Calculating the Blood Oxygen Content

         
          Calculating the amount of oxygen carried by the blood takes into account several factors:  the hemoglobin content of the blood, the degree to which the oxygen is bound to the hemoglobin (oxygen saturation), and the amount of oxygen dissolved in the blood based on the PO2.  The formula is as follows: 

     # mL O2 carried by each 100 mL blood = (1.39 X Hgb concentration - O2 Sat/100) + (0.003 X PO2)

             One gram of hemoglobin can carry 1.39 mL of oxygen.  Some sources will cite that hemoglobin can carry 1.34 to 1.36 mL of oxygen per gram based on the assumption that a portion of the oxygen will be bound to methemoglobin rather than hemoglobin.  For simplicities sake, our calculations will use 1.39 mL per gram. 

            The hemoglobin concentration provides us with the amount of hemoglobin the patient has available to bind with oxygen.  It is normally reported in mg/dL.  A deciliter is a tenth of a liter, or 100 mL.  Because the hemoglobin concentration carries the term 100 mL in the denominator, our answer will also have 100 mL in the denominator, meaning that our answer will be in the form 'X number of milliliters of oxygen per 100 milliliters of blood').

            The oxygen saturation is included in the calculation to determine how much of the hemoglobin in the bloodstream is actually carrying oxygen.  For example, one gram of hemoglobin that is 100% saturated with oxygen will carry 1.39 mL of oxygen.  If the oxygen saturation is only 50%, however, it will only carry half as much oxygen.  

If you own PACES version 2.1 and would like 10 FREE CCNA practice questions, you may download the Oxygen Content Calculation expansion pack here.

 

"...a small amount
 of hemoglobin has
 a more dramatic effect than a large amount
 of oxygen..."

         
          Oxygen can also be carried dissolved in the blood.  That raises an interesting question:  if oxygen can be carried in the blood, then why is hemoglobin even important at all?  The answer is that hemoglobin is far more efficient at carrying oxygen.  If you look at the formula, you'll notice that the factor to calculate the amount of oxygen dissolved in the blood is 0.003 mL per 100 mL of blood (at a PO2 of .  Hemoglobin, as we noted, can carry 1.39 mL of oxygen per 100 mL of blood.  In normal conditions, that's over 1000 times more!  Obviously, hemoglobin is crucial in oxygen transport.

            As an example, let's calculate the blood oxygen content for a patient with a hemoglobin of 15 gm/dL, an O2 saturation of 98%, and a PO2 of 105 mmHg.  Our equation would be: 

(1.39 X 15 gm/dL X 98/100) + (0.003 X 105) 
Or 
(20.433) + (0.315)
Or 
20.75 mL O2/dL 

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