Page 81 - Fundamentals of anatomy physiology
P. 81
68 Unit 1 Levels of Organization
2 cell and tissue functions. As a result, the concentration of hy- Pure water has a pH of 7, but solutions display a wide range
drogen ions in body fluids must be regulated precisely. of pH values, depending on the nature of the solutes involved.
A few hydrogen ions are normally present even in a A solution with a pH of 7 is said to be neutral, because it
sample of pure water, because some of the water molecules
dissociate spontaneously, releasing cations and anions. The contains equal numbers of hydrogen and hydroxide ions.
dissociation of water is a reversible reaction. We can repre-
sent it as: A solution with a pH below 7 is acidic (a-SI-dik), meaning
H2O ∆ H + + OH - that it contains more hydrogen ions than hydroxide ions.
Notice that the dissociation of one water molecule yields a A pH above 7 is basic, or alkaline (AL-kuh-lin), meaning
h ydrogen ion (H+) and a hydroxide (hı.-DROK-sı.d) ion, OH−.
that it has more hydroxide ions than hydrogen ions.
Very few water molecules ionize in pure water, so the
number of hydrogen and hydroxide ions is small. The quanti- The normal pH of blood ranges from 7.35 to 7.45.
ties are usually reported in moles, making it easy to keep track Abnormal fluctuations in pH can damage cells and tissues by
of the numbers of hydrogen and hydroxide ions. One liter of breaking chemical bonds, changing the shapes of proteins, and
pure water contains about 0.0000001 mol of hydrogen ions a ltering cellular functions. Acidosis is an abnormal physiological
and an equal number of hydroxide ions. In other words, the state caused by low blood pH (below 7.35). A pH below 7 can
concentration of hydrogen ions in a solution of pure water is produce coma. Likewise, alkalosis results from an abnormally
0.0000001 mol per liter. This can be written as high pH (above 7.45). A blood pH above 7.8 generally causes
uncontrollable and sustained skeletal muscle contractions.
3H + 4 = 1 * 10 - 7mol/L
Checkpoint
The brackets around the H+ signify “the concentration of,” 1 5. Define pH, and explain how the pH scale relates to
another example of chemical notation.
acidity and alkalinity.
The hydrogen ion concentration in body fluids is so 16. What is the significance of pH in physiological systems?
important to physiological processes that we use a special
shorthand to express it. The pH of a solution is defined as See the blue Answers tab at the back of the book.
the negative logarithm of the hydrogen ion concentration
in moles per liter. So instead of using the equation [H+] = 2-8 Acids, bases, and salts are
1 × 10−7 mol/L, we say that the pH of pure water is –(–7), or 7.
inorganic compounds with important
Using pH values saves space, but always remember that the physiological roles
pH number is an exponent and that the pH scale is logarithmic.
For instance, a pH of 6 ([H+] = 1 × 10−6, or 0.000001 mol/L) Learning Outcome Describe the physiological roles of inorganic
means that the concentration of hydrogen ions is 10 times compounds.
greater than it is at a pH of 7 ([H+] = 1 × 10−7, or 0.0000001
mol/L). The pH scale ranges from 0 to 14 (Figure 2–10). The body contains both inorganic and organic acids and bases
that may cause acidosis or alkalosis, respectively. An acid is
any solute that dissociates in solution and releases hydrogen
Figure 2–10 The pH Scale Indicates Hydrogen Ion Concentration. The pH scale is logarithmic; an increase or decrease of one unit
c orresponds to a tenfold change in H+ concentration.
1 mol/L Beer, Urine PuBrelooEdggsOwcaeteanr Household Household 1 mol/L
hydrochloric vinegar, water bleach ammonia sodium
Saliva, hydroxide
acid wine, Tomatoes, milk
pickles grapes Oven
Stomach cleaner
acid
Extremely Increasing concentration of Hϩ Neutral Increasing concentration of OHϪ Extremely
acidic basic
pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
[Hϩ] 100 10Ϫ1 10Ϫ2 10Ϫ3 10Ϫ4 10Ϫ5 10Ϫ6 10Ϫ7 10Ϫ8 10Ϫ9 10Ϫ10 10Ϫ11 10Ϫ12 10Ϫ13 10Ϫ14
(mol/L)
