As a byproduct of the metabolic processes providing our bodies with energy, carbon dioxide needs to be constantly expelled from our body. Blood plasma and red blood cells provide the means to carry this metabolic byproduct from various tissues in your body to your lungs, where it is exhaled and removed from your body. There are three main methods your blood uses to transport carbon dioxide, binding the carbon dioxide to blood plasma proteins or hemoglobin, transporting carbon dioxide using the bicarbonate buffer system, or simply by dissolving carbon dioxide within itself.
The simplest method of carbon dioxide transport is to be carried as a dissolved form in the blood. Carbon dioxide is more soluble in blood than oxygen (1). Once in the bloodstream, carbon dioxide is carried to the lungs where it diffuses into the lungs and is exhaled and expelled from the body. It is estimated between 5-7% of the carbon dioxide produced by your body is carried to the lungs in this manner (1).
Carbon dioxide entering the blood from the tissues can also bind to a free blood plasma protein or enter red blood cells. After entering a red blood cell, carbon dioxide reacts with hemoglobin to form a carbaminohemoglobin compound (1). As blood moves from your tissues to the lungs, carbon dioxide is carried along with the plasma proteins or hemoglobin. This attachment is reversible, so once these compounds arrive in the lungs, carbon dioxide detaches itself, diffuses into the lungs, and is exhaled and expelled from your body. This method of transport is estimated to account for 25% of the carbon dioxide transported by the blood (1).
The majority of carbon dioxide transported by the blood is through the bicarbonate buffer system. Carbon dioxide diffusing into a red blood cell is converted to carbonic acid by the enzyme carbonic anhydrase via the following chemical equation: CO2 + H2O –> H2CO3 (1). The carbonic acid then dissociates into hydrogen and bicarbonate ions. The hydrogen ion reacts and attaches itself to hemoglobin while the bicarbonate ion is pumped out of the red blood cell in exchange for a chloride ion, a process known as a chloride shift (1). The result is blood carries both the bicarbonate (now in the blood plasma) and the hydrogen ion to the lungs.
Once in the lungs, bicarbonate is pumped back into the red blood cell in exchange for a chloride ion, the hydrogen ion dissociates from hemoglobin, carbonic acid is reformed, and carbonic anhydrase converts the carbonic acid into carbon dioxide and water (1). Carbon dioxide then diffuses from the blood into the lungs and is exhaled and expelled from the body. It is estimated 70% of all carbon dioxide transported by the blood uses this method of transport (1).
Thus these are the three main methods of transporting carbon dioxide within the blood. I hope this article has given you a better understanding of how your body moves carbon dioxide from your tissues to the lungs where it is ultimately exhaled and expelled from your body.
References
1. Brooks, George A., Thomas D. Fahey, and Kenneth M. Baldwin. Exercise Physiology: Human Bioenergetics and Its Applications. 4th ed. New York: McGraw-Hill, 2005. 249-251.
