RSPT 1207 Cardiopulmonary Anatomy & Physiology
UNIT 5.2 lecture notes
CARBON DIOXIDE TRANSPORT


C02 is carried in the blood in the plasma and in the RBC. Of these two routes, each carries C02 in 3 ways...so C02 is carried in the blood in 6 ways.



PLASMA CO2

C02 is carried in the plasma in three ways:

1. Bound to a plasma protein as CARBAMINO COMPOUND.  This is insignificant amount, only 1%,  and it  cannot be measured.

2. C02 is dissolved as a gas in the plasma PaC02 in artery or PvC02 in vein .  It exerts a pressure which can be measured. This amount is significant.
 

3. C02 is carried in the blood plasma as bicarbonate. This amount is significant because the relationship between carbonic acid to bicarbonate determines the pH of the blood.

The C02 HYDROLYSIS into C02 + H20,

which in turn, can become H2C03 (carbonic acid.)

This reaction can continue to go forward into HC03 - and H+

or it could go back to H20 and C02.


C02   +  H20  [turns into]  H2CO3  [turns into]   HC03-  +  H+

Which direction the reaction goes toward is depends on the accumulation of substances on the ends of the equation.

When there is more C02 [on the left of the equation], the reaction moves to the right toward creation of more H+.

But if there is more H+ [on the right side], the reaction moves to the left so that more C02 is created.

The practical results of this attempt to maintain balance is that C02 does ‘create’ acid [H+ ] but also it means that as H+ accumulates and the pH become acidic, this reaction results eventually in increased C02 which is a volatile acid…that can be removed by breathing deeper and faster.



HYDROLYSIS:


When some salts (C02) are placed in water, the molecule is split up. Carbon dioxide hydrolysis into carbonic acid, and because carbonic acid is so unstable it  returns to water and C02--- or it can turn into bicarbonate and H+.

 The ratio of carbonic acid to bicarbonate determines the Ph of the blood. The normal ratio of HC03- to carbonic acid is  20 : 1. This will result in a Ph of 7.40.

 As the C02 in the blood rises, more carbonic acid is created and the ratio changes with rising H2C03… so the Ph drops to more acid levels.

 As the blood C02 drops, the ratio again changes with H2C03 dropping also, so that the pH rises to more alkalotic levels.

 This relationship is the basis of the HENDERSON-HASSELBALCH EQUATION:

Ph = 6.1 + the log of   [ HC03- ]                                                      pH=       6.1 + log of [HC03-]
                                              H2C03                       or                                                                (PaC02 X .03)

The Henderson/Hasselbalch equation describes the relationship between the pH of the blood and the balance of bicarbonate and carbonic acid. As HC03- rises the pH rises.  As H2C03 rises, the Ph drops.



PaC02 AND pH

While the carbonic acid molecules and the H+   are not actually measured, the PaC02 is measurable.
 

For every 1000 molecules of C02 dissolved as a gas [as PaC02] there is only one molecule which is hydrolyzed. Because this relationship between PaC02 and H2C03  is so stable, the PaC02  can be measured and the carbonic acid can be derived from the PaC02.

So H2CO3 = PaC02 x 0.03.


CARBON DIOXIDE CARRIED IN THE RED BLOOD CELL

The C02 is carried in the RBC in three compartments:
 

1. C02 is dissolved in the water of the RBC. This exerts a pressure but it insignificant and basically unmeasurable.

2. C02 binds to reduced hemoglobin to form CARBAMINO HEMOGLOBIN. The C02 binds to the protein portion of the Hb, rather than to the iron as 02 does.

3. C02 is carried in the RBC's plasma as CARBONIC ACID. This compartment carries so much C02 because the enzyme CARBONIC ANHYDRASE.
 

 Carbonic Anhydrase quickens the hydrolysis of C02 into H2CO3 by 13,000 times.  As the C02 rises in the RBC,  the carbonic acid rises and the RBC'S intracellular pH drops, so that Hb affinity for 02 decreases.

Remember, the Hb needs a trigger to let go of the 02.


CHLORIDE SHIFT

 Because intracellular carbonic acid is rising so fast, it immediately converts into HC03- and H+. These  levels rise quickly and the reduced hemoglobin combines with the H+(HHb) (thus buffers it).

 

The HC03 - is transported out of the cell so fast that a electrostatic gradient exists between the inside of the RBC and  the blood plasma..so Cl- enter the cell. This is called the CHLORIDE SHIFT or THE HAMBURGER EFFECT.


HALDANE EFFECT

 The C02 dissociation curve is linear, but the Hb C02 relationship can be altered.

The level of Sa02 of the Hb affects the ability of the hemoglobin to carry C02. The less 02 on the Hb,  the quicker the C02 loads up.

The affinity for 02 of the reduced hemoglobin is less because of the decreased pH, but the presence of C02 bound to the globulin portion of the Hb will also alter the 02 affinity. This is called the HALDANE EFFECT.

So when pH (or C02) affects the 02 curve, it is due to the Borh effect.  When the Hb02 affects the ability of the Hb to hold C02, it is due to the Haldane Effect.

The amount of C02 carried in the blood as:

Bicarbonate is                       80%
carbamino-compounds is     12%
dissolved                                   8%
 


CLINICAL USE FOR 02 AND CO2 TRANSPORTATION

 When looking at Arterial blood gases, you assess the patient’s ability to transport gases and to maintain Acid/Base

 

1.         You ASSESS THE ABILITY to oxygenate the body based on the Pa02 and the Sa02. Calculations of the P (A-a) D02 and of the a/A ratio and of the Ca02 will help to determine the adequacy of oxygenation.

2.          When one looks at the PaC02, and the pH, one is assessing the ability of the body to ventilate properly. The major stimulation to breathe is due to the presence of C02 in the cerebral spinal fluid.

3.         The relationship between the HC03- and the Pa02 is viewed  to determine the ability of the lungs and the kidneys to maintain acid base balance.