Biomedical Engineering Reference
In-Depth Information
Fig. 2
Pressure-volume
graphs of cat excised lungs.
Lungs inflated with saline
(
solid line
)andair(
dashed
line
)
Saline
Air
200
150
100
50
0
4
8
Pressure (cmH
2
O)
12
16
20
During inspiration, an excess surfactant molecules are recruited to the interface
when the concentration there drops below the saturation level, and at expiration
the decrease in interface area causes the expulsion of surplus molecules [
15
]. The
dynamics of interface area, concentration of surfactant at the interface and surface
tension causes part of the lung hysteresis, see Fig.
2
, although much of the hysteresis
is caused by successive collapses and recruitments of pieces of lung parenchyma.
The surface tension also has structural function, because it increases the stiffness
of the lung parenchyma and supports part of the load. Fortunately, this increased
stiffness is much higher in lung volumes near total lung capacity, since the
increased surface area of parenchyma reduces the concentration of surfactant at the
interface [
16
].
1.5
Atelectasis
Atelectasis is a lung condition in which part of the lung collapses preventing air
flow there. Atelectasis can be acute or chronic and can affect the entire lung or part
of it. From the physical point of view atelectasis occurs when moist membranes of
the parenchyma touch themselves, leading to the fusion of the liquid layers, what
consequently reduces the surface energy. The collapsed region can only be recruited
if subjected to a higher contrary pressure to the adhesion pressure. The surfactant is
very important to detach the adhered membranes because it makes easier to increase
the area of the interface liquid-air with lower surface tension.
Despite the biomedical literature addressing the surface tension as the cause
of atelectasis, no tribological studies of the septa surfaces were found. The
mechanism of adhesion of the septa and the forces involved in it are still not well
understood [
17
].
