CHAPTER 8 • EXCHANGE FUNCTION OF THE MICROCIRCULATION
185
Arteriole
Capillaries
Venule
O
2
Delivery: DO
2
= F • CaO
2
O
2
Consumption: VO
2
= F • (CaO2 - CvO2)
V02
Venous O2 Content: CvO2 = CaO2
■ FIGURE 8.4 Model for oxygen delivery and balance in tissues. Oxygen delivery (
DO
2) is the product of blood
flow
(F)
and the arterial oxygen content
(CaO2).
Oxygen consumption (VO2) is the product of flow and arte-
rial-venous oxygen difference (oxygen extraction;
CaO2
-
CvO2)
according to the Fick principle. Rearranging
the equation shows that venous oxygen content
(CvO2)
depends on CaO2 minus the ratio of VO2 to F.
(e.g., ischemia) or arterial oxygen content
(e.g., anemia, hypoxemia).
Oxygen delivery represents only what is
available to the tissue, not what is utilized by
the tissue. As arterial blood enters the micro-
circulation, and particularly the capillaries,
oxygen diffuses from the blood into the tis-
sues, and this reduces the oxygen content of
the blood (see Fig. 8.4). The greater the oxy-
gen consumption of the tissue, the greater the
amount of oxygen that diffuses from the blood.
Therefore, as the blood leaves the tissues, the
venous blood has a lower oxygen content than
the arterial blood. For example, if 5 mL O2/100
mL blood were removed as the blood passes
through a tissue (i.e., oxygen extraction),
then the venous blood oxygen content (CvO2)
will be 15 mL O2/100 mL blood if CaO2 is 20
mL O2/100 mL blood. Note, that as previously
described in Chapter 7, oxygen extraction dif-
fers among organs and depends on their oxy-
gen consumption and blood flow. W hen the
oxygen extraction (CaO2 - CvO2; mL O2/mL
blood) is multiplied by the blood flow (F; mL/
min), this represents the amount of oxygen
consumed by the tissue (VO2; mL O2 /m in),
which is described by the Fick Principle (see
Equation 4-3 and below). Note that oxygen
content needs to be expressed as mL O2/mL
blood instead of mL O2/100 mL blood when
calculating values using the Fick Principle.
VO2 = F (CaO2 - CvO2)
If this equation is solved for the oxygen
extraction, then we see that oxygen extrac-
tion is determined by the ratio of oxygen con-
sumption to blood flow.
V O2
(CaO2 - CvO2) =
Therefore, oxygen extraction is increased if
oxygen consumption increases or blood flow
decreases. Because the arterial oxygen content
normally does not change significantly, then
increased extraction reduces the venous oxygen
content. This is more clearly seen by solving the
previous equation for venous oxygen content:
V O2
CvO2 = CaO2 - ^ F 2
Venous oxygen measurements are used for
monitoring patients in intensive care settings,
and therefore, the above relationship helps to
explain what can cause venous oxygen levels
(usually measured as venous oxygen saturation
or PO2) to fall. Venous oxygen saturation meas-
ured in the pulmonary artery (SvO2) is nor-
mally about 75% and has a PO2 of about 40 mm
Hg. If it is abnormally low, this can be caused
by elevated organ consumption by organs,
reduced organ blood flow, or reduced arterial
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