In considering the development of this system it is important to review the development of the foregut, in specific and the development of the endoderm overall. It is also important to consider the development of the respiratory system in terms of its many constituent components. The respiratory system begins at the nasal cavity and consists of a conducting portion and a respiratory portion. The conducting portion includes nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. The respiratory portion consists of the respiratory bronchioles, alveolar ducts, alveolar sacs and the alveoli.
Gaseous exchange occurs in the alveoli.
The development of the respiratory system involves the endoderm and the mesoderm that surrounds it. As discussed previously, the embryo undergoes lateral body folding and during this process the endoderm forms into a gut tube. This G.I. tube begins cephalically at the oral plate and continues until it reaches the cloacal or anal plate. The early development of the lungs lags behind the development of the heart and great vessels. However, as development proceeds the lungs will eventually occupy more of the thoracic cavity than the heart.