Page 82 - Fundamentals of anatomy physiology

ions, lowering the pH. A hydrogen atom that loses its electron
consists solely of a proton, so we often refer to hydrogen ions
simply as protons, and to acids as proton donors.
A strong acid dissociates completely in solution, and the
reaction occurs essentially in one direction only. Hydrochloric
acid (HCl) is a representative strong acid. In water, it ionizes
as follows:
HCl ¡ H + + Cl - weak base. For example, the carbonic acid–bicarbonate buffer
The stomach produces this powerful acid to help break down
food. Hardware stores sell HCl under the name muriatic acid,
for cleaning concrete and swimming pools.
A base is a solute that removes hydrogen ions from a so-
lution, raising the pH. It acts as a proton acceptor. In solution,
many bases release a hydroxide ion (OH−). Hydroxide ions
have an attraction for hydrogen ions and react quickly with
them to form water molecules. A strong base dissociates com-
pletely in solution. Sodium hydroxide, NaOH, is a strong base.
In solution, it releases sodium ions and hydroxide ions:
NaOH ¡ Na+ + OH - adding hydrochloric acid to sodium hydroxide, you neutralize
Strong bases have a variety of industrial and household uses.
Drain openers (Drano) and lye are two familiar examples.
Weak acids and weak bases do not dissociate completely. At
equilibrium, a significant number of molecules remains intact
in the solution. For the same number of molecules in solution,
weak acids and weak bases have less impact on pH than do
strong acids and strong bases. Carbonic acid (H2CO3) is a weak
acid found in body fluids. In solution, carbonic acid reversibly
dissociates into a hydrogen ion and a bicarbonate ion, HCO3−:
H2CO3 ∆ H + + HCO3- discomfort?
Salts
A salt is an ionic compound containing any cation except a
hydrogen ion, and any anion except a hydroxide ion. Because
they are held together by ionic bonds, many salts dissociate
completely in water, releasing cations and anions. For exam-
ple, sodium chloride (table salt) dissociates immediately in
water, releasing Na+ and Cl−. Sodium and chloride are the
most abundant ions in body fluids. However, many other ions
are present in lesser amounts as a result of the dissociation of
other inorganic compounds. Ionic concentrations in the body
are regulated in ways we will describe in Chapters 26 and 27.
The ionization of sodium chloride does not affect the
local concentrations of hydrogen ions or hydroxide ions. For
this reason, NaCl, like many salts, is a “neutral” solute. It does with nitrogen, phosphorus, sulfur, iron, or other elements.
not make a solution more acidic or more basic. Through their
interactions with water molecules, however, other salts may in-
directly affect the concentrations of H+ and OH− ions. Thus,
the dissociation of some salts makes a solution slightly acidic
or slightly basic.
Chapter 2 The Chemical Level of Organization 69
Buffers and pH Control
Buffers are compounds that stabilize the pH of a solution by
2removing or replacing hydrogen ions. Buffer systems usually
involve a weak acid and its related salt, which functions as a
system (detailed in Chapter 27) consists of carbonic acid and
sodium bicarbonate, NaHCO3, otherwise known as baking
soda. Buffers and buffer systems in body fluids help maintain
the pH within normal limits. The pH of several body fluids is
included in Figure 2–10.
The use of antacids such as Alka-Seltzer is one exam-
ple of the type of reaction that takes place in buffer systems.
­Alka-Seltzer uses sodium bicarbonate to neutralize excess
hydro­chloric acid in the stomach.
Note that the effects of neutralization are most evident
when you add a strong acid to a strong base. For example, by
both the strong acid and the strong base.
HCl + NaOH ¡ H2O + NaCl
This neutralization reaction produces water and a salt—in this
case, the neutral salt sodium chloride.
Checkpoint
1 7. Define the following terms: acid, base, and salt.
1 8. How does an antacid help decrease stomach
See the blue Answers tab at the back of the book.
2-9 Carbohydrates contain carbon,
hydrogen, and oxygen in a 1:2:1 ratio
Learning Outcome Discuss the structures and functions
of carbohydrates.
Carbohydrates are one type of organic compound. Organic
compounds always contain the elements carbon and hydro-
gen, and generally oxygen as well. Many organic molecules are
made up of long chains of carbon atoms linked by covalent
bonds. The carbon atoms typically form additional covalent
bonds with hydrogen or oxygen atoms and, less commonly,
Many organic molecules are soluble in water. Our previ-
ous discussion focused on inorganic acids and bases, but there
are also important organic acids and bases. For example, active
muscle tissues generate lactic acid, an organic acid. It must be
neutralized by the carbonic acid–bicarbonate buffer system to
prevent a potentially dangerous pH decline in body fluids.
Organic compounds are diverse, but certain groupings
of atoms occur again and again, even in very different types
of molecules. These functional groups greatly influence the