Final answer:
The fetal respiratory system develops in four key periods: the initial formation of lung structures up to week 16, vascularization and formation of alveolar ducts from weeks 16 to 24, significant growth and maturation of alveolar precursors from weeks 24 to birth, and the inflation of the lungs after birth with alveolar maturation continuing into childhood.
Step-by-step explanation:
The development of the respiratory system in a fetus occurs over four distinct periods. The first period starts around week 4, when initial structures like the lung bud and bronchial buds begin to form. This lasts until about week 16, which marks the end of the Embryonic stage and the beginning of the Pseudoglandular period. Over this time, bronchial buds continue to branch and form all of the segmental bronchi, and by week 16, the respiratory bronchioles are developed.
During the Canalicular phase, which spans weeks 16 to 24 of gestation, extensive vascularization and the formation of alveolar ducts and precursors take place. Respiratory bronchioles are formed by about week 19, and cells begin to differentiate to form type I and type II pneumocytes. Around week 20, fetal breathing movements may begin, important for muscle toning and indicating good health.
The Saccular period follows from weeks 24 to birth (term). Major growth and maturation of the respiratory system occur, more alveolar precursors develop, and larger quantities of pulmonary surfactant are produced. A significant milestone is achieved around week 28, where the maturity of the alveolar precursors enables premature babies to breathe independently.
After birth, an important event is the inflation of the lungs. The thoracic cavity's compression during delivery forces fluid out of the lungs, and the first breaths a newborn takes further inflates the lungs. The lungs continue to mature well into childhood, with a full complement of functional alveoli appearing around 8 years of age.