ory system is divided into two portions namely the conducting portion and the respiratory portion. The conducting portion includes the nasal cavity, the pharynx, larynx, trachea and the bronchi while the respiratory portion consists of the bronchioles, the alveolar sacs and the alveoli. [Thomas A Marino] At around the fourth week of the embryonic stage, the laryngotracheal bud emerges from the laryngotracheal groove. The proximal end of the bud connects into the pharynx through a narrow opening called the glottis while the middle portion develops into the trachea. The lateral end of the laryngotracheal bud bifurcates into two distinct buds that develop into the right and left bronchi of the lungs. These lateral lung buds continue to divide and develop into bronchial tubes that constitute the lower respiratory system. During the eighth week, the cartilage, the connective tissues, and the pleural sacs also start to develop from the mesoderm.
             There are four distinct phases in the development of the lungs. During the first phase or the pseudoglandular period, which starts from the second month, the bronchi start to differentiate and develop into bronchioles. The second phase or the canalicular period starts from the fifth month and continues over the next ten weeks during which time the terminal bronchioles or the respiratory bronchioles are formed. [Thomas A Marino] At six months time, the terminal region of the bronchial tubes differentiate and develop into air sacs called alveoli. By the end of six months, an estimated one million terminal branches are formed and each bronchiole starts developing hundreds of alveoli over the next few months. Thus a minimum of 26-week period for sufficient development of the lungs is required for survival outside the womb. [John F. Neas] From the seventh month, the terminal sac phase begins where the capillaries proliferate rapidly and approach the resp
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