What are the Bronchi? Left, Right Main Bronchus, Bronchioles

The bronchi (singular ~ bronchus) are the two branches (left and right) at the bottom of the trachea that lead into the lungs. These main bronchi then divide into lobar and segmental bronchi. Together the trachea and the main left and right bronchi, along with all its subdivisions, are  referred to as a the tracheobronchial tree.

The main bronchi are tube like structures that allow for the passage of air between the trachea and lung. It enters the hilum of the lung and divides into a number of branches and further subdividions. It eventually terminates in the bronchioles which lead into the alveolar sacs of the lung.

Trachea –> Main Bronchi –> Lobar Bronchi –> Segmental Bronchi –> Conducting Bronchiole –> Terminal Bronchiole –> Respiratory Bronchiole

Mainstem and Lobar Bronchi

The walls of the bronchi are lined with ciliated respiratory epithelium that secretes mucus (goblet cells) and traps dust particles. Mucus is also secreted from the submucosal glands. Smooth muscle in the bronchi wall extends all the way to the bronchioles. This can widen and narrow the airway depending on the degree of contraction or relaxation.

Right Main Bronchus

This is the wider but shorter of the two bronchi. It subdivides into three  lobar branches :

1. Right superior lobar bronchus, also known as the eparterial bronchus
2. Right middle lobar bronchus
3. Right lower lobar bronchus

Left Main Bronchus

This is longer than the right main bronchus. It passes under the aortic arch to subdivide into two lobar branches  :

1. Left superior lobar bronchus
2. Left inferior lobar bronchus

Segmental Bronchi

The segmental bronchi are subdivisions of the lobar bronchi. It supplies the bronchopulmonary segments (10 in the right lung, 8 to 10 in the left lung) and is named according to the segment it supplies.

Right Bronchopulmonary Segment

• Right Superior Lobe
  • Apical
  • Posterior
  • Anterior

• Right Middle Lobe
  • Lateral
  • Medial

• Right Inferior Lobe
  • Superior
  • Anterior basal
  • Medial basal
  • Lateral basal
  • Posterior basal

Left Bronchopulmonary Segment

• Left Superior Lobe
  • Apical *
  • Posterior *
  • Anterior
  • Superior lingular
  • Inferior lingular

• Left Inferior Lobe
  • Superior
  • Anterior basal **
  • Medial basal **
  • Lateral basal
  • Posterior basal

* Both lobes may be combined into the apicoposterior segment.

** Both lobes may be combined into the anteriomedial basal segment.

Bronchioles

Bronchioles are lined with simple epithelium that lack the mucus secreting glands of the bronchi. Smooth muscle is also present in the wall of the bronchioles except for the most distal respiratory bronchioles.

Conducting and Terminal Bronchioles

The small bronchial tubes branch out directly from segmental bronchi.The distal parts of a conducting bronchiole are referred to as the terminal bronchiole.

Respiratory Bronchioles

These are the smallest part of the bronchial tree and lead into the alveolar ducts which empty into the alveolar sacs.

Related Articles

1. What is Bronchitis?
2. What is the Trachea?