Biomedical Engineering Reference
In-Depth Information
hemoglobin becomes oxygenated, it is termed as oxyhemoglobin (HbO
2
). Each red blood
cell carries over 250 million hemoglobin molecules. Oxygen-hemoglobin binding is a
reversible process that can be described by kinetic rate constants that are dependent on
the partial pressure of oxygen within the blood. The hemoglobin-oxygen reaction can be
defined with the following kinetic reaction:
HbO
2
As the oxygen partial pressure increases, the rate of oxygen-hemoglobin binding increases,
such that the equilibrium is shifted to favor more oxyhemoglobin. This is typically quanti-
fied by the percent saturation of hemoglobin. At a normal alveolar oxygen partial pressure
of approximately 100 mmHg, nearly all of the hemoglobin in the blood becomes saturated
with oxygen (approximately 98%). In tissues surrounding systemic capillaries, the partial
pressure of oxygen is close to 40 mmHg, which would lead to nearly 75% hemoglobin sat-
uration. Therefore, under normal conditions, hemoglobin remains fairly saturated with
oxygen, even within the capillaries. However, if the tissue is under slightly hypoxic condi-
tions, the percent saturation drops rather quickly to bring the tissue oxygen level back to
normal (at an oxygen partial pressure of 20 mmHg the hemoglobin saturation is close to
35%, meaning that a significant portion of oxygen has dissociated from hemoglobin under
these conditions).
Carbon dioxide transport is slightly more complicated than oxygen transport within the
blood. Upon entering the blood, carbon dioxide can follow one of three pathways at a car-
bon dioxide partial pressure of approximately 45 mmHg, which is commonly found
within the systemic capillaries. (
Figure 9.8
). The first is the conversion of carbon dioxide to
Hb
1
O
2
2
FIGURE 9.8
CO
2
Enters
the blood
5% Remains
dissolved in blood
Pathways for carbon dioxide trans-
portation in blood at a carbon dioxide partial pres-
sure of 45 mmHg. The majority of carbon dioxide is
converted to hydrogen and bicarbonate ions. About
25% of the carbon dioxide binds directly to free
amino groups in the hemoglobin protein. The
remaining carbon dioxide is transported directly
within the blood.
95% Diffuses into
red blood cells
70% Converted to
H
2
CO
3
by
carbonic anhydrase
25% Binds to
hemoglobin
H
+
Binds to
hemoglobin
H
2
CO
3
Dissociates
to H
+
and HCO
3
−
HCO
3
−
enters
blood
Cl
−
enters red
blood cell
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