Biomedical Engineering Reference
In-Depth Information
Fig. 1
14-hedra tessellation
filling the space
space when tessellated and having the smallest ratio of surface to volume among
the space filling polyhedra [
12
]. The airways are then formed by removing some
walls from the tessellated 14-hedra. The estimated difference for the surface-volume
ratio between the parenchyma modelled this way and the values found in literature
for real parenchyma is very small, around 2.7%
[
13
], which corroborates this
geometrical approximation.
1.3
Surface Tension in Alveoli
Because the lungs are very efficient mass exchangers with very thin wall between air
and blood, some water diffuses from the blood vessels to alveolar cavity. The surface
tension of the water film on the septa affects significantly the alveolar mechanics.
To reduce surface tension of the air-water interface, some epithelial cells produce
a very efficient surfactant mixture. Surfactant molecules have a water affinity polar
side and an air affinity apolar side [
14
].
1.4
Structural Effect and Hysteresis
The surface tension magnitude depends on the concentration of the surfactant
molecules at the air-liquid interface. During the respiratory cycle changes in the
total area of the lung alter the air-liquid interface area. Consequently the surfactant
concentration at the interface vary together with the surface tension magnitude. That
is, surface tension increases during inspiration and decreases during expiration.
