Vascular Biology, Structure and Function
31
P=C,RT
(2.3.3)
The partial pressure of a gas mixture,
p,,
can be calculated knowing
the molar fraction of the gas,J;, and the total pressure,
P,
i.e.
P,
=f,p
(2.3.4)
When a gas with partial pressure
pi
is in contact with a liquid, some
of
the gas will be dissolved in the liquid. Here we can define the solubility
which
is
related to the concentration
c1
of
the gas, and its partial pressure:
(2.3.5)
Solubility in general is dependent on the total pressure above the liquid
and temperature. Solubility
of
some gases in blood plasma
is
shown in
Table 2.3.1.
Table
2.3.1
Solubility
of
gases
in blood plasma.
Gas
(Solubility in MoladmmHg
02
11.4
x
COZ
co
N2
He
3.3
10.~
7
lo-'
4.8
1.2
x
2.3.2
Oxygen Saturation
Curves
and Hemoglobin
The binding of oxygen with hemoglobin provides an efficient transport
system to deliver and maintain a desirable amount of tissue and organ
oxygenation. The oxygen saturation curve follows an S-shape as shown
in Fig. 2.3.1. At a partial pressure of
100
mmHg, typical in the lungs and
in arteries, hemoglobin is about 97% saturated. In veins and some
tissues, the partial pressure
of
oxygen is about 40 mmHg, here the
saturation decreases to 75% or
so.
Since the slope of the curve changes
greatly at this level, hemoglobin can easily give up its carrying oxygen
readily when the metabolic need arises. This is accompanied by
a
drop
