Essentials of Anatomy and Physiology

(NaHCO 3 ), a weak base. Each of these molecules par-
ticipates in a specific type of reaction.
If a potential pH change is created by a strong acid,
the following reaction takes place:

HClNaHCO 3 →NaCl  H 2 CO 3
(strong acid) (weak acid)
The strong acid has reacted with the sodium bicar-
bonate to produce a salt (NaCl) that has no effect on
pH and a weak acid that has little effect on pH.
If a potential pH change is created by a strong base,
the following reaction takes place:

NaOHH 2 CO 3 →H 2 O  NaHCO 3
(strong base) (weak base)
The strong base has reacted with the carbonic acid
to produce water, which has no effect on pH and a
weak base that has little effect on pH.
The bicarbonate buffer system is important in both
the blood and tissue fluid. During normal metabolism,
these fluids tend to become more acidic, so more
sodium bicarbonate than carbonic acid is needed. The
usual ratio of these molecules to each other is about 20
to 1 (NaHCO 3 to H 2 CO 3 ).

Phosphate Buffer System

The two components of this buffer system are sodium
dihydrogen phosphate (NaH 2 PO 4 ), a weak acid, and
sodium monohydrogen phosphate (Na 2 HPO 4 ), a weak
base. Let us use specific reactions to show how this
buffer system works.
If a potential pH change is created by a strong acid,
the following reaction takes place:

HClNa 2 HPO 4 →NaCl  NaH 2 PO 4

(strong acid) (weak acid)

The strong acid has reacted with the sodium mono-
hydrogen phosphate to produce a salt that has no
effect on pH and a weak acid that has little effect on
pH.
If a potential pH change is created by a strong base,
the following reaction takes place:

NaOH NaH 2 PO 4 →H 2 O  Na 2 HPO 4

(strong base) (weak base)

The strong base has reacted with the sodium dihy-
drogen phosphate to form water, which has no effect
on pH and a weak base that has little effect on pH.

The phosphate buffer system is important in the

regulation of the pH of the blood by the kidneys (see

Fig. 19–4). The cells of the kidney tubules can remove

excess hydrogen ions by forming NaH 2 PO 4 , which is

excreted in urine. The retained Naions are returned

to the blood in the peritubular capillaries, along with

bicarbonate ions.

Protein Buffer System

This buffer system is the most important one in the

intracellular fluid. You may recall from Chapter 15

that hemoglobin buffers the hydrogen ions formed

during CO 2 transport. The amino acids that make up

proteins each have a carboxyl group(COOH) and an

amine(or amino) group(NH 2 ) and may act as either

acids or bases.

The carboxyl group may act as an acid because it

can donate a hydrogen ion (H) to the fluid to coun-

teract increasing alkalinity:

The amine group may act as a base because it can

pick up an excess hydrogen ion from the fluid to coun-

teract increasing acidity:

The buffer systems react within a fraction of a sec-

ond to prevent drastic pH changes. However, they

have the least capacity to prevent great changes in pH

because a limited number of molecules of these buffers

are present in body fluids. When an ongoing cause is

disrupting the normal pH, the respiratory and renal

mechanisms will also be needed.

RESPIRATORY MECHANISMS

The respiratory system affects pH because it regulates

the amount of CO 2 present in body fluids. As you

COOH C NH 2

H

R

H

R

COOH C NH 3 +

NH 2 C COOH NH 2 C COO- + H+

H

R

H

R

448 Fluid–Electrolyte and Acid–Base Balance