184
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
determines the rate of oxygen diffusion, the
total amount of oxygen that is available per
unit time for diffusion is determined by the
amount of hemoglobin-bound oxygen in the
blood and the rate of blood flow into the tissue.
The amount of oxygen in the blood (oxygen
content) is determined by the PO2 of the blood,
along with the amount of hemoglobin in the
red cells and the hemoglobin binding affinity
for oxygen (Fig. 8.3). This relationship is called
the hemoglobin-oxygen dissociation curve. At
normal arterial PO2 values (95 mm Hg), about
97% of the hemoglobin is bound to oxygen (97%
hemoglobin saturation; SaO2). If the blood con-
tains 15 g of hemoglobin per 100 mL of blood
(normal value), and a gram of hemoglobin can
bind to 1.34 mL oxygen, then 20.1 mL oxygen
(15 g/100 mL x 1.34 mL O2/g) will be bound to
hemoglobin in 100 mL of blood when 100% sat-
urated, and 19.5 mL O2/100 mL blood is bound
at 97% saturation. A small amount of oxygen
(~0.3 mL O2/100 mL blood) is dissolved in the
free water of the plasma and cells at normal
arterial PO2 values. Therefore, the total amount
of hemoglobin-bound and dissolved oxygen
■ FIGURE 8.3 Hem oglobin-oxygen dissociation
curve. Percent oxygen saturation of hemoglobin
(%
HbO2)
has a sigmoidal relationship w ith the par-
tial pressure of oxygen (PO2). In this example, 100%
saturation corresponds to an arterial blood oxygen
content
(CaO2)
of about 20 mL O2/ 100 mL blood.
This assumes that the hemoglobin concentration
is 15 g/100 mL blood and that 1.34 mL O2 bind to
each gram of hemoglobin. Note that the amount
of dissolved oxygen is very small relative to the
amount of oxygen bound to hemoglobin. The
dissociation curve shifts to the right (decreased
hemoglobin affinity for oxygen) w ith increased
temperature and PCO2, and decreased pH.
at normal arterial PO2 values is approximately
20 mL O2/100 mL blood (or, 20 vol %).
The
hem oglobin-oxygen
dissociation
curve is sigmoidal in shape; therefore, small
decreases in arterial PO2 from normal values
do not significantly reduce the oxygen content
of the arterial blood. However, as the arterial
PO2 begins to fall below 80 mm Hg, and espe-
cially in the range of tissue PO2 values (20 to
40 mm Hg), the curve becomes very steep and
there is a large decrease in the amount of oxy-
gen bound to hemoglobin as PO2 decreases.
At a PO2 of about 25 mm Hg, hemoglobin is
only 50% saturated (P50 = 25 mm Hg). There-
fore, as blood flows into tissues, the relatively
low PO2 in the tissue results in oxygen diffus-
ing from the blood into the tissue. This lowers
the blood PO2 and causes oxygen to dissoci-
ate from the hemoglobin so that it can diffuse
into the tissue. Unloading of oxygen from
hemoglobin can also be enhanced by factors
that cause a rightward shift in the oxygen dis-
sociation curve. For example, increased tem-
perature and PCO2, and decreased pH shift
the curve to the right, which shifts the P50 to
the right. Therefore, at any given tissue PO2
value, a rightward shift causes more unload-
ing of oxygen from the hemoglobin because
of reduced binding affinity to oxygen. This
is an important mechanism to increase tissue
oxygenation when the metabolic activity of
a tissue increases (e.g., contracting muscle),
which increases tissue temperature and CO2
production, and decreases pH.
The oxygen content of the arterial blood
(CaO2; mL O2/100 mL blood) multiplied by
the arterial blood flow (F; mL/min) represents
the oxygen delivery (DO2; mL O2/m in) to the
tissue (Fig. 8.4).
DO
2
= F ■
CaO2
Therefore, the
oxygen delivery to a tissue is
determined by the arterial blood flow and the
arterial oxygen content
.
Because arterial blood
is normally near its maximal oxygen capac-
ity (>95% saturated), oxygen delivery to a
tissue can only be enhanced by increasing
blood flow. On the other hand, oxygen deliv-
ery can be reduced by decreasing either flow
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