Key Difference – PAO2 vs SAO2
The transport of oxygen (O2) by blood in the arteries is a critical process and is governed by many factors such as pH of the blood, partial pressures of the gases in blood, saturation levels of O2, concentration of available hemoglobin and cardiac efficiency. The balance of these factors will assure the efficient transport of O2 to peripheral tissue based on the requirement of the particular tissue. The partial pressure and the saturation of O2 are two very important parameters which determine healthy transport of O2 in blood that is characterized by the Oxygen-Hemoglobin dissociation curve which depicts the saturation of hemoglobin with O2, the partial pressure and the concentration of O2 in blood. The partial pressure of O2 (PAO2) is the pressure exerted by O2 on the arterial walls while saturation of O2 (SAO2) is the overall percentage of hemoglobin binding sites occupied by O2. This is the key difference between PAO2 and SAO2.
CONTENTS
1. Overview and Key Difference
2. What is PAO2
3. What is SAO2
4. Similarities Between PAO2 and SAO2
5. Side by Side Comparison – PAO2 vs SAO2 in Tabular Form
6. Summary
What is PAO2?
Partial Pressure is defined by Dalton’s law of Partial pressures, where it is stated that the total pressure of a system is equal to the sum of the individual pressures exerted by the gases present in the mixture. The partial pressures of dissolved gases in blood are measured by assuming that the blood was allowed to equilibrate with a volume of gas. Thus, Partial Pressure of O2 (PAO2) also known as the O2 tension in blood, is the pressure exerted by O2 on the arterial wall. It is important to note that O2 in blood is dissolved in a mixture of other gases such as carbon dioxide and carbon monoxide, but O2 is the only gas that exerts a pressure on the arterial wall.
When the concentration of O2 in the blood is higher, the PAO2 also rises, allowing the blood to carry higher amounts of O2 in comparison with other fluids such as water. Measuring and recording PAO2 is important during disease states because there are certain physiological processes that depend on changes in O2 in their microenvironments which are characterized by the changes in PAO2.
What is SAO2?
Saturation of O2 (SAO2) in blood defines the percentage of hemoglobin binding sites that are occupied by O2. Each hemoglobin molecule can occupy four O2 molecules as it can allosterically modify its conformation to facilitate binding of O2 to its binding site. During 100% saturation, all hemoglobin binding sites are occupied by O2, and any increase in partial pressure or the concentration of O2 in blood would not cause an increase in saturation. This is depicted by the plateau area of the oxygen-hemoglobin dissociation curve. This saturation pattern is the reason for the characteristic sigmoid shaped curve of the O2 – Hemoglobin curve.
Figure 01: Oxygen-Hemoglobin dissociation curve
What are the Similarities Between PAO2 and SAO2?
- PAO2 and SAO2 depend on the concentration of O2 present in the blood and lungs.
- Both parameters can be used as indicators to suggest imbalances of hemoglobin, O2, cardiac efficiency and respiratory efficiency.
- PAO2 and SAO2 are directly proportional until O2 reaches its maximum saturation.
What is the Difference Between PAO2 and SAO2?
PAO2 vs SAO2 |
|
| PAO2 is the pressure exerted by O2 on the arterial wall. | SAO2 is the percentage of hemoglobin binding sites that are occupied by O2. |
| Units of Expression | |
| PAO2 is expressed in Pascal (pressure measuring units). | SAO2 is expressed as a percentage. |
| Depending Factor | |
| Dissolved O2 concentration affects the PAO2. | The number of available O2 binding sites and the PAO2 affect the SAO2. |
Summary – PAO2 vs SAO2
PAO2 and SAO2 define cardiac efficiency and are considered as markers to assess the metabolic conditions of the lungs and heart in terms of Oxygen levels. PAO2 is the pressure exerted by O2 on the arterial wall. SAO2 is the percentage of hemoglobin binding sites that are occupied with O2. This is the main difference between PAO2 and SAO2. The normal PAO2 of a healthy person should lie above 17 kPa or 128 mmHg which will result in a 100% SAO2 whereas the normal SAO2 is greater than 90%. Deviations of these levels act as markers and are important in analyzing the abnormalities in hemoglobins and Carbon monoxide poisoning.
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References:
1.Collins, Julie-Ann, et al. “Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve.” Breathe, European Respiratory Society, Sept. 2015, Available here. Accessed 21 Aug. 2017.
Image Courtesy:
1.” Oxyhaemoglobin dissociation curve” By Ratznium at English Wikipedia Later versions were uploaded by Aaronsharpe at en.wikipedia. – Transferred from en.wikipedia to Commons. (Public Domain) via Commons Wikimedia
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