Blood serves to transport the respiratory gases. Oxygen, which is essential for the cells is transported from alveoli of lungs to the cells. Carbon dioxide, which is the waste product in cells is transported from cells to lungs.
TRANSPORT OF OXYGEN
Oxygen is transported from alveoli to
the tissue by blood in two forms:
1. As simple physical solution
2. In combination with hemoglobin.
AS SIMPLE SOLUTION
Oxygen dissolves in water of plasma
and is transported in this physical form. Amount of oxygen transported in this way is
very negligible. It is only 0.3 mL/100 mL of plasma. It forms only about 3% of
total oxygen in blood. It is because of poor solubility of oxygen in water
content of plasma. Still, transport of oxygen in this form becomes important
during the conditions like muscular exercise to meet the excess demand of oxygen
by the tissues.
IN COMBINATION WITH
HEMOGLOBIN
Oxygen combines with hemoglobin in
blood and is transported as oxyhemoglobin. Transport of oxygen
in this form is important because, maximum
amount (97%) of oxygen is transported by this method.
Oxygenation of
Hemoglobin
Oxygen combines with hemoglobin only
as a physical combination. It is only oxygenation and not
oxidation. This type of
combination of oxygen with hemoglobin has got some advantages. Oxygen can be
readily released from hemoglobin when
it is needed. Hemoglobin accepts oxygen readily whenever the partial pressure
of oxygen in the blood is more. Hemoglobin gives out oxygen whenever the
partial pressure of oxygen in the blood is less. Oxygen combines with the iron
in heme part of hemoglobin. Each molecule of hemoglobin contains
4 atoms
of iron. Iron of the hemoglobin is present in ferrous form.
Each iron atom combines with one molecule of oxygen. After combination, iron
remains in ferrous form only. That is why the combination of oxygen with hemoglobin
is called oxygenation and not oxidation.
Oxygen Carrying
Capacity of Hemoglobin
Oxygen carrying capacity of hemoglobin
is the amount of oxygen transported by 1 gram of hemoglobin. It is 1.34 mL/g.
Oxygen Carrying
Capacity of Blood
Oxygen carrying capacity of blood
refers to the amount of oxygen transported by blood. Normal hemoglobin content
in blood is 15 g%. Since oxygen carrying capacity of hemoglobin is
1.34 mL/g, blood with 15 g% of
hemoglobin should carry 20.1 mL% of oxygen, i.e. 20.1 mL of oxygen in 100 mL of
blood.
But, blood with 15 g% of hemoglobin
carries only 19 mL% of oxygen, i.e. 19 mL of oxygen is carried by 100 mL of
blood. Oxygen carrying capacity of blood is only 19 mL% because the hemoglobin
is not
fully saturated with oxygen. It is
saturated only for about 95%.
Saturation of
Hemoglobin with Oxygen
Saturation is the state or condition
when hemoglobin is unable to hold or carry any more oxygen. Saturation of
hemoglobin with oxygen depends upon partial pressure of oxygen. And it is
explained by oxygenhemoglobin dissociation curve.
OXYGEN-HEMOGLOBIN DISSOCIATION
CURVE
Oxygen-hemoglobin dissociation curve
is the curve that demonstrates the relationship between partial
pressure of oxygen and the percentage
saturation of hemoglobin with oxygen. It explains hemoglobin’s
affinity for oxygen. Normally in the
blood, hemoglobin is saturatedwith oxygen only up to 95%. Saturation of
hemoglobin with
oxygen depends upon the partial pressure of
oxygen. When the partial pressure of
oxygen is more hemoglobin accepts oxygen and when the partial pressure of
oxygen is less, hemoglobin releases oxygen.
Method to Plot
Oxygen-hemoglobin
Dissociation Curve
Ten flasks or
tonometers are taken. Each one is filled with a known quantity of blood with
known
concentration of hemoglobin. Blood in
each tonometer is exposed to oxygen at different partial pressures.
Tonometer is rotated at a constant
temperature till the blood takes as much of oxygen as it can. Then, blood is
analyzed to measure the percentage saturation of hemoglobin with oxygen.
Partial pressure of oxygen and saturation of hemoglobin are plotted to obtain
the oxygen-hemoglobin dissociation curve.
Normal
Oxygen-hemoglobin Dissociation Curve
Under normal conditions,
oxygen-hemoglobin dissociation curve is ‘S’ shaped or sigmoid shaped.
Lower part of the curve indicates
dissociation of oxygen from hemoglobin. Upper part of the curve indicates the
uptake of oxygen by hemoglobin depending upon partial pressure of oxygen.
P50
P50 is the partial pressure of oxygen at which
hemoglobin saturation with oxygen is 50%. When the partial pressure of oxygen
is 25 to 27 mm Hg, the hemoglobin is saturated to about 50%. That is, the blood
contains 50% of oxygen. At 40 mm Hg of partial pressure of oxygen, the
saturation is 75%. It becomes 95% when the partial pressure of oxygen is 100 mm
Hg.
Factors Affecting
Oxygen-hemoglobin
Dissociation Curve
Oxygen-hemoglobin dissociation curve
is shifted to left or right by various factors:
1. Shift to left indicates acceptance (association) of oxygen by
hemoglobin
2. Shift to right indicates dissociation of oxygen from hemoglobin.
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