Biomedical Engineering Reference
In-Depth Information
2.6.2
Mechanics of Breathing
Inhalation is initiated by the contraction of the diaphragm which contracts and de-
scends about 1 cm during normal breathing and up to 10 cm on forced breathing. The
diaphragm lines the lower part of the thorax, sealing it off air-tight from the abdom-
inal cavity below. Its contraction causes muscles in the thorax to pull the anterior
end of each rib up and outwards enlarging its volume. As a result, the pressure inside
the thorax (intrathoracic pressure) and inside the lungs (intrapulmonary pressure)
decreases relative to the outside atmospheric air pressure. The pressure difference
induces the inhaled air from a higher pressure to a lower pressure in order to equalise
the pressure. During exhalation the lung and chest wall return to its equilibrium
position and shape. The thoracic cavity volume is reduced and the pressure builds
up to release the air from the lungs. In quiet breathing only the elastic recoil of the
lung and chest walls is needed to return the thorax to equilibrium (a passive process).
However in forceful expiration additional muscles (intercostals) in the thorax and
abdomen are also used to further increase the pressure.
A pressure-volume curve can be used to obtain information about how the lung
deforms during breathing. It can describe the mechanical behaviour of the lungs
and chest walls such as the elasticity of the lung, and its ability to expand and
stretch (distensibilty) through the slope of the pressure-volume curve. This is re-
ferred to as lung compliance (C
L
) and has units of mL/cm H
2
O to reflect the change
in lung volume (
V
) as a result of a change in the pressure (
P
) of the lung
(e.g.
C
L
=
V
/
P
). A pressure-volume curve from the literature (Harris 2005) is
shown in Fig.
2.17
. A high compliance value refers to a lung that is easily distended
and is reflected with a steep pressure-volume curve. A low compliance means that
Fig. 2.17
Pressure-volume
curve taken from Harris
(2005) acquired with the
super-syringe method. The
open circles are the data
points plotted continuously
during the maneuver. The
solid lines show the
quasi-static points connected
to form a smooth P-V curve.
The inflation and deflation
points are not connected,
because they were performed
separately in this example
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