Biomedical Engineering Reference
In-Depth Information
1.2.2 Material, polymer and technical requirements
Gas transfer
Membranes for blood oxygenation need to provide sufficient gas exchange and
at the same time prevent any mixing of blood and gas (oxygen or oxygen-
enriched air). The driving force for the gas transport across the membrane wall is
the difference in chemical potential of the gas on both sides of the membrane,
i.e. the different partial pressures or concentrations in the blood and on the gas
side. Transport of the gas will always occur into the direction of the lower
concentration or pressure.
The exchanged amount of gas on a given contact surface follows Fick's law:
V
a
1
ÿa
2
d
K
d
At
where V is the amount of gas exchanged; a
1
and a
2
are activities (pressures) of
gas on either side; d is the diffusion distance; K
d
is the diffusion constant, A is
the surface area; and t is time.
In the human lung, gas exchange occurs on a surface (A) of 50±200 m
2
, at
blood film thicknesses of 6±15m, and with contact times (t) in the sub-second
level.
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Most of all, there is no direct contact of blood and air, but both media are
always separated by the alveolar capillary membrane with a diffusion distance
(d) of only 1±2m. The important point in this is that blood only `sees'
endothelial cells, thus no foreign surface. Blood is oxygen enriched and carbon
dioxide deprived without any unphysiological interference.
In air, oxygen has a volume share of 21%, which corresponds to a partial
pressure of oxygen of 160mmHg. In the human lungs, due to mixing with dead
space air and the increased water vapour pressure, in the alveoli this is reduced
to slightly more than 100mmHg at the alveolar capillary interface. As venous
blood has an oxygen partial pressure of only 40 mmHg, oxygen is easily taken
up and the oxygen partial pressure for arterial blood goes up to 100mmHg,
which means that equilibrium with the breathed air is almost reached. For
carbon dioxide, the gradients are exactly opposite: the partial pressure of carbon
dioxide in air is close to zero. The partial pressure in the blood that enters the
lungs is about 46mmHg, and thus carbon dioxide diffuses through the alveoli
capillary membrane, so that in the alveoli the carbon dioxide partial pressure
rises to about 40mmHg. Although the driving force is smaller than for oxygen,
the exchange of carbon dioxide takes place readily, so that arterial blood has a
partial pressure of carbon dioxide of 35±45mmHg.
The main difficulty in the construction of any blood contacting biomedical
device is that no foreign material can be as gentle to blood as the endo-
thelium. Whenever blood touches a surface that is not endothelium, this
means to the body defence system that there is an injury that needs to be
counteracted and treated. The larger the foreign surface the blood touches and
