Oxygen Transport Blood Buffers  Elmhurst College
Carbon Dioxide Transport Respiratory Acidosis Metabolic Acidosis  Chemistry Department
Buffers in the Kidneys Respiratory Alkalosis Metabolic Alkalosis  Virtual ChemBook

 Blood Buffer Equation:

The two equilibriums on the right may be combined into one equation as follows:
CO2 + HOH <===> H2CO3 <===> H+ + HCO3-

Carbon Dioxide Transport

Bicarbonate Buffer:

Carbon dioxide produced in the tissue cells diffuses into the blood plasma. The largest fraction of carbon dioxide diffuses into the red blood cells. The carbon dioxide in the red blood cells is transported as: dissolved CO2, combined with hemoglobin, or as bicarbonate,(largest fraction).

The formation of bicarbonate ions, (HCO3- ) takes place by the following reactions:

Hydration of CO2: CO2 + HOH === H2CO3
Dissociation of H2CO3: H2CO3 === H+ + HCO3-

The H2CO3/HCO3- combination acts as the primary buffer of the blood. The hydration of carbon dioxide is a slow process but occurs rapidly in the red blood cells because a high concentration of the enzyme carbonic anhydrase catalyzes the reaction.

Bicarbonate diffuses out of the red blood cells into the plasma in venous blood and visa versa in arterial blood. Chloride ion always diffuses in an opposite direction of bicarbonate ion in order to maintain a charge balance. This is referred to as the "chloride shift".

The changes in concentration of CO2 or HCO3- ion can influence slight pH changes in the blood even though it is buffered. At the same time the concentration of H+ ions will influence the concentrations of CO2 and HCO3- ions.



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Combined Oxygen and Carbon Dioxide Transport:

The reactions for both oxygen and carbon dioxide are coupled together and work in cooperation with each other. The main reason for this "coupled" effect is that both systems are influenced by hydrogen ions and equilibrium principles.

At the lungs, the diffusion of oxygen into the blood triggers the reactions. The oxygen reacts with and attaches to hemoglobin. This oxygenation reaction with hemoglobin produces excess H+ ions which react with HCO3- to produce H2CO3. The carbonic acid decomposes to CO2 which diffuses out of the blood.

QUES. 1: At the tissue cells, the excess of carbon dioxide triggers the reaction. Describe in your own words the sequence of reactions occurring at the area of the tissue cells until oxygen enters the cells.

 


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Equilibrium at the Tissue Cells:

Example: Use equilibrium principles to explain the reactions occurring at the tissue cells.

Solution: Two equilibrium equations are needed

(Rx. 1)Carbon dioxide- Carbonic Acid - Bicarbonate Buffer reaction
CO2 + HOH <===> H2CO3 <===> H+ + HCO3-:

(Rx. 2) Hemoglobin / oxygenation and deoxygenation reaction
HHgb + O 2 <===> HgbO 2 + H+

At cells CO2 diffuses out of the tissue cells and causes an increase of CO2 in the blood. Rx. 1 shifts right (all the way). H2CO3 increases and H+ increases. The increase of H+ causes Rx. 2 to shift left which causes O2 to be released to the blood and can then diffuse into the tissue cells.

QUES. 2: Use equilibrium principles to explain the reactions occurring at the region of the lungs. Start with the diffusion of oxygen from the lungs into the blood and finish with carbon dioxide being exhaled from the lungs.

   

pH Changes in the Blood:

The changes in concentration of CO2 or H+ + HCO3- ion can influence slight pH changes in the blood even though it is buffered. At the same time the concentration of H+ ions will influence the concentrations of CO2 and H+ + HCO3- ions.

Example:

Predict the change in pH in venous blood caused by the diffusion of CO2 from tissue cells using equilibrium principles.

Solution:

Always write the blood buffer equations first.

CO2 + HOH === H2CO3 === H+ + H+ + HCO3-

Increased carbon dioxide causes reaction(1) to shift right causing H2CO3 to increase. The increased H2CO3 causes reaction (2) to shift right causing H+ to increase and finally causing pH to decrease. Using reaction (3), increased carbon dioxide causes a shift in equilibrium all the way to the right with consequent H+ increase, which causes the pH to decrease in the venous blood.

QUES. 3: Predict the change in pH in arterial blood caused by the diffusion of CO2 out of the blood into the lungs. Use equilibrium principles