NMWH
07-21-2010, 10:24 AM
Oxygen-Hemoglobin Dissociation Curve
demonstrates a progressive increase in the percentage
of hemoglobin bound with oxygen as blood Po2 increases, i
which is called the per cent saturation of hemoglobin. i
Because the blood leaving the lungs and entering the
systemic arteries usually has a Po2 of about 95 mm Hg, one
can see from the dissociation curve that the usual oxygen
saturation of systemic arterial blood averages 97 per cent. i
Conversely, in normal venous blood returning from the
peripheral tissues, the Po2 is about 40 mm Hg, and the
saturation of hemoglobin averages 75 per cent. i
http://www.mda-sy.com/up//uploads/images/mda-sy-73ddc27f43.jpg
Adjustment to the Metabolic Needs of
Individual Tissues
Hemoglobin does not unload the same amount of oxygen to
all tissues. Some tissues need more and some less,
depending on their state of activity. Hemoglobin responds to
such variations and unloads more oxygen to the tissues that
need it most. i
In exercising skeletal muscles, for example, the utilization
coefficient may be as high as 80%. Four factors adjust the
rate of oxygen unloading to the metabolic rates of different
tissues: i
1. Ambient PO2
Since an active tissue consumes oxygen rapidly, the PO2 of
its tissue fluid remains low. From the oxyhemoglobin
dissociation curve , you can see that at a low PO2, HbO2
releases more oxygen.
2. Temperature.
When temperature rises, the oxyhemoglobin dissociation
curve shifts to the right; in other words, elevated
temperature promotes oxygen unloading. Active tissues are
warmer than less active ones and thus extract more oxygen
from the blood passing through them.
3. The Bohr effect.
Active tissues also generate extra CO2, which raises the H+
concentration and lowers the pH of the blood. Like elevated
temperatures, a drop in pH shifts the oxygen-hemoglobin
dissociation curve to the right and promotes oxygen
unloading. The increase in HbO2 dissociation in
response to low pH is called the Bohr effect. It is less
pronounced at the high PO2 present in the lungs, so pH has
relatively little effect on pulmonary oxygen loading. In the
systemic capillaries, however, PO2 is lower and the Bohr
effect is more pronounced. i
4. BPG.
Erythrocytes have no mitochondria and meet their energy
needs solely by anaerobic fermentation. One of their
metabolic intermediates is bisphosphoglycerate (BPG), i
which binds to hemoglobin and promotes oxygen unloading. i
An elevated body temperature (as in fever) stimulates BPG
synthesis, as do thyroxine, growth hormone, testosterone, i
and epinephrine. All of these hormones thus promote oxygen
unloading to the tissues.
http://www.mda-sy.com/up//uploads/images/mda-sy-d9bf6e674a.jpg
demonstrates a progressive increase in the percentage
of hemoglobin bound with oxygen as blood Po2 increases, i
which is called the per cent saturation of hemoglobin. i
Because the blood leaving the lungs and entering the
systemic arteries usually has a Po2 of about 95 mm Hg, one
can see from the dissociation curve that the usual oxygen
saturation of systemic arterial blood averages 97 per cent. i
Conversely, in normal venous blood returning from the
peripheral tissues, the Po2 is about 40 mm Hg, and the
saturation of hemoglobin averages 75 per cent. i
http://www.mda-sy.com/up//uploads/images/mda-sy-73ddc27f43.jpg
Adjustment to the Metabolic Needs of
Individual Tissues
Hemoglobin does not unload the same amount of oxygen to
all tissues. Some tissues need more and some less,
depending on their state of activity. Hemoglobin responds to
such variations and unloads more oxygen to the tissues that
need it most. i
In exercising skeletal muscles, for example, the utilization
coefficient may be as high as 80%. Four factors adjust the
rate of oxygen unloading to the metabolic rates of different
tissues: i
1. Ambient PO2
Since an active tissue consumes oxygen rapidly, the PO2 of
its tissue fluid remains low. From the oxyhemoglobin
dissociation curve , you can see that at a low PO2, HbO2
releases more oxygen.
2. Temperature.
When temperature rises, the oxyhemoglobin dissociation
curve shifts to the right; in other words, elevated
temperature promotes oxygen unloading. Active tissues are
warmer than less active ones and thus extract more oxygen
from the blood passing through them.
3. The Bohr effect.
Active tissues also generate extra CO2, which raises the H+
concentration and lowers the pH of the blood. Like elevated
temperatures, a drop in pH shifts the oxygen-hemoglobin
dissociation curve to the right and promotes oxygen
unloading. The increase in HbO2 dissociation in
response to low pH is called the Bohr effect. It is less
pronounced at the high PO2 present in the lungs, so pH has
relatively little effect on pulmonary oxygen loading. In the
systemic capillaries, however, PO2 is lower and the Bohr
effect is more pronounced. i
4. BPG.
Erythrocytes have no mitochondria and meet their energy
needs solely by anaerobic fermentation. One of their
metabolic intermediates is bisphosphoglycerate (BPG), i
which binds to hemoglobin and promotes oxygen unloading. i
An elevated body temperature (as in fever) stimulates BPG
synthesis, as do thyroxine, growth hormone, testosterone, i
and epinephrine. All of these hormones thus promote oxygen
unloading to the tissues.
http://www.mda-sy.com/up//uploads/images/mda-sy-d9bf6e674a.jpg