Transport of Carbon Dioxide

 TRANSPORT OF CARBON DIOXIDE

• Carbon dioxide is produced in cells, during glycolysis and the citric acid cycle in the cytoplasm and mitochondria. As it is produced in the cells, carbon dioxide dissolves into the water of the cytoplasm and continues to build up the pressure until it reaches a partial pressure greater than 40mmHg. 

• Due to this concentration gradient, carbon dioxide molecules can freely diffuse from the extracellular space through the capillary walls, rapidly equilibrating and raising the partial pressure of carbon dioxide in the blood from about 40 mmHg on the arterial side of a capillary to 46mmHg on the venous side.

• At every minute,100mL of blood is entered in systemic capillaries from the arterial side which contains 48mL carbon dioxide & 4mL carbon dioxide is added to 100mL blood passes through the capillary. Then the Content of carbon dioxide in 100mL of venous blood is raised to 52mL.

There are three ways carbon dioxide is carried in the blood from the peripheral tissues back to the lungs: 

1. As dissolved gas, 

• carbon dioxide diffuses from peripheral tissues into the bloodstream, approximately 10% of it remains dissolved in the plasma, or the extracellular fluid matrix of the blood, to a partial pressure of about 45 mmHg

2. As bicarbonate, 

• The carbon dioxide that diffuses through the capillaries and, ultimately, into the red blood cells combines with water in a chemical reaction catalyzed by the enzyme carbonic anhydrase to form carbonic acid. Carbonic acid almost immediately dissociates into a proton and a bicarbonate anion (HCO3-). 

• bicarbonate is the principal means by which carbon dioxide is transported throughout the bloodstream according to the equation CO2 + H2O --> H2CO3 --> H+ + HCO3-.

• The proton formed by this reaction is buffered by hemoglobin while the bicarbonate anion diffuses out of the red blood cell into the plasma in exchange for a chloride anion through a special HCO3-/Cl- transporter. Thus, venous blood has both a higher concentration of bicarbonate and a lower concentration of chloride due to chloride shift.

3. As Carbamino compounds. 

• The remaining carbon dioxide that diffuses into the capillary goes into the red blood cells, binds to the amino terminus of proteins, predominantly hemoglobin, to form carbaminohemoglobin.

• Small amounts of carbon dioxide bind with plasma proteins such as albumin, globulin, & fibrinogen to form carbinoprotiens.

Bohr effect states that the increase of carbon dioxide in the blood causes a right shift in the oxygen-hemoglobin dissociation curve and, consequently, increased oxygenation of the tissues. 

Once the carbon dioxide-enriched blood reaches the lungs, 

The partial pressure of carbon dioxide in venous blood is 46mmHg, so first dissolve carbon dioxide diffuse from high-pressure blood to low pressure in alveoli.

As oxygen binds to hemoglobin, the hemoglobin becomes more acidic. 

This has two effects:

1. It decreases the binding affinity of the hemoglobin for carbon dioxide, making the carbon dioxide more likely to dissociate from the hemoglobin and diffuse out of the red blood cell into the alveolar space. 

2. Acidic hemoglobin can release a proton H+ that will combine with bicarbonate to form carbonic acid.

          Again, Le Chatelier’s Principle:

 H+ + HCO3- --> H2CO3 --> CO2 + H2O. 

The carbon dioxide produced here continually diffuses into the alveoli until the partial pressure of carbon dioxide in the blood becomes 40mmHg and alveolar carbon dioxide is exhaled.

Haldane effect: when hemoglobin is loaded with oxygen that helps to deload the proton & that react with bicarbonate & release carbon dioxide from RBCs to the plasma of blood & then diffuse to alveoli.