InSight
THE PLANNER
T h e p H s c a le
Figure 2.9
The strengths of acid and bases are measured on the pH scale. Some common examples
are shown.
such as pure water, is neutral—neither acidic nor alkaline.
An acidic solution has a pH below 7, while a basic solu-
tion has a pH above 7. A change of one whole number on
the pH scale represents a
10-fold
change in H+ concen-
tration. Thus, a solution with a pH of 6 is 10 times more
acidic than one with a pH of 7 and 100 times more acidic
than one with a pH of 8.
Although the pH of various body fluids may differ,
the normal limits for each are quite narrow. Figure 2.9
shows the pH values for certain body fluids compared
with those of common household substances. Homeostatic
mechanisms maintain the pH of blood between 7.35 and
7.45, so that it is slightly more basic than pure water.
Even though strong acids and bases may be taken
into the body or be formed by body cells, the pH of fluids
inside and outside cells remains almost constant, in part
because of the presence of buffers. Buffers are chemical
compounds that act quickly to temporarily bind H+, re-
moving the highly reactive, excess H+ from solution (but
not from the body). Buffers can also release H+ that they
have stored into the body to maintain pH. Buffers prevent
rapid, drastic changes in the pH of a body fluid by convert-
ing strong acids and strong bases into weak acids and weak
bases. Strong acids release H+ more readily than weak ac-
ids and thus contribute more free H+. Similarly, strong
bases raise pH more than weak ones.
Life Uses Important Chemicals 33
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