CHAPTER 8 • EXCHANGE FUNCTION OF THE MICROCIRCULATION
183
of about 95 mm Hg. Direct measurements of
PO2 in small arterioles (20 to 80 |Jm diameter)
of some tissues reveal that the PO2 is only 25
to 35 mm Hg, which corresponds to a 30%
to 60% loss of oxygen content of the blood.
Therefore,
substantial
amounts
of oxygen
can diffuse out of the blood before the blood
reaches
the
capillaries;
however,
capillar-
ies are still the most important site for tissue
oxygenation because the relatively high cap-
illary density ensures that diffusion distances
between the blood and tissue cells are short.
Figure 8.2 illustrates the diffusion of oxy-
gen from blood within a capillary, across the
capillary endothelium, and then into a cell.
The PO2 is only slightly reduced just out-
side of the capillary (from 25 to 24 mm Hg)
because little oxygen is consumed as it dif-
fuses through the endothelial cell and into
the interstitial fluid surrounding the capillary.
The oxygen in the interstitium then diffuses
down a concentration gradient into nearby
cells. Because the mitochondria inside a cell
are consuming oxygen, the PO2 may be very
low inside the cytoplasm of the cell. Although
an intracellular PO2 of 5 mm Hg is shown in
Figure 8.2, the value depends on where the
PO2 is measured within the cell, the rate of
mitochondrial
oxygen
consumption,
and
the capillary blood PO2. Just inside the cell
membrane, the PO2 is much higher than at
the center of the cell; the lowest PO2 is found
within the mitochondria. Therefore, signifi-
cant oxygen gradients exist within cells.
In Figure 8.2, the overall concentration
gradient driving oxygen diffusion into the
cell is 20 mm Hg. According to Fick’s first
law (Equation 8-1), the rate of oxygen dif-
fusion ( JO2) is proportionate to the concen-
tration difference of oxygen (expressed as
PO2 difference) between the capillary blood
and inside the cell, assuming a fixed diffu-
sion constant, diffusion distance, and surface
area.
Therefore, increasing capillary blood
PO2 (as occurs when a person breathes pure
oxygen) or decreasing tissue PO2 (as occurs
with increased tissue oxygen consumption)
increases the rate of oxygen diffusion into
the tissue. Capillary PO2 is also increased by
dilation of resistance vessels. This increases
microvascular blood flow, thereby delivering
more oxygen to the capillaries per unit time,
which results in higher PO2 values in the cap-
illary blood. If vasodilation is accompanied by
an increase in the number of flowing capillar-
ies (as occurs during skeletal muscle contrac-
tion), this increases the surface area available
for oxygen diffusion and further enhances
oxygen transport into the tissue. For exam-
ple, if the cell shown in Figure 8.2 were sur-
rounded by three capillaries instead of one,
then there would be an increase in the rate of
oxygen diffusion into the cell, which would
be necessary if the mitochondrial oxygen con-
sumption increased significantly.
Oxygen Delivery and Extraction
The previous discussion described oxygen dif-
fusion from blood into tissue cells, and how
the PO2 gradient from the blood to the tissue
cell plays an important role in determining
the rate of diffusion. While the PO2 gradient
Capillary
Interstitium
2
24
mmHg
Cell
AC = 25 - 5 mmHg
= 20 mmHg
f f lW ) '
QÎÎ2) 5 mmHg
Œ2D
(0>
®5D
JO2 = DA (AC/AX)
JO
2
oc AC
■ FIGURE 8.2 Diffusion of oxygen
(JO2)
from capil-
laries into the tissue follows Fick's first law of diffu-
sion. Because the diffusion constant
(D),
the area
for exchange (A), and the diffusion distance (AX)
remain relatively constant in a single capillary, the
diffusion of oxygen is governed primarily by the dif-
ference in partial pressure of oxygen
(PO2)
between
the blood and cells (AC), which is 20 mm Hg in this
illustration. Most of this PO2 gradient is between the
interstitium and cell when mitochondria are actively
consuming oxygen; there is only a small gradient
across the capillary endothelium. Increasing the
overall PO2 gradient increases the rate of diffusion.
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