32
Dynamics
of
the
Vascular System
in the partial
pressure of oxygen, hence a reduced
saturation
of
hemoglobin. Thus, the oxygen transport system is ideally designed to
perform the tasks
of
on-demand metabolic adjustments. This is even
better
illustrated when oxygen
is
transferred
from hemoglobin to
myoglobin during greater muscle tissue demand.
The affinity
of
myoglobin for oxygen
is
significantly greater than that
of
hemoglobin.
0
Arterial
Fig.
2.3.1
:
Oxygen saturation curve displaying
its
S-shaped characteristics. Normal
arterial and venous blood 02 saturations are
also
indicated.
The oxygen affinity decreases with decreasing pH. This is termed the
Bohr effect by which changes in blood
PCO2
which affects blood pH,
indirectly also influence hemoglobin-oxygen affinity.
Hemoglobin consists of four polypeptide chains or globins and four
disc-shaped molecular ring
or
heme
groups, allowing binding of four
oxygen molecules.
Once bound with oxygen, the iron atoms in
hemoglobin gives it the red color.
Optical absorptions of hemoglobin
and oxy-hemoglobin (Fig.
2.3.2)
can be readily monitored by near-
infrared spectroscopy. The isobestic point, when the
two
absorptions are
equal which can be used as a reference, is at
805
nm.
