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Ventilation, or breathing, is the movement
of air through the conducting passages between the atmosphere
and the lungs. The air moves through the passages because
of pressure gradients that are produced by contraction
of the diaphragm and thoracic muscles.
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Pulmonary ventilation
Pulmonary ventilation is commonly referred to as breathing.
It is the process of air flowing into the lungs during inspiration
(inhalation) and out of the lungs during expiration (exhalation).
Air flows because of pressure differences between the atmosphere
and the gases inside the lungs.
Air, like other gases, flows from a region with higher pressure
to a region with lower pressure. Muscular breathing movements
and recoil of elastic tissues create the changes in pressure
that result in ventilation. Pulmonary ventilation involves
three different pressures:
- Atmospheric pressure
- Intraalveolar (intrapulmonary) pressure
- Intrapleural pressure
Atmospheric pressure is the pressure of the air outside the
body. Intraalveolar pressure is the pressure inside the alveoli
of the lungs. Intrapleural pressure is the pressure within
the pleural cavity. These three pressures are responsible
for pulmonary ventilation.
Inspiration
Inspiration (inhalation) is the process of taking air into
the lungs. It is the active phase of ventilation because it
is the result of muscle contraction. During inspiration, the
diaphragm contracts and the thoracic cavity increases in volume.
This decreases the intraalveolar pressure so that air flows
into the lungs. Inspiration draws air into the lungs.
Expiration
Expiration (exhalation) is the process of letting air out
of the lungs during the breathing cycle. During expiration,
the relaxation of the diaphragm and elastic recoil of tissue
decreases the thoracic volume and increases the intraalveolar
pressure. Expiration pushes air out of the lungs.
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