**1. Introduction**

The respiratory system is anatomically divided into the following two parts: upper respiratory tract (organs outside the chest: nose, pharynx, and larynx) and lower respiratory tract (organs inside the chest: trachea, bronchi, bronchioles, alveolar ducts, and alveoli). This system that performs three basic functions, i.e., air transmission, air filtration, and gas exchange (respiration), is functionally divided into two zones. These are the conductive zones (from the nose to the bronchioles) that act as a pathway for the delivery of inhaled gases, and the respiratory zone (from the alveolar canal to the alveoli) where gas exchange occurs. The branching pattern of the conducting passages is known as the tracheobronchial tree as it resembles the branching of a tree [1].

The lungs, the main organ of the respiratory system, are divided into two sections depending on the functions of their structural parts. These are the tubes that conduct air (bronchi and bronchioles) and respiratory tissue (alveolar ducts, alveolar sacs, and alveoli). Ventilated by a secondary (lobar) bronchus, each lobe of the lung is divided into smaller pyramidal-shaped segments known as the bronchopulmonary segments and is ventilated by a tertiary (segmental) bronchus [2].

The bronchi of the lower respiratory tract are vital in terms of respiratory aspects because they are responsible for the transmission and filtration of air as well as for key immunological functions.
