tree: order of branches

Trachea
  │
  └─ Carina
      ├─ Right main (primary) bronchus
      │    ├─ Superior lobar bronchus
      │    ├─ Middle lobar bronchus
      │    └─ Inferior lobar bronchus
      │         └─ Segmental (tertiary) bronchi
      │
      └─ Left main (primary) bronchus
           ├─ Superior lobar bronchus
           └─ Inferior lobar bronchus
                └─ Segmental (tertiary) bronchi

Segmental bronchi
  → smaller bronchi
  → bronchioles
  → terminal bronchioles
  → respiratory bronchioles
  → alveolar ducts
  → alveolar sacs and alveoli

Important points

• The right main bronchus is wider, shorter, and more vertical than the left. Aspirated foreign bodies therefore commonly enter the right bronchial tree.
• Lobar bronchi supply lung lobes: 3 on the right and 2 on the left.
• Segmental bronchi supply bronchopulmonary segments.
• The trachea divides at the carina, usually around vertebral level T4-T5.

Labeled diagram: bronchial smooth muscle

• SM: Bronchial smooth muscle or muscularis. It forms a circular to spiral layer around the airway and changes airway diameter.
• E: Respiratory epithelium, mainly pseudostratified ciliated columnar epithelium.
• C: Hyaline cartilage plates that support the bronchial wall.
• V: Blood vessels.
• G: Seromucous gland, where visible.

• It lies beneath the mucosa, forming a mainly circular or spiral layer around the bronchus.
• In larger bronchi it is more continuous; in smaller bronchi it appears as spirally arranged bundles.
• Contraction narrows the bronchial lumen: bronchoconstriction.
• Relaxation widens the lumen: bronchodilation.
• It is controlled involuntarily by the autonomic nervous system. Beta-2 receptor stimulation relaxes it.

Bronchial wall muscle: key information

1. Respiratory epithelium
2. Lamina propria
3. Smooth-muscle layer (muscularis)
4. Submucosa with seromucous glands
5. Hyaline cartilage plates
6. Adventitia and surrounding lung tissue

• Smooth muscle forms a generally circular, often spirally arranged layer around the bronchial lumen.
• In larger bronchi it is a more continuous layer; it becomes thinner and less organized in smaller bronchi.
• Contraction decreases lumen diameter, causing bronchoconstriction and greater airway resistance.
• Relaxation increases lumen diameter, causing bronchodilation.
• Cartilage plates support the bronchi, while smooth muscle regulates their diameter. Cartilage is absent in bronchioles, where smooth muscle has a larger role in controlling airway caliber.

Labeled bronchial-wall histology image

• SM = smooth muscle
• E = respiratory epithelium
• C = hyaline cartilage
• V = blood vessel

• The heading has an error: “Functi tional Arrangement Wall.”
• Labels 5 and 7 are duplicated.
• It presents unusually specific named layers, such as “inner helical” and “outer helical” smooth-muscle layers, which are not standard labels used in the major undergraduate anatomy and histology texts.

Closest reliable textbook sources for the same information

Best source for a labeled bronchial-wall image

• Figure 17-7: tertiary or segmental bronchus
• Figure 17-8: bronchial wall
• Labels include:
  • SM: smooth muscle
  • E: epithelium
  • C: cartilage
  • G: seromucous glands
  • V: blood vessels

Standard description to write in notes

The bronchial wall has a smooth-muscle layer, called the muscularis, arranged in interlacing circular and spiral bundles beneath the mucosa. It becomes relatively more prominent as bronchi become smaller. Contraction narrows the airway lumen and produces bronchoconstriction. Cartilage changes from rings in large bronchi to discontinuous plates in intrapulmonary bronchi, and is absent in bronchioles.