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 PO2 of 160, and alveoli have a PO2 is 104, oxygen easily makes it's way through the air passages to the alveoli. Similarly, venous capillary blood has a PO2 of 40, so oxygen easily diffuses across the respiratory membrane to the capillary system. Capillary PO2 is now 104.
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 alveolar PO2 is usually 104 under normal conditions, reduced hemoglobin 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 normal is 95-98%.
Hemoglobin does not affect the partial pressure of oxygen, and so the PaO2 is 104. 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. As more oxygen molecules enter the capillary bloodstream, this increases the PO2, and therefore increases hemoglobin's affinity for oxygen. As the PO2 increases, more and more oxygen molecules bind with hemoglobin until hemoglobin becomes completely saturated.
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 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 disease conditions can shift this curve to the right or to the left, and this affects hemoglobin's affinity for oxygen, and this will have a direct affect on a patient's SpO2 and PO2.
Certain conditions cause the hemoglobin to pick up more oxygen from the blood stream, and certain conditions cause the hemoglobin to release oxygen to the blood stream. These events, in effect, will cause the curve to shift to the right or left.
A curve shifts to the right when the hemoglobin has a decreased affinity for oxygen, and has a "harder" time making the bond with oxygen. This decreases hemoglobin's affinity for, allowing oxygen to leave hemoglobin to 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 (we'll describe this below)
A curve shifts to the left when the 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)
- CO2 poisoning
- Alkalotic (tissues cold)
- Decreased 2.3 DPG
- Fetal Hemoglobin (fetus needs less oxygen and can live off lower PO2s)
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)
- Cystic Fibrosis
- Congenital heart diseases
- Anything that increases metabolism (HEAT), such as acidosis, exercise, fever, etc.
- 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.