Human embryos develop in a head-to-tail (cephalo-caudal) direction. Therefore, upper respiratory structures develop first.
Effects
of Aging on the Respiratory System
Human embryos develop in a head-to-tail (cephalo-caudal)
direction. Therefore, upper respiratory structures develop first. By week four
of development, two thickened ectodermal plates called olfactory placodes are present on the head’s anterior aspect. These
quickly form the olfactory pits, from
which the nasal cavity develops. The pits extend posteriorly, connecting with
the early pharynx, which simultaneously forms, from the endodermal germ layer.
The lower respiratory organ epithelia develop as outpockets of the foregut endoderm, eventually becoming the pharyngeal mucosa. This protruding laryngotracheal
bud is pres-ent by week five of development. Its proximal section forms the
lining of the tracheal. Its distal end splits to form the bronchial mucosae and
eventually the alveoli. These linings are covered by mesoderm, which forms the
respiratory passage walls and lung stroma.
By week 28, the respiratory system is developed enough to
allow a premature baby to breathe by itself. Infants born before week 28
usually have IRDS due to inadequate surfactant production. During fetal
development, the lungs are filled with fluid and respiratory exchange is
handled by the placenta. The lungs are mostly bypassed by vascular shunting of circulating
blood. At birth, the respiratory passages become filled with air. The partial
pressure of CO2 rises in the blood, exciting the respiratory
centers. This causes the first breath to be taken by the baby. The alveoli
inflate and begin gas exchange. However, it will take almost two weeks until
the lungs are fully inflated.
Aging causes the lungs to lose elasticity, lower-ing their
VC. The ribs play a role in affecting breath-ing because they may become
arthritic and the costal cartilages may become less flexible. Respira-tory
volume is therefore impaired, resulting in the inability to exercise as long or
as hard compared with earlier in life. Emphysema risk is much higher in smokers
than in nonsmokers, but some evidence of the disease is present in most people
over age 50 regardless of their history of smoking. Although aging affects the
respiratory system of every adult, smokers experience greatly increased
problems as aging continues.
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