Oxyhemoglobin Dissociation Curve made easy

The Oxyhemoglobin Dissociation curve shows how blood carries oxygen through the body.  It also shows the relationship between SpO2 and PaO2 as determined by hemoglobin's affinity for oxygen.

Here is a simple way of understanding the curve.

Fick's law:  According to this law, a gas travels from areas of high pressure to areas of low pressure.

So, since room air has a partial pressure of oxygen (PaO2) of 160, and alveoli have a PaO2 is 104, oxygen easily makes it's way through the air passages to the alveoli. Similarly, venous capillary blood has a PaO2 of 40, so oxygen easily diffuses across the respiratory membrane to the capillary system. Capillary PaO2 now becomes 104, thus becoming part of the arterial circulation.

Affinity of Hemoglobin.  This is a protein present in red blood cells (erythrocytes).  In the capillary system oxygen comes into contact with a reduced hemoglobin, or a hemoglobin that has no oxygen molecule on it. This makes it so the hemoglobin has a high attraction, or high affinity, for oxygen. Oxygen then binds with a hemoglobin

The amount of oxygen bound to hemoglobin at any time is based on the partial pressure of oxygen that it is exposed to.  Since freshly oxygenated capillary blood has a PaO2 of 104 under normal conditions, reduced hemoglobin (hemoglobin that does not have oxygen) has a high affinity for it.

Some oxygen molecules will remain in the plasma, although a majority is picked up and transported by a hemoglobin molecule. The normal percentage of hemoglobin carrying oxygen is referred to as the oxygen saturation (SpO2), and a normal value is generally considered to be in the 95-98%.  However, as you will soon see, 90% or greater can be considered acceptable.

Hemoglobin does not affect the partial pressure of oxygen, and so the PaO2 of arterial blood remains 104. The accepted range of PaO2 is generally considered to be in the 80-100 range.  When a cell uses up it's oxygen molecules, it has a PO2 that ranges somewhere around 22-35. As the hemoglobin approaches this cell with a lower PO2, it releases it's hemoglobin.  The cell on this end is said to have a high affinity for oxygen.

The Oxyhemoglobin Dissociation Curve. Venous blood has a PO2 of 40 and is generally referred to as PvO2.  As more oxygen molecules enter the capillary bloodstream, this increases the PO2, and therefore increases reduced hemoglobin's affinity for oxygen.  It is here where the capillary system converts from being part of the unoxygenated venous system to the oxygenated arterial system.

As the PO2 of capillary blood increases, more and more oxygen molecules bind with hemoglobin until a majority of hemoglobin becomes completely saturated.  A normal SpO2 value here is about 98%, or a range of 95-98%.  An acceptable range is 90%.  The capillary then becomes part of the arterial system, with its PO2 now being referred to as PaO2.

So, the curve has an s-shape because, at lower PO2s, oxygen binds to hemoglobin at a high rate, and this slows down as hemoglobin become more saturated. At PO2s above 60 the curve is relatively flat, meaning that the oxygen content of the blood will not change much with subsequent increases in PO2. In other words, the only way to get more oxygen to tissues would require adding more hemoglobin molecules to the blood, which would require a blood transfusion. Or, another simpler method would be to add more oxygen to the plasma by increasing the fraction of inspired oxygen (FiO2).

4-5-6-7-8-9 Rule.  It is because of this curve that we can use SpO2 to estimate PaO2.

• PO2 40 = SpO2 70%
• PO2 50 = SpO2 80%
• PO2 60 = SpO2 90%

Shifting of the curve.  Certain conditions cause hemoglobin to release more oxygen into the bloodstream, shifting the curve to the right; certain conditions cause hemoglobin to pick up more oxygen from the bloodstream, shifting the curve to the left.  Both of these conditions will have a direct affect on SpO2 and PaO2

1. Shift to the right:  The curve shifts to the right when hemoglobin has a decreased affinity for oxygen, and has a "harder" time making the bond with oxygen.  This decreases hemoglobin's affinity for oxygen, causing it to un-bond with hemoglobin and enter tissues.  
• Lower SpO2 for a given PO2
• Requires a higher PO2 to achieve the desired SpO2
• Hemoglobin more likely to dump oxygen into tissues (active muscles need more oxygen)
• Think Heat. Anything that creates heat will move curve to right. Acidosis or low pH (heat)
• High CO2 causes Acidosis (heat)
• Exercise (heats up body)
• Increased 2.3 DPG (I describe this below)
2. Shift to the left:  The curve shifts to the left when hemoglobin has an increased affinity for oxygen, and has an "easier" time making the bond with oxygen.  
• Higher SpO2 for a given PO2
• Hemoglobin is more likely to cling to O2 and not let go (activity is minimal)
• Think Cold. The colder your body, the slower activity will be.
• Hypothermia (cold tissues)
• Rest (minimal exertion)
• Hypocarbia
• CO2 poisoning
• Alkalotic (tissues cold)
• Decreased 2.3 DPG
• Fetal Hemoglobin (fetus needs less oxygen and can live off lower PO2s)

What is 2.3 DPG? It's a substance in the blood that controls movement of oxygen from the blood to tissues.

1. Increasing 2.3 DPG: This is your bodies way of responding to lack of oxygen.  It lowers hemoglobin's affinity for oxygen, causing hemoglobin to release oxygen into the bloodstream for tissues to use.   This moves the curve to the right. The following conditions cause the body to increase production of 2.3 DPG:

• Anemia (it may take 24 hours after transfusion to replenish supply, and return curve to normal)
• Chronic Obstructive Pulmonary Disease (COPD)
• Cystic Fibrosis
• Congenital heart diseases
• Anything that increases metabolism (HEAT), such as acidosis, exercise, fever, etc.

1. Decreasing 2.3 DPG: This results from lack of DPG enzymes to make 2.3 DPG. The body responds by increasing red blood cells (RBCs) that are weak and burst easy. This moves the curve to the left. When this happens your body will increase 2.3 DPG production to try to move it back to normal. The following conditions cause this:

• Erythrocytosis
• Anemia
• Large blood transfusion

Conclusion.  A bit of a complicated topic.  If you can master it, or even slightly comprehend it, you should better understand how the body carries oxygen, and the relationship between SpO2 and PO2.  If you have comments or questions, or even ideas to make this easier, please leave a comment below (updated 8/11/14)

Further reading:

• Why is normal SpO2 98%?
• Diffusion of oxygen from air to tissues