Development of Respiratory System
It begins at the end of third week by formation of endodermal laryngotracheal groove at floor of pharynx caudal to hypobranchial eminence. Laryngotracheal tube is separated from foregut by formation of tracheo-esophageal septum. This tube elongates till the thorax. Upper part of the tube forms the mucus membrane of larynx while lower part form mucus membrane of trachea and bronchi. The lower end of the tube will divides to form 2 lung buds. Surrounding mesoderm will form muscles and cartilages of larynx and trachea.Dr. Sherif Fahmy
Formation of LungsEach lung bud divides into 3 branches, but on left side one of the branches disappears. Right lung has 3 lobes, left lung has 2 lobes. Each branch of the lung bud divides repeatedly to form bronchioles and lastly alveoli. Blood vessels are formed from surrounding mesoderm. Lung invades pleura from medial aspect, so pleura surrounds lung by 2 layers. Expansion of lung occurs after birth when the baby takes his first inspiration