The lungs develop in the embryo as a ventral evagination of the foregut; thus, the epithelium of the respiratory system is of endodermal origin. This initial respiratory diverticulum grows into the thoracic mesenchyme. The bronchial cartilages, smooth muscle, and the other connective tissue elements are derived from the thoracic mesenchyme.
The air passages of the respiratory system consist of a conducting portion and a respiratory portion.
The conducting portion of the respiratory system consists of those air passages that lead to the sites of respiration within the lung where gas exchange takes place. The conducting passages include those located outside as well as within the lungs.
The passages external to the lungs consist of the following:  Nasal cavities that represent two large air-filled spaces in the uppermost part of the respiratory system (and, during forced breathing, the oral cavity residing inferior to the nasal cavities).
 Nasopharynx that lies behind the nasal cavities and above the level of the soft palate and communicate inferiorly with oropharynx that is posterior to the oral cavity.
 Larynx that is a hollow tubular organ containing cartilaginous framework responsible for producing sounds.
 Trachea that is a flexible air tube that extend from the larynx to the thorax. It serves as a conduit for air and in the mediastinum it birurcates into paired main bronchi.
 Paired main (primary) bronchi that enter root of the right or left lung.
FIGURE 19.1 • Diagram of respiratory passages. The nasal cavities, nasopharynx, oropharynx, larynx, trachea, bronchi, and bronchioles constitute the conducting portion of the respiratory system. The respiratory portion of the system, where gas exchange occurs, is composed of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. (Based on Boileau G. A Method of Anatomy. Baltimore: Williams & Wilkins, 1980.)
Within the lungs, the main bronchi undergo extensive branching to give rise ultimately to the distributing bronchioles. The bronchioles represent the terminal part of the conducting passages. Collectively, the internal bronchi and the bronchioles constitute the bronchial tree.
The respiratory portion is that part of the respiratory tract in which gas exchange occurs. Sequentially, it includes these:  Respiratory bronchioles that are involve in both air conduction and gas exchange.  Alveolar ducts that are elongated airways formed from the confluence openings to alveoli.  Alveolar sacs that respresnt spaces surrounded by clusters of alveoli.  Alveoli that are the primary sites of gas exchange.
Blood vessels enter the lung with the bronchi. The arteries branch into smaller vessels as they follow the bronchial tree into the substance of the lung. Capillaries come into intimate contact with the terminal respiratory units, the alveoli. This intimate relationship between the alveolar air spaces and the pulmonary capillaries is the structural basis for gas exchange within the lung parenchyma. The essential features of the lung blood supply are described on page 687.
Air passing through the respiratory passages must be conditioned before reaching the terminal respiratory units.
Conditioning of the air occurs in the conducting portion of the respiratory system and includes warming, moistening, and removal of particulate materials. Mucous and serous secretions play a major role in the conditioning process. These secretions moisten the air and also trap particles that have managed to slip past the special short thick hairs, called vibrissae, in the nasal cavities. Mucus, augmented by these serous secretions, also prevents the dehydration of the underlying epithelium by the moving air. Mucus covers almost the entire luminal surface of the conducting passages and is continuously produced by goblet cells and mucus-secreting glands in the walls of the passages. The mucus and other secretions are moved toward the pharynx by means of coordinated sweeping movements of cilia and are then normally swallowed.