26.18 : 血液中の二酸化炭素輸送

Carbon dioxide is transported from tissues to the lungs in three ways.

About 7-10% travels dissolved in plasma.

Over 20% is bound to hemoglobin, forming carbaminohemoglobin triggered by a relatively higher  P_CO2 in the tissues.

Conversely, relatively lower P_CO2  in pulmonary capillaries prompts carbaminohemoglobin dissociation, releasing CO₂. 

The remaining CO₂ travels in the blood as bicarbonate ions.

Most CO₂ entering Red Blood Cells is catalyzed into carbonic acid by the enzyme carbonic anhydrase. The carbonic acid then dissociates into bicarbonate and hydrogen ions.

The bicarbonate ions quickly diffuse out of the red blood cells.

In exchange, chloride ions enter the Red Blood Cells through the chloride shift to neutralize charges in the Red Blood Cells.

Bicarbonate ions travel with the blood to the lungs, where the low P_CO2  triggers their re-entry into the Red Blood Cells. The whole process reverses, releasing the CO₂.