The lung is the essential respiration organ. Human lungs are located in two
cavities on either side of the heart, however they are not identical. Both lungs are
separated into lobes by fissures, with 3 lobes on the right and 2 on the left. Each
lobe is surrounded by a pleural cavity, which consists of 2 pleurae. The parietal
pleura lies against the rib cage, and the visceral pleura lies on the surface of the
lungs. In between the pleura is pleural fluid. The pleural cavity helps lubricate
the lungs, as well as provide surface tension to keep the lung surface in contact
with the rib cage.
Their principle function is to transport oxygen from the atmosphere into the
bloodstream, and to release carbon dioxide from the bloodstream into the
atmosphere. A large surface area is needed for this exchange of gases which is
accomplished by specialized cells that form thin-walled air sacs, alveoli.
In humans, the trachea divides into the two main bronchi that enter the roots of
the lungs. The bronchi continue to divide within the lung, and after multiple
divisions, give rise to bronchioles. The bronchial tree continues to branch until
its reaches the level of terminal bronchioles, which lead to alveolar sacs. Alveolar
sacs are made up of clusters of alveoli, where the individual alveoli are tightly
wrapped in blood vessels and it is here that gas exchange actually occurs.
There are 4 stages in lung development: embryonic stage, pseudoglandular stage,
canalicular stage, and terminal sac stage.
Embryonic stage is where the developing lungs grow into and begin to fill the
pleural cavities. Embryonic development starts at 4-7 weeks after conception.
After 4 weeks, the gut tube of the primitive gut grooves into the anterior wall,
and that groove goes deeper, so it forms a pouch at the front, which is derived
from the endoderm. It can be said that the lung is developed from the gut.