Biomedical Engineering Reference
In-Depth Information
and:
dC
O
2
2
blood
dx
D
Ru
B
h
ð
C
O
2
2
gas
2
C
O
2
2
blood
Þ
5
ð
9
:
11
Þ
respectively. To prove the assumption of the oxygen concentration being constant, add
Equations 9.10 and 9.11
together to get
dC
O
2
2
gas
dx
dC
O
2
2
blood
dx
1
5
0
which leads to
C
O
2
2
gas
1
C
O
2
2
blood
5
constant
if you integrate the differential equation with respect to distance.
Equations 9.7 through
9.11
can be used to model the gas movement across any semipermeable membrane if the
assumption regarding a constant concentration is valid.
9.5 OXYGEN/CARBON DIOXIDE TRANSPORT IN THE BLOOD
Oxygen and carbon dioxide have a very low solubility in plasma, which limits the
amount of oxygen and carbon dioxide that the blood can carry. Red blood cells, however,
can remove the dissolved oxygen and carbon dioxide from the blood. In the case of oxy-
gen, hemoglobin directly binds to oxygen molecules. Carbon dioxide, however, is modi-
fied into plasma soluble molecules by proteins carried within red blood cells and is
therefore not directly transported by red blood cells. These processes solve the problem of
the solubility of oxygen and carbon dioxide in plasma. It also allows for the gases to con-
tinually diffuse into the bloodstream as long as the red blood cells are not saturated,
because the concentration gradient is never quenched.
Every 100 mL of blood leaving the alveolar capillaries carries approximately 20 mL of
oxygen, of which only approximately 0.3 mL is pure oxygen dissolved in blood. The
remaining 19.7 mL of oxygen is bound to hemoglobin (Hb). Each hemoglobin molecule
consists of four protein subunits with one iron ion per protein subunit (
Figure 9.7
). As
β
chain 2
α
chain 1
Heme group composed
of an iron core
β
chain 1
α
chain 2
FIGURE 9.7
The protein structure of hemoglobin, which consists of four globular protein subunits. Each sub-
unit contains one heme molecule, which is a non-protein compound that surrounds an iron core. It is this iron
molecule that facilitates the transport of oxygen and carbon dioxide. Each red blood cell contains close to 300 mil-
lion hemoglobin molecules. Adapted from Martini and Nath (2009).
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