Physiology
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Boundless Anatomy and Physiology
APPENDIX B: Development and Aging of the Organ Systems
Development of the Respiratory System
Physiology Textbooks Boundless Anatomy and Physiology APPENDIX B: Development and Aging of the Organ Systems Development of the Respiratory System
Physiology Textbooks Boundless Anatomy and Physiology APPENDIX B: Development and Aging of the Organ Systems
Physiology Textbooks Boundless Anatomy and Physiology
Physiology Textbooks
Physiology
Concept Version 11
Created by Boundless

Development of the Respiratory System

Lung development can be divided into distinct stages: the pseudoglandular period, the canalicular period, and the terminal saccular period.

Learning Objective

  • Evaluate the ability of a fetus to survive birth based on the development of the respiratory system


Key Points

    • During the pseudoglandular period (also known as "glandular period") all of the major lung elements, except those required for gas exchange (e.g. alveoli) will develop.
    • During the canalicular period the lumen of the bronchi enlarge, lung tissue becomes highly vascularized, and respiratory bronchioles and alveolar ducts develop from the terminal bronchioles.
    • The terminal saccular period will establish the important blood-air barrier and specialized cells of the respiratory epithelium will emerge.
    • The lungs expand after the first breath.

Terms

  • terminal saccular period

    The important blood-air barrier is established during the terminal saccular period. Specialized cells of the respiratory epithelium appear at this time, including type I alveolar cells across which gas exchange occurs, and type II alveolar cells which secrete pulmonary surfactant.

  • first breath

    At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. When the newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. This triggers it to take the first breath, within about 10 seconds after delivery.

  • canalicular period

    The lumens of the bronchi enlarge and lung tissue becomes highly vascularized during the canalicular period.


Full Text

The respiratory system lies dormant in the human fetus during pregnancy. The development of the lungs can be divided into different stages. The pseudoglandular period (also known as "glandular period") spans weeks six to 16 and during this time the developing lung resembles an endocrine gland. By the end of this period, all of the major lung elements, except those required for gas exchange (e.g. alveoli), have appeared. Respiration is not possible during this phase and fetuses born during this period are unable to survive. The canalicular period spans weeks 16 to 26 and during this time the lumens of the bronchi enlarge, lung tissue becomes highly vascularized, and respiratory bronchioles and alveolar ducts develop from the terminal bronchioles. Respiration is possible towards the end of this period, but few fetuses born during this time will survive. The terminal saccular period spans from week 26 to birth. During this time, the important blood-air barrier is established.

Specialized cells of the respiratory epithelium appear including type II alveolar cells which secrete pulmonary surfactant. This surfactant is important in reducing the surface tension at the air-alveolar surface, allowing expansion of the terminal saccules. During this time, the lungs are rock-like and will sink if placed in water but will expand after the first breath, a trait which is used to determine if babies were born alive. Figure 1 shows the right and left lung buds from which the bronchi and lungs will develop. At birth, the respiratory system becomes fully functional upon exposure to air, although some lung development and growth continues throughout childhood. Pre-term birth can lead to infants with under-developed alveolar type II cells which produce surfactant. The lungs of pre-term infants therefore may not function well because the lack of surfactant leads to increased surface tension within the alveoli leading to alveoli collapse and no gas exchange, a condition known as respiratory distress syndrome. Lastly, the alveolar period spans from birth to eight years of age and during this stage the terminal saccules, alveolar ducts, and alveoli increase in number. True alveoli appear as indentations in the saccular wall and septae form to produce divisions in the wall.

Respiratory buds

Lung buds from a human embryo of about four weeks, showing commencing lobulations.

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