organ systems: gas transport

this unit covered a few basic biochemical ideas about the way that oxygen and carbon dioxide are transported by the blood. oxygen is mainly transported in the bound form to hemoglobin, oxyhemoglobin, and thus the uptake and release of oxygen to and from the blood is governed by the oxyhemoglobin saturation curve, a sigmoidal shaped curve which describes the relative saturation of oxygen for a given PO2. this graph ties in with the previous lecture on gas exchange, in which we covered all the PO2 and PCO2s for the atmosphere, arteries, body tissues, and veins. looking at these numbers using the dissociation curve, we find that at the arterial PO2 of 95 mmHg, hemoglobin is ~97% saturated and at the venous PO2 of 40 mmHg, hemoglobin is ~70% saturated -- this difference in saturation represents oxygen's unloading from the blood. we then look at factors that can shift the dissociation curve to the right (essentially saying that for a given partial pressure, hemoglobin affinity for oxygen has decreased). these factors are: increased H+ production, which causes conformational changes in hemoglobin that reduces its affinity to oxygen, increased temperature, and increased 2,3-DPG, which competes for O2 binding sites on Hb.

carbon dioxide is transported as a dissolved gas (7%), bound to hemoglobin (23%), but mainly in the form of bicarbonate ion (70%). this conversion to bicarbonate is facilitated by carbonic anhydrase, which converts dissolved CO2 into carbonic acid, H2CO3, which then dissociates into H+ and bicarbonate ion, HCO3-. this reaction is important because it also is the common pathway which describes the mechanism for both the bohr and the haldane effect. the bohr effect can be summed up as: increased CO2 production in body tissues and uptake into the blood facilitates O2 release into tissues. this occurs when CO2 is converted to carbonic acid, and thus H+ and bicarbonate-- the H+ binds to hemoglobin and as described above decreases its affinity for O2, allowing it to be released into the tissues. the haldane effect is the opposite: increased O2 uptake in the lungs facilitates CO2 release. this happens because O2 binds to hemoglobin and causes the release of H+ protons, which then combine with bicarbonate and form carbonic acid, which is converted back into CO2 via carbonic anhydrase, which is then released into the air.


questions
1. what are the ways in which O2 is transported in the blood and the relative percentage of each?
2. what is the saturation level of hemoglobin in arterial blood?
3. what is the saturation level of hemoglobin in venous blood?
4. what would the saturation level of hemoglobin be if PO2 was at 160mmHg, the atmospheric PO2?
5. what are three factors that can shift the hemoglobin saturation curve to the right?
6. what are the ways in which CO2 is transported in the blood and the relative percentage of each?
7. describe the formation of bicarbonate ion from dissolved CO2.
8. what is the enzyme that catalyzes the formation of carbonic acid?
9. what is the "chloride shift"?
10. describe the bohr effect.
11. describe the haldane effect.

answers
1. 3% as a dissolved gas, 97% bound to hemoglobin.
2. ~97%
3. 70%
4. >99%
5. decreased pH, increased temperature, increased concentration of 2,3-DPG
6. 7% as a dissolved CO2 gas, 23% bound to hemoglobin, 70% as bicarbonate ion.
7. dissolved CO2 + H2O -> H2CO3 (carbonic acid) -> H+ and bicarbonate ion.
8. carbonic anhydrase.
9. Cl- moving into cells to balance the H+ that is being dissociated from carbonic acid.
10. the bohr effect refers to hemoglobin's decreased affinity for O2 in the tissues which is caused by the increase in H+ concentration which binds to hemoglobin and changes its conformation. the increased H+ concentration in the tissues is due to higher CO2 levels from metabolism, which is converted into carbonic acid, which dissociates into H+ and bicarbonate. in short: increased CO2 in the tissues causes decreased hemoglobin affinity for O2.
11. the haldane effect is analogous to the bohr effect except in the reverse order; increased O2 in the lungs causes CO2 to be released from the blood.