PROCESS DIAGRAM
Breathing Involves Changes
in Pressures and Volumes
LEARNING OBJECTIVES
1.
Describe
the steps in the process of pulmonary
ventilation.
2.
Identify
the various lung volumes and capacities.
3.
Explain
the different breathing patterns.
ulmonary ventilation, which as we have seen
is the flow of air between the atmosphere
and the lungs, occurs due to differences in
air pressure.
To understand the mechanics of breathing, as shown
in Figure 13.3, you need to know some physical and ana-
tomical principles:
1.
Like fluid, air moves in response to changes in pressure;
specifically, air moves from regions of higher pressure
to regions of lower pressure. We inhale, or breathe
in, when the pressure inside the lungs is less than the
atmospheric air pressure. We exhale, or breathe out,
when the pressure in the lungs is greater than the
atmospheric air pressure.
2.
According to Boyle’s law, at a constant temperature,
the pressure of a gas is inversely related to its volume.
So when the volume of a gas increases the pressure
decreases, and vice versa.
3.
The lungs and chest wall are elastic. The elastic
properties of the chest wall and lungs oppose each
other: The chest tends to pull outward, while the lungs
T he b re ath in g p ro cess • Figure 13.3
Various respiratory muscles contract and expand the thoracic cavity during normal
breathing. Additional muscles are recruited for deep inhalations and forced exhalations.
The contraction and relaxation of the respiratory muscles change the volume and pres-
sure in the thoracic cavity and alveoli, causing air to flow into and out of the lungs.
Atmospheric pressure = 760 mmHg
At rest, when the
diaphragm is relaxed
alveolar pressure is
equal to atmospheric
pressure, and there
is no air flow.
Atmospheric pressure = 760 mmHg
During exhalation, the diaphragm and external
intercostals relax. The chest and lungs recoil, the
chest cavity contracts, and the alveolar pressure
increases above atmospheric pressure. Air flows
out of the lungs in response to the pressure
gradient, and the lung volume decreases. During
forced exhalations, the internal intercostals and
abdominal muscles contract, thereby reducing
the size of the chest cavity further and creating
a greater increase in alveolar pressure.
Alveolar
pressure =
762 mmHg
During inhalation, the diaphragm and
external intercostals contract. The chest
cavity expands, and the alveolar pressure
drops below atmospheric pressure. Air
flows into the lungs in response to the
pressure gradient and the lung volume
expands. During deep inhalation, the
scalene and sternocleidomastoid muscles
expand the chest further, thereby creating
a greater drop in alveolar pressure.
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