Cell membranes are completely permeable to the passage of carbon dioxide and oxygen. On diffusing into the red blood cell these gases interact with other compounds. Once beyond the capillaries no further gain or loss of these gases can occur and the compounds involved reach an equilibrium.
While in the systemic capillaries, the RBCs are subjected to increasing levels of carbon dioxide and decreasing levels of oxygen. The opposite is the case in the pulmonary capillaries. The reactions are the same but in reverse directions in these two locations. In the mind map the blue arrows represent the direction of the reactions occurring in the systemic capillaires; the red arrows represent the direction in the pulmonary capillaires.
The enzyme carbonic anhydrase (CA in circle) is found inside red blood cells. This enzyme catalyzes the combination of carbon dioxide (CO2) and water (H2O) into carbonic acid (H2CO3). Once formed within the RBC a small proportion of carbonic acid molecules ionize to produce hydrogen ions (H+) and bicarbonate ions (HCO3-).
The RBC membrane is also freely permeable to bicarbonate ions (HCO3-). These ions diffuse into the plasma along their concentration gradient in exchange for chloride ions (Cl-). This is called the 'chloride shift.'
The hydrogen ions (H+) in the RBC are buffered, that is, they are not allowed to diffuse into the plasma. Instead, they bind to hemoglobin (Hb) but only after they displace oxygen (O2) from that compound. Hemoglobin, when it binds to hydrogen ions, is said to be 'reduced.' It is in this form that hemoglobin also binds with carbon dioxide to form carbaminohemoglobin (HbH+CO2).
Carbon dioxide is transported in three forms. The smallest amount is in the form of dissolved carbon dioxide located in the plasma. There is somewhat more carbaminohemoglobin confined to the RBC. The majority is in the form of bicarbonate ions that equilabrate across the RBC membrane.
As oxygen (O2) enters the RBC it interacts with hemoglobin (Hb) and displaces hydrogen ions (H+) from it. This also causes any bound carbon dioxide to dislodge. In normal situations hemoglobin becomes almost 100% saturated with oxygen. In this form it is called oxyhemoglobin (HbO2).
The 'freed' hydrogen ions (H+) combine with bicarbonate ions (HCO3-) to form carbonic acid (H2CO3). This reduces the bicarbonate ions within the RBC causing more to diffuse in from the plasma. This, in turn, causes a chloride shift from the RBC into the plasma.
The concentration of carbonic acid (H2CO3) within the RBC does not increase because of the presence of carbonic anhydrase (CA in circle). This enzyme converts carbonic acid to carbon dioxide and water. The increasing carbon dioxide in the plasma diffuses out of the plasma and into the lungs.
Last Updated: 7/18/2005