Biology · Ch 15 — Breathing and Exchange of Gases
Transport of Carbon Dioxide
Transport of Carbon Dioxide
Carbon dioxide, generated continuously as a waste product of cellular respiration in every tissue of the body, is carried back to the lungs by blood through three separate, parallel routes, rather than by a single dominant mechanism as with oxygen -- and, unlike oxygen, these three routes carry sharply unequal, but each individually significant, shares of the total.
The largest single share, accounting for approximately 70% of the carbon dioxide transported, travels in the form of bicarbonate ions (HCO3-) dissolved in the blood plasma. This route begins when carbon dioxide diffuses from a respiring tissue into a red blood cell, where the enzyme carbonic anhydrase -- present there in large quantity, and essentially absent from plasma itself -- very rapidly catalyses the combination of this CO2 with water to form carbonic acid (H2CO3); this carbonic acid, being a weak acid, almost immediately dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). The bicarbonate ion then diffuses out of the red blood cell into the surrounding plasma, where it is carried onward to the lungs, while the hydrogen ion released at the same step is largely buffered within the red blood cell by binding to haemoglobin (a process itself assisted, at the tissue level, by the fact that haemoglobin having just released its bound oxygen is somewhat better able to buffer H+, an effect complementary to, and often described alongside, the Bohr effect).
A second, smaller but still substantial, share -- approximately 20 to 25% -- travels bound directly to haemoglobin itself, forming a compound called carbaminohaemoglobin. Here, carbon dioxide binds not to the iron-containing haem group (which is instead the binding site for oxygen) but to the globin (protein) portion of the haemoglobin molecule, at a chemically distinct site, so the two gases do not compete directly with one another for the very same binding site on haemoglobin.
The remaining, smallest share -- approximately 7% -- is simply dissolved directly in the blood plasma, in much the same manner, though rather more readily, as the small dissolved fraction of oxygen discussed in the previous section. …
Worked out. A short explanatory note box on the chloride shift, the ionic exchange that keeps red blood cell transport of carbon dioxide as bicarbonate electrically balanced. Inside the red blood cell, the enzyme carbonic anhydrase rapidly converts dissolved CO2 and water into carbonic acid, which immediately dissociates into a bicarbonate ion (HCO3-) and a hydrogen ion (H+); the H+ is buffered by binding to haemoglobin, but the negatively charged HCO3- diffuses out of the red blood cell into the plasma for transport to the lungs. Because this outward movement of a negative ion would otherwise leave the inside of the red blood cell with a net positive charge, a chloride ion (Cl-) simultaneously diffuses in from the plasma to replace it -- the chloride shift, or Hamburger shift -- keeping the red blood cell electrically neut …