How iis carbondioxide transported from tissue to lungs in human being?

Carbon dioxide is transported  from tissue to lungs  through blood in human beings.
There are three different ways by which carbondioxide is transported in the body-
  • In combination with the amine group of haemoglobin proteins (20-25%)
  • In the form of bicarbonate ions (70%)
  • In dissolved form through plasma (7%)
The exchange of gases between blood and body tissues depends on their respective partial pressure. They flow from their high partial pressure to low partial pressure by detaching from the haemoglobin molecule.
Transport of carbon dioxide through haemoglobin:
  • Carbon dioxide binds to the amino group of polypeptide chain of hemoglobin to form carbamine-haemoglobin. This is a reversible reaction.
  • The formation of carbamino-haemoglobin depends upon − pO2 and pCO2.When pCO2 (high) and pO2 (low)  as in tissues,  binding of CO2 with free amine group in haemoglobin occurs and carbamino-haemoglobin is formed.
  • When pCO2 (low) and  pO2 (high) as found in alveloli , dissociation of CO2  from carbamino-haemoglobin takes place, i.e., CO2 which is present in deoxygenated blood. Thus, CO2 is bound with haemoglobin in tissue and dissociates in alveoli where it is released.
Transport of carbon dioxide in the form of bicarbonate ions

 Most of carbon dioxide is transported in the form of bicarbonate ions with the help of enzyme called  carbonic anhydrase present in RBC and plasma.
  •  At the tissue site where partial pressure of CO2 is high due to catabolism, CO2 diffuses into blood (RBCs and plasma) and combines with water to form carbonic acid. This reaction is catalyzed by carbonic anhydrase in both the directions.
Carbonic acid dissociates into hydrogen ions and bicarbonate ions.This reaction is also catalyzed by carbonic anhydrase in both the directions.
At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2O. Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2.

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through veins and pulmonary arteries

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Co2 gets combined with heamoglobin in the form of carbaminohaemoglobin and is so transported but majority of Co2 is transported in the form of bicarbonates ion.

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