Oxygen and carbon dioxide are carried in the blood, pumped by the heart, and shifted through membranes by partial pressure gradients. Oxygen dissolves to a much smaller degree than carbon dioxide, so that at rest, the PO2 would only help provide 6% of the body’s O2 requirements. There must be another way. The hemoglobin molecule comes to the rescue.
Hemoglobin is made of globin, a protein, and four heme groups consisting of an iron atom surrounded by an organic group. Four polypeptide chains make up the globin, each bearing a red-pigmented, disk-shaped heme. Each heme group binds one oxygen molecule, so each hemoglobin molecule binds four oxygens. Considering one red blood cell holds ~ 250 million Hb molecules, it can carry ~ 1 BILLION oxygen molecules (Marieb 581). The entire molecule looks like a strange knot (not shown here), but its functioning is crucial.
Hemoglobin serves to transport oxygen and carbon dioxide in the blood. 24% of carbon dioxide is transported as carbaminohemoglobin (Kapit 49). 97% of oxygen in the blood is transported as oxyhemoglobin (HbO2).
(1) The reaction at the lungs: 02 + HHb ——> HbO2– + H+
(2) The reaction at tissues: H+ + HbO2– ——> HHb + 02
Apparently, this reaction can take place in either direction. At equilibrium, the reaction takes place in both directions, but when a stress (such as increased concentration of a reactant) is placed on the reaction, it shifts. Thus, the transport of oxygen, and hydrogen ions is governed by La Chatelier’s principle.
THE PRINCIPLE OF LE CHATELIER
If a stress is applied to a system in equilibrium, the system will respond in such a way as to relieve that stress and restore equilibrium under a new set of conditions (Hein 401).
67% of carbon dioxide travels as bicarbonate (HCO3–). The transport of carbon dioxide is also ruled by Le Chatelier’s principle:
If you take a moment to see how these four (actually two) reactions interplay, gas movements will make more sense. At the tissues (4), the concentration of CO2 is higher than that of O2. A reaction takes place whereby bicarbonate and hydrogen ions are produced. In turn (2), hydrogen ions combine with HbO2 to release the oxygen from oxyhemoglobin. Interestingly, hydrogen ions decrease the strength of the HbO2 bond; this is called the Bohr effect. Operation (3) ensures that hydrogen ions produced when O2 is “loaded” onto red blood cells (1) do not contribute to blood acidosis. H+ binds with bicarbonate to form CO2 and H2O. Their concentrations are kept high at the lungs in order to maintain the partial pressure gradient that ensures we breathe them OUT.
So, H+, normally a pH concern, is actually a boon to the equilibrium: it helps release oxygen at the tissues and drive CO2 off bicarbonate at the lungs. During the cycle, H+ is swapped back and forth HHb and HCO3–. Any shifts in hydrogen ion concentrations will create a shift in blood pH, and that’s bad news.