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Name: Jim
Status: student
Grade: 9-12
Location: TX
Country: N/A
Date: 2/4/2005


Question:
We learned that blood pH is neutral at 7.4. We also learned that higher than 7.8 or lower than 7.0 is really bad and leads to death.

But why is there a significant difference between 7.4 and 7.0 or 7.4 and 7.8? It seems really close, but I'm told there is quite a large difference. Would you explain it better please?


Replies:
While I'm not sure about the specific numbers, keep in mind that pH is a "log" scale, that is, a change of 7 to 8 represents a decrease in the concentration of (H+) of a factor of 10, that is pH = 10^-(H+) -- the concentration being in the exponent. In addition, blood serum is highly buffered so that this decrease in the (H+) represents a large change in the electrolyte concentration.

Vince Calder


Well, you know pH is a logarithmic unit, right? 1.0, the difference between 7.0 and 8.0, represents a factor of 10 in the concentration of H+ ions. So 0.4 represents about a factor of 2.5 in H+ concentration. A range of 0.8 represnts a factor of 6 or so. That can be pretty large if you are dependent on the linearly-expressed concentration of some chemical involved in an equilibrium.

There are lots of proteins in the body whose shape and chemical state depends on an equilibrium they keep with H+ ions. For example, if your blood is much too acidic, many Fe2+'s in hemoglobin will be replaced by a 2 to 4 H+ ions. Your body's oxygen transport would be reduced or disabled. I'm not sure that particular equilibrium is in its transition at normal pH, but some other important things might be. Unfortunately I don't know enough biochemistry to know what they are.

The body is a complex system with many different now-necessary inventions in it, and all the protein mechanisms it's built on depend nakedly on pH. It's not really surprizing some important sub-system has a fairly narrow working range. I imagine that if you plotted tolerable pH range versus life-form complexity, the lower forms would more often tolerate wider ranges of pH.

I suppose many micro-organisms and cells try to maintain a constant pH inside regardless of pH of the fluid outside. The body's blood is supposed to be reglated already, to gain the advantage that the all the individual cells don't have to maintain heavy standing assets for self-protection, as a stand-alone micro-organism might have to do. When you measure or change the pH of the blood, you have stepped inside that line of defense.

Jim Swenson


While it seems like a 0.4 difference is not that much, to our bodies it is quite large. That is because the human body is so complex in the way it works that it requires the blood pH to be perfect or close to it to work correctly. Basically, the body is really picky. If the pH goes to low, it is called acidosis. Examples would be respiratory acidosis where there is inadequate ventilation, creating a rise in carbon dioxide in the plasma. In metabolic acidosis an example would be large amounts of ketone bodies because of uncontrolled diabetes. Basically, if the blood pH gets too low or too high it is because of an abnormal amount of hydrogen ions or bicarbonate ions. While a blood pH of 7.2 is considered acidosis, remember that acid-base levels of blood are different than talking about the standard acid-base levels. The 7.2 blood would not be considered acidic, but would be labeled as acidosis for blood.

Grace Fields


It all depends on what kind of scale you use. If you are plotting data on a graph for example, and your tick marks on the Y axis are in whole numbers, you wouldn't see a noticeable difference in pH effect. But if your scale is in hundredth's, and you go from 6.0 to 8.0 in that scale, then there is a noticeable difference. So, in the body, the "scale" is set to a very low tolerance on either side of 7.4 There is a little leeway, 7.35-7.45 is normal for most people, but when you start getting away from that on either side, it begins to effect parameters in the body such as enzyme kinetics (the way your enzymes function and how fast they function). Our bodies have all kinds of "sensors", security alarms if you will, that monitor the pH of the blood and if it wanders off too much, mechanisms kick in that try to bring it back into balance. If your body can't adjust, disease results.

Van Hoeck



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