The to come lower than the atmospheric pressure, which

The alveolar
wall contains macrophages which engulf phagocytes and removes any debris and
dust particles that may be in the walls. The pulmonary arterioles and the
venules separate into capillaries in the outer surface of the alveoli. They
have a single layer of endothelial cells and a basement membrane. (Nhlbi.nih.gov, 2017)

During inhalation
the air pressure inside the lungs gets equal to the atmospheric pressure. When
air will be flowing into the lungs it would cause the pressure inside the
alveolus to come lower than the atmospheric pressure, which is done by the
increase in the volume of lungs. So inhalation can take place the lungs must
expand. This is done by the alveoli expanding with the participation of the
diaphragm /external intercostal.
The diaphragm is dome shaped having skeletal muscles that contract, allowing it
to flatten which will increase the diameter of the thoracic cavity and is accountable
for the movement of about 75% of the air that enter the lungs. As the diaphragm
increase its dimeter the external intercostal contract causing the ribs to be
pulled up and the sternum to be pushed down. As this happens the walls of the
lungs are pulled outward due to low atmospheric pressure the parietal and
visceral pleurae will start to keep close to each other due to the surface tension
that gets created by the touching surfaces. As the thoracic cavity gets more
expanded so does the parietal and pleura lining (getting pulled in all
direction) which in return increases lung volume. But during exhalation the
pressure in the lungs is greater than the atmosphere. Exhalation has two main
factors which is the recoil of the elastic fibers and the inward pull of the
surface tension from the alveolar fluid. Exhalation is the process of
inhalation but reversed. The external intercostal, ribs, diaphragm and thoracic
cavity go back to their original space and also the surface tension will exert
and inward pull between the parietal and visceral pleurae and the elastic membrane
which would recoil. This will make the lung volume decrease and the alveolar
pressure to increase, causing the air from the alveoli to flow into the low pressure
air in the atmosphere. (Nhlbi.nih.gov, 2017)

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During
breathing there’s a process called gas exchange that takes place. This is
Oxygen and Carbon Dioxide moving between the lungs and blood. Diffusion
occurs during gas exchange as the blood in the capillaries
surrounding the alveoli has a lower oxygen concentration compared to the air in
the alveoli which has just been inhaled. The walls of the alveoli and
capillaries are only one cell thick, this allows gasses to diffuse across them.
The blood in capillaries will have a higher concentration of CO2 than the
inspired air due to CO2 being a waste product. CO2 will then diffuse the other
way and into the alveoli where it can be exhaled.

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