explain the development of the lungs.

Lungs are the spongy lower respiratory tract organs in pairs and are filled with air and lie in humans' thoracic cavity. They function to facilitate gas exchange in the body by taking oxygen from the outer environment during breathing, supplying it to the bloodstream, and releasing carbon dioxide that they have taken from the bloodstream into the external environment.

The development of the lungs in the organism starts at its fetal stage and continues after birth. It occurs in five stages:

• Embryonic stage- It starts from the third week of gestation and continues till the sixth week of gestation. It begins with lung bud formation, also known as a respiratory diverticulum from the foregut's ventral wall. The lung bud separates from the foregut and forms the esophagus, which later the lung bud bifurcates from its distal end, giving rise to the left and right primary bronchial buds. The bifurcation at the proximal end gives rise to the trachea and larynx. Three secondary bronchial buds are formed from the primary bronchial buds by the fifth week of gestation. This later give rise to ten tertiary buds, and their formation marks the end of this phase.
• Pseudoglandular stage- Occuring between the sixth and sixteenth week of gestation, in this phase, more divisions occur in the buds, forming a respiratory tree resulting in terminal bronchiole formation. The respiratory epithelium lines the respiratory passage.
• Canalicular stage- Occuring between the sixteenth and the twenty-eighth week of gestation, in this stage, the respiratory bronchioles are formed from terminal bronchioles. Around these structures, the vascularization also increases. The epithelium walls become thin, causing lumen enlargement. Specialized cell types that are associated with lungs are formed by differentiation in this phase.
• Saccular stage- It occurs between the gestation's twenty-eighth and the thirty-sixth week and involves the generation of terminal branches from the respiratory bronchioles. The primitive alveoli, which are lined with type I and type II alveolar cells, formation from the branches also occur during the saccular stage.
• Alveolar stage- This stage starts during the thirty-sixth week of gestation and continues till the child reaches eight years of age. It involves the maturation of the alveoli leading to the formation of mature alveoli, which have established contacts with the capillary walls for gaseous exchange.