The Bohr Effect

As you might imagine, the Bohr effect was first discovered by a guy named Bohr! He discovered that there were other factors that affected the loading/unloading of oxygen by Hb. One of the factors that he discovered was pH. He found that if the pH was lower than normal (normal physiological pH is 7.4), then Hb does not bind oxygen as well. This is shown in the following % Saturation graph.

This plot includes the % Saturation of Hb at normal pH (7.4) and at a lower pH value (7.2). You should notice that the curves are not identical suggesting that pH affects the binding of oxygen to Hb. Let us examine the % saturation at the lungs and at the tissues when only the pH is changed. In the lungs (pO2 = 100 mm Hg), Hb is 98% saturated at both pH 7.4 and 7.2. Thus the binding of oxygen to Hb in the lungs is not affected by changing the pH and oxygen will load normally. The situation is different at the tissues. The change in the pH results is a lower % saturation of Hb even though the pO2 in the tissues has remained at 40 mm Hg for this example. Reading from the graph, the % saturation at pH 7.4 with a pO2 of 40 mm Hg is about 70% (meaning 30% of the oxygen coming to the tissue is released). At a pH of 7.2, the value is close to 60% (meaning 40% of the oxygen coming to the tissue is released). Thus more oxygen is delievered to tissues at a lower pH even when the pO2 remains unchanged. Lowering the pH appears to shift the entire red curve to the right.

Now, is this just a boring piece of information or does it have physiological significance. Remember, if we were to design an oxygen delivery system, we would like to deliver more oxygen to the more active tissues, the ones that need the oxygen. How would we know if a tissue is more active and requires more oxygen? One way is by the amount of oxygen present in a tissue. If the tissue is using more oxygen, then one would expect the amount of oxygen (pO2) would be lower. We have seen that when this is the case, more oxygen is delivered to the tissue (previous page). Another indicator that a tissue has a high metabolic rate (and thus a need for increased oxygen delivery) is the production of carbon dioxide. When a tissue is more active, the amount of carbon dioxide produced will be increased (pCO2 is higher). Carbon dioxide reacts with water as shown in the following equation:

CO2+ H2O <---------> H+ + HCO-3

What this means is that as the amount of carbon dioxide increases, more H+ are formed and the pH will decrease. Thus, a lower pH in the blood is suggestive of an increased carbon dioxide concentration which in turn is suggestive of a more active tissue that requires more oxygen. According to Bohr, the lower pH will cause Hb to deliver more oxygen! Just what we want to occur.

Note that if pO2 and pH should drop together, even more oxygen will be delivered then if only one of the parameters were changed (for example, pH = 7.2, pO2 = 30 mm Hg compared to pH = 7.4, pO2 = 30 mm Hg).

A few more items to add to your knowledge base: If the pH of the tissues should rise (due to a drop in the carbon dioxide concentration), the the % Saturation curve shifts to the left (not shown) and results in less oxygen being delivered. Does that make sense? Also, there are several other factors that will also affect the binding of oxygen to Hb. One of these is temperature. A more active tissue will be producing more heat and will be warmer. This increased temperature shifts the curve to the right (not shown) just as decreased pH does. Is this beneficial to the tissue? Why?

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