Key Points to the Curve

Oxyheamoglobin Dissociation Curve

Key Points to the curve:

1. Shows how much oxygen our blood is carrying at different pressures…so how much oxygen can bind with hemoglobin and become saturated (1 hemoglobin with 4 oxygen molecules is fully saturated).

2. The flat portion of the curve:

Represents how much oxygen is bound to hemoglobin at high partial pressures. Each hemoglobin molecule is saturated with 4 oxygen molecules. So at rest, the oxygen molecules remain bound to the hemoglobin molecule because the partial pressure is high enough to keep the bonds between the 2 molecules. At these high partial pressures, hemoglobin’s affinity for oxygen is still high, which means that hemoglobin will attract lots of oxygen and bind with it. This is good because at rest, our muscles do not need the oxygen to be offloaded into the tissues, because the energy requirements at rest are low.

3. The flat portion means that even if the partial pressure of oxygen drops from 100mmHg to 60mmHg, it will not change the amount of hemoglobin that is saturated with oxygen... i.e. hemoglobin will still be highly saturated with oxygen (e.g. 95%). So the amount of oxygen in our blood will not change much even though there is a large drop in partial pressure. This is good for our bodies because it allows us to keep our oxygen levels fairly constant at high altitudes (e.g. at the top of Mount Everest). If we cannot keep our hemoglobin saturated with oxygen at high altitudes, then we would die because our body would have no oxygen. 4. The steep portion of the curve:

When the partial pressure of oxygen drops below 40mmHg, the amount of oxygen that will bind with hemoglobin is drastically reduced. This leaves hemoglobin’s affinity for oxygen drastically low which means that not a lot of oxygen will stay bound with hemoglobin. If oxygen cannot stay bound to hemoglobin, it will be unloaded into the surrounding tissues. So in the steep portion, even a small change in the partial pressure of oxygen will result in a large amount of oxygen that is released (unloaded) from hemoglobin. This is very good because during exercise, when our active muscle tissues require large amount of oxygen for energy, then hemoglobin will release (unload) the oxygen into our tissues to meet the high oxygen demands.

5. Therefore our bodies will change and adapt when there are differences in partial pressure to release oxygen or keep it bound with hemoglobin, depending on our needs for oxygen.

6. Factors that affect the curve:

A rightward shift indicates that the hemoglobin has a decreased affinity for oxygen. This makes it more difficult for hemoglobin to bind to oxygen (requiring a higher partial pressure of oxygen to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release oxygen bound to it. The effect of this rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, e.g. during exercise.

The curve is shifted to the left by the opposite of these conditions. This leftward shift indicates that the hemoglobin has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly. Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (e.g. at the lungs).

Changes in pH – if the pH of our blood changes (becomes more acidic or more alkaline), then it will shift the curve to the right. This means that oxygen will be released from hemoglobin much quicker (as the steep portion begins at higher partial pressures). For example, when you are exercising your body produces lactic acid. The lactic acid lowers your blood pH, causing more oxygen to be released from hemoglobin and unloaded into the exercising tissues.

Temperature – As temperature increases, the curve shifts to the right. The increased body temperature allows more oxygen to be released to meet the oxygen requirements while exercising. As we continue exercise, more ATP is needed to replace the energy that has been used and lost as heat.

Carbon Dioxide – An increase in the partial pressure of carbon dioxide (PCO2), will also cause the curve to shift to the right. Accumulation of CO2 in the blood will lead to acidosis and lower the pH of the blood. This in turn, will shift the curve to the right as would a decreased pH.

Left shift Right shift

(high affinity for O2) (low affinity for O2)

Temperature decrease increase

p(CO2) decrease increase

pH (Bohr effect) increase (alkalosis) decrease (acidosis)