Hemoglobin Oxygen Saturation Simulation

The Effects of Altitude

What The Oxyhemoglobin Dissociation Curve Represents

The oxyhemoglobin dissociation curve illustrates the relationship between the arterial partial pressure of oxygen (PaO₂) and the levels of hemoglobin oxygen saturation.

:: Oxygen Loading in the Lungs

The left side of the curve is relatively flattened compared to its middle portion, indicating that a relatively significant increase in PaO₂ is needed to bind the first oxygen molecule. This occurs because unsaturated hemoglobin is in a tense (T) conformational state and has a low affinity for oxygen.

The middle portion of the curve is the steepest, indicating that relatively small increases in PaO₂ are needed to saturate the hemoglobin molecules further.

The binding of the first oxygen molecule to hemoglobin, which results in 25% saturation, shifts the molecule to a relaxed (R) state. This change in shape increases hemoglobin’s affinity for oxygen, facilitating the binding of the second (50% saturation) and third (75% saturation) oxygen molecules. This phenomenon is referred to as cooperative binding.

The right portion of the oxyhemoglobin dissociation curve is the flattest. With only one binding site available, a relatively large increase in PaO₂ is required to bind the fourth oxygen to hemoglobin, making it 100% saturated.

Summary: When no O₂ molecules are bound to hemoglobin, it assumes a tensed (T) shape, which has a low affinity for O₂. While in this state, it requires a significant increase in PaO₂ to bind the first O₂. This binding triggers a conformational change in the hemoglobin molecule, causing it to shift to a relaxed (R) state. This conformational state increases hemoglobin’s affinity for oxygen, making it easier for the second and third O₂ molecules to bind. However, binding the fourth O₂ molecule to hemoglobin is the most difficult, requiring the largest increase in PaO_2.

:: Oxygen Unloading in the Tissues

The PaO₂ near the tissues drops significantly because tissue cells remove the O₂ from the extracellular environment to produce ATP. Under resting conditions, the PaO₂ is ~40 mmHg, which causes hemoglobin to release the fourth (last-loaded) O₂.

Tissue cells consume more O₂ as their activity increases, reducing the PaO₂. The lower oxygen pressure causes the second and third oxygen molecules to readily unbind from hemoglobin, as indicated in the steep portion of the oxyhemoglobin dissociation curve. This ensures efficient oxygen unloading, making more O₂ available to the active cells.

:: Other Affecting Factors

In addition to the partial pressure of oxygen (PaO₂), hemoglobin’s affinity for oxygen (O₂) is influenced by conditions that shift the oxyhemoglobin dissociation curve to the right or left, making it easier or more difficult for hemoglobin to bind O₂. Three common factors are:

• Fluctuating levels of pH and/or CO₂ (Bohr Effect) resulting from changes in cell activity.
• Variations in temperature due to changes in cell activity, environmental conditions, or fever.
• Changing amounts of 2,3-DPG (2,3-diphosphoglycerate) in red blood cells caused by hypoxemia or anemia.

Assessment

Oxyhemoglobin Dissociation Curve Simulator

Use this activity to create content.

1. Use the oxyhemoglobin dissociation curve simulator to determine the arterial O₂ pressure (PaO₂) needed to load the first O₂ (25% saturation) onto hemoglobin.
2. Determine how much PaO₂ is needed to load the second O₂ (50% saturation) and third O₂ (75% saturation) onto hemoglobin.
3. Explain why less pressure is needed to load the second and third O₂ molecules than the first O₂ molecule.
4. How much pressure is needed to load the fourth O₂ molecule (100% saturation)?
5. How many O₂ molecules will be unloaded from hemoglobin if the tissue cells are in a resting state and the PaO₂ is 40 mmHg?
6. How many O₂ molecules will be unloaded from hemoglobin molecules if the tissue cells become active and the PaO₂ drops to ~ 18 mmHg?
7. How do the acids and CO₂ released from active tissue cells affect hemoglobin’s affinity for oxygen?
8. How would the increase in acids and CO₂ affect the oxyhemoglobin dissociation curve?
9. How would this situation affect oxygen delivery to the tissues?
10. How does a decrease in PaO₂ caused by exposure to high altitude affect the oxyhemoglobin dissociation curve?
11. How does this situation affect hemoglobin’s O₂ affinity and the delivery of O₂ to the tissue cells?

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