Bronchi
Bronchi
Also known as: bronchial tree, bronchus
The cartilage-supported branching airways from the carina to the bronchioles: main (primary) → lobar (secondary) → segmental (tertiary) bronchi and their subdivisions.
Overview
The trachea divides into right and left main bronchi, which enter the lungs at the hila. Each main bronchus gives lobar bronchi (three on the right, two on the left), and each lobar bronchus divides into segmental bronchi (ten on the right, eight to ten on the left), each ventilating a bronchopulmonary segment. Repeated division follows through ≈6–12 further generations of smaller bronchi until the airway is ≈1 mm wide and loses its cartilage, becoming a bronchiole.
As bronchi become intrapulmonary, the C-shaped rings are replaced by irregular cartilage plates that surround the wall, and a continuous layer of spirally arranged smooth muscle lies between the mucosa and cartilage. Each bronchus travels with a branch of the pulmonary artery (bronchovascular bundle), while pulmonary veins run between segments. Medium-sized bronchi are the main site of airway resistance.
- Location
From the carina (T4/T5) through the hila into the lungs.
Parts & subdivisions
- Shorter, wider and more vertical.
- Longer, narrower and more horizontal.
- One per lobe: right superior, middle and inferior; left superior and inferior.
- One per bronchopulmonary segment.
Anatomy
- Innervation
Pulmonary plexus: vagal (parasympathetic) fibres cause bronchoconstriction and mucus secretion via M3 muscarinic receptors; sympathetic fibres (T1–T5) and circulating adrenaline relax bronchial smooth muscle via β2 receptors. Vagal afferents carry cough, irritant and stretch signals.
- Arterial supply
Bronchial arteries: usually two left bronchial arteries from the descending thoracic aorta and one right bronchial artery from the third right posterior intercostal artery (or a common trunk with the upper left bronchial artery). They supply airways down to the respiratory bronchioles, the visceral pleura and the walls of pulmonary vessels.
- Venous drainage
Bronchial veins → azygos (right) and accessory hemiazygos/superior intercostal (left) veins; deeper bronchial veins drain into pulmonary veins, adding a small amount of deoxygenated blood to systemic arterial blood (physiological shunt).
- Lymphatic drainage
Intrapulmonary → bronchopulmonary (hilar) → tracheobronchial → paratracheal nodes → bronchomediastinal trunks.
- Histology
Respiratory epithelium (ciliated pseudostratified columnar with goblet cells); lamina propria with elastic fibres; smooth muscle layer; submucosa with seromucous glands; cartilage plates; adventitia. Goblet cells and glands diminish distally.
Function
- Conduct air to and from the lungs (part of the anatomical dead space).
- Regulate airflow resistance via bronchial smooth muscle tone.
- Clear mucus and particles via the mucociliary escalator; initiate cough.
Clinical significance
Bronchiectasis
Permanent dilatation of bronchi with impaired clearance and chronic purulent sputum, following severe infection, cystic fibrosis, primary ciliary dyskinesia, immunodeficiency or allergic bronchopulmonary aspergillosis (which causes central bronchiectasis). CT shows the "signet-ring" sign (bronchus wider than its accompanying artery).
Bronchodilator pharmacology
β2 agonists (salbutamol, formoterol) and muscarinic antagonists (ipratropium, tiotropium) relax bronchial smooth muscle; non-selective β-blockers can provoke bronchospasm in asthma.
Test yourself
Which airway segment is the first to contain alveoli opening from its wall?
- Terminal bronchiole
- Segmental bronchus
- Respiratory bronchiole
- Lobar bronchus
Answer: Respiratory bronchiole
Respiratory bronchioles (generations ≈17–19) have scattered alveoli budding from their walls and mark the start of the respiratory zone. Terminal bronchioles (≈generation 16) are the last purely conducting airways.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.