Biomedical Engineering Reference
In-Depth Information
B
RI
B
IM
q
O
M
q
O
R
q
O
I
B
IR
B
MI
Red Blood Cell
Interstitial
Fluid/
Capillaries
Cytosol of
Slow Muscle
Fiber
FIGURE 8.18
The diffusion of oxygen from the red blood cell into the cytosol of the slow muscle fiber.
Note that we have assumed that the reverse reactions do not involve oxygen and that oxy-
gen is treated as a molecule and not two oxygen atoms, which would have introduced a
2
O
q
term in Eq. (8.72).
Oxygen is transported through the arterial system to the capillaries, where it diffuses out
of the red blood cell into the interstitial fluid. It then moves into the cytosol (the liquid part
of the cytoplasm that does not contain any organelles) of slow muscle fibers, as shown in
Figure 8.18, where
q
O
I
is the quantity of
O
2
in the interstitial fluid,
q
O
M
is the quantity of
O
2
in the cytosol of the slow muscle fiber, and the other quantities are defined as before.
The equation describing the diffusion process and reactions with
Hb
is given by
q
O
R
¼
B
IR
q
O
C
B
RI
q
O
R
þ
K
q
H
1
K
q
H
0
q
O
R
þ
K
q
H
2
K
q
H
1
q
O
R
10
01
21
12
þ
K
32
q
H
3
K
q
H
2
q
O
R
þ
K
q
H
4
K
q
H
3
q
O
R
23
43
34
ð
8
:
73
Þ
q
O
I
¼
B
RI
q
O
R
þ
B
MI
q
O
M
B
IR
q
O
I
B
IM
q
O
I
q
O
M
¼
B
IM
q
O
I
B
MI
q
O
M
2
is approximately 100 mm Hg and
98 percent saturated. On the venule side of the capillary membrane,
On the arterioles side of the capillary membrane,
PO
PO
2
is approximately
40 mm Hg and 75 percent saturated. When
PO
2
is high, oxygen quickly binds with hemo-
globin, and when
PO
2
is low, oxygen is quickly released from hemoglobin. As the red blood
cell moves through the capillary, the
PO
2
gradient causes oxygen to be released into the
interstitial fluid.
Once inside the slow muscle fiber, oxygen is moved to the mitochondria using a different
mechanism than that used in other cells. After oxygen diffuses across the cell membrane, it
quickly binds to myoglobin (
), a protein-like hemoglobin whose function is to transport
and store oxygen, and forms oxymyoglobin
Mb
. By storing oxygen in oxymyoglobin,
oxygen is driven into the cell by a large concentration gradient until it binds with all avail-
able myoglobin. At this point, the oxygen concentration on either side of the membrane
equilibrates. Slow muscle fibers also have many more mitochondria than other cells, which
allows higher levels of oxidative metabolism. Thus, the muscle fiber is able to store a large
quantity of oxygen in oxymyoglobin, and when needed, it is readily available to the mito-
chondria to create ATP. This greatly increases oxygen transport to the mitochondria than
if the cell just depended on oxygen diffusion in the absence of myoglobin.
When oxygen is bound to myoglobin to create oxymyoglobin in the cytosol, oxymyoglo-
bin then diffuses from the cytosol into the mitochondria, and once in the mitochondria, a
ð
MbO
2
Þ
