O2 TRANSPORT IN BLOOD
Despite the relatively low solubility of O2
in plasma (O2
is 20 times
less soluble than CO2
), the volume of gaseous O2 per liter of blood
remarkably is almost the same as that in air. This is possible because
red blood cells contain a large quantity of the O2
-binding protein
hemoglobin.
O2
is carried in two forms:
- 98% of O2
in blood is carried bound to hemoglobin within red blood
cells.
- Only 2% of O2
in arterial blood is present as dissolved O2
. At an
arterial PO2 of 100 mm Hg, only 0.3 mL of O2
is dissolved in every
100 mL of blood.
Hemoglobin is the red-colored iron-containing oxygen-transport metalloprotein.
Hemoglobin can be saturated with oxygen molecules (oxyhemoglobin, Hb04
), or
desaturated with oxygen molecules (deoxyhemoglobin, Hb).
The process of hemoglobin
saturation is described by the
oxyhemoglobin dissociation
curve.
This curve relates oxygen
saturation (SO2
) and partial
pressure of oxygen in the blood
(PO2
), and is determined by what
is called "hemoglobin affinity for
oxygen"; that is, how readily
hemoglobin acquires and releases
oxygen molecules into the fluid
that surrounds it.
Figure shows the normal
relationship between percent
hemoglobin saturation and PO2
.
In healthy lungs with an alveolar
partial pressure of O2 of 100 mm
Hg, pulmonary capillary blood is
100% saturated with O2
. When
PO2 decreases below 60 mm Hg,
there is a rapid decline in
hemoglobin saturation.
A left shift in the oxyhemoglobin
dissociation curve causes hemoglobin to
become saturated at lower PO2
.
Therefore, loading O2
into blood at the
lung is easier, but it is more difficult to
unload O2 at the tissues.
If the curve shifts right, O2
loading at the
lungs is reduced and the proportion of
hemoglobin saturated with O2 may be
reduced, but O2
is unloaded more
readily at the tissues.
Carbon monoxide (CO) is a product of incomplete
combustion and is a colorless, odorless, tasteless,
nonirritating gas. CO is a poisonous gas because it
binds irreversibly to hemoglobin and prevents O2
binding. It binds to hemoglobin with very high affinity
to produce carboxyhemoglobin.
CO poisoning occurs when a person inspires toxic
levels of CO, which can result in brain damage or even
death. Symptoms are very nonspecific and often
mimic viral illness, with headache, malaise, nausea,
vomiting, and drowsiness.
This curve relates oxygen
saturation (SO2
) and partial
pressure of oxygen in the blood
(PO2
), and is determined by what
is called "hemoglobin affinity for
oxygen"; that is, how readily
hemoglobin acquires and releases
oxygen molecules into the fluid
that surrounds it.
A left shift in the oxyhemoglobin
dissociation curve causes hemoglobin to
become saturated at lower PO2
.
Therefore, loading O2
into blood at the
lung is easier, but it is more difficult to
unload O2 at the tissues. 
CO poisoning occurs when a person inspires toxic
levels of CO, which can result in brain damage or even
death. Symptoms are very nonspecific and often
mimic viral illness, with headache, malaise, nausea,
vomiting, and drowsiness.
