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 pCO_2.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 CO_2.