Carbon Dioxide

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    image showing the three ways carbon dioxide travels in the bloodYou already began to learn how CO2 travels through the blood.  There are three main ways, and they are, in order from most used to least used:

1.  as bicarbonate ions
2.  as carbamino- hemoglobin
3.  dissolved in plasma

These are all depicted in this figure from your book.  I will now discuss each of these ways in a little more detail.

Bicarbonate Ions

    The majority (approximately 70%) of carbon dioxide is carried in the blood this way.

    Most of the time that carbon dioxide encounters water, it undergoes a chemical reaction with the water.  This chemical reaction is:

CO2 + H2O -----> HCO3- + H+

If you count, you'll see that this equation is balanced (only one C atom on each side, 3 O atoms on each side, and 2 H atoms on each side).  HCO3- is the bicarbonate ion.  The H+ is a regular hydrogen ion.  If the hydrogen ion is left to hang out in the plasma, this would cause problems.  You see, any increase in hydrogen ions leads to a decrease in pH, and so the plasma would get more acidic.  This is not a problem, however, because the hydrogen ion readily associates with hemoglobin; therefore it is not floating around affecting the pH.

    That means that when carbon dioxide and water combine, their main product is the bicarbonate ion.  This is rather handy for us.  The bicarbonate ion acts as a buffer for our blood, maintaining blood pH at approximately 7.4.  I'll explain how this works some more when we get to Chapter 21... so review your pH information to be ready for that chapter!

    It is extremely useful that since we have to transport carbon dioxide anyway, we have a way to use it while it is being transported!  We use it to maintain our blood pH.  Pretty cool, huh?

    As I noted in the oxygen section, transport is only good if we can release the transported material.  Remember how this was NOT the case for carbon monoxide?  The transport of carbon monoxide served us in no way.  Well, we are able to release carbon dioxide by reversing the chemical reaction listed above.

    Again, whether we transport CO2 or not depends on the partial pressures of CO2.  When the partial pressure of carbon dioxide is higher in the tissues than in the blood, the reaction proceeds as drawn above, and bicarbonate ions are formed.  However, when the partial pressure of carbon dioxide is higher in the blood than outside of the blood (as is the case in the lungs), the equation reverses, and bicarbonate ions recombine with hydrogen ions to release carbon dioxide (and water).

Carbaminohemoglobin

    15% - 25% of carbon dioxide is carried this way.

    CO2 can also attach to hemoglobin.  However, it does not attach to the iron within the heme group like oxygen and carbon monoxide do.   Instead, it attaches to the globin protein itself.  Therefore, one hemoglobin molecule can carry 4 oxygen and 4 carbon dioxide molecules at the same time!  They do not interfere with one another.

    As described for the carbon dioxide - to - bicarbonate transition, the carbaminohemoglobin - to - carbon dioxide - plus - hemoglobin transition also depends on the partial pressures of carbon dioxide.  When the partial pressure of CO2 in the tissues is higher than in the blood, carbaminohemoglobin forms, while if the partial pressure of CO2 in the blood is higher, carbaminohemoglobin releases its CO2.

Dissolved in plasma

    Only 7% of carbon dioxide is carried this way.

    This is a rather inefficient way to carry carbon dioxide, but it does occur.  Carbon dioxide merely diffuses from one tissue to another based solely on the pressure gradient of carbon dioxide partial pressures.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.