The lungs are the organs responsible for gas exchange. Humans require oxygen for cellular respiration, and they produce carbon dioxide as a waste product. The lungs are composed of a number of multiple levels of branching airways, called bronchi. These open into tiny, microscopic air sacs, the alveoli, in which the gas exchange takes place. The lungs begin to develop during gestation and continue to grow throughout childhood.
Fetal Development
The lungs begin to develop during the fourth week of prenatal life. The early lung arises as an outgrowth of the early gastrointestinal system. This initial growth period in the lung is called the “embryonic period.” The next phase, termed the pseudoglandular phase, is when the majority of the bronchi, the air conducting passageways in the lungs, develop. This occurs from weeks 5 to 16 of gestation. This phase is followed by the canalicular phase and saccular phases, which continue until birth. These phases are characterized by continued development of the respiratory tree.
After Birth
At birth, lung development is still incomplete. The alveoli are the most distal endpoints of the respiratory tree. These tiny air sacs are where the oxygenation of the blood occurs. At birth, an infant has approximately 300 million alveoli. Further branching of the respiratory tree continues after birth to form more alveoli. According to the an article published in 2006 in the “American Journal of Physiology,” only half of the of an individual’s alveoli are present at birth. The lungs continue to form alveoli throughout childhood. This so-called “alveolar period” continues until a child is 10.
Lung Volumes
As children grow into adults, their lung volumes slowly increase. The capacity of the lungs can be measured by a technique known as plethysmography. The increase in lung volume roughly corresponds to the gradual increase in height as children grow. During puberty, the growth of the lungs increases dramatically in males. In females, while the lungs continue to grow until women reach their adult size, the growth of the lungs progresses more gradually.
Diseases of Lung Development
Respiratory distress syndrome is a disease that affects lung development. This condition occurs more frequently in babies who are born preterm, especially before 34 weeks of gestation. The risk is greater in male babies, babies born to diabetic mothers and babies delivered by Cesarean section. The lungs may be immature, and because of deficiency of an essential chemical, surfactant, the infant’s lungs collapse with each breath. As a result, these children often need intensive respiratory support following birth.
References
- Human Embryology: Phases of Lung Development
- "American Physiological Society"; Alveoli Increase in Number But Not Size from Birth to Adulthood in Rhesus Monkeys; Dallas M. Hyde, et al.; June 2007
- Langman's Medical Embryology; Thomas W. Sadler Ph.D.; 2009
- "Thorax"; Lung Function in White Children Aged 4 to 19 Years; M. Rosenthal, et al.; 1993
