pH Significance of Blood
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
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?
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
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
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
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.
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.
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.
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Update: June 2012