1004
UNIT 4
Maintenance of the Body
26
Optimal pH varies from one body fluid to another, but not
by much. Te normal pH of arterial blood is 7.4, that of venous
blood and IF is 7.35, and that of ICF averages 7.0. Te lower pH
in cells and venous blood reflects their greater amounts of acidic
metabolites and carbon dioxide, which combines with water to
form carbonic acid, H
2
CO
3
.
Whenever the pH of arterial blood rises above 7.45, a person
is said to have
alkalosis
(al
0
kah-lo
9
sis) or
alkalemia
. A drop
in arterial pH below 7.35 results in
acidosis
(as
0
ĭ-do
9
sis) or
acidemia
. Because pH 7.0 is neutral, chemically speaking 7.35
is not acidic. However, it is a higher-than-optimal H
1
concen-
tration for most cells, so any arterial pH between 7.0 and 7.35 is
called
physiological acidosis
.
Although small amounts of acidic substances enter the body
via ingested foods, most hydrogen ions originate as metabolic
by-products or end products. For example:
Breakdown of phosphorus-containing proteins releases
phosphoric acid
into the ECF.
Anaerobic respiration of glucose produces
lactic acid.
Fat metabolism yields other organic acids, such as fatty acids
and
ketone bodies.
Loading and transport of carbon dioxide in the blood as
HCO
3
2
liberates hydrogen ions.
Te H
1
concentration in blood is regulated sequentially by
(1) chemical buffers, (2) brain stem respiratory centers, and (3)
renal mechanisms. Chemical buffers, the first line of defense, act
within a fraction of a second to resist pH changes. Within 1–3
minutes, changes in respiratory rate and depth occur to com-
pensate for acidosis or alkalosis. Te kidneys, the body’s most
potent acid-base regulatory system, ordinarily require hours to
a day or more to alter blood pH.
Chemical Buffer Systems
List the three major chemical buffer systems of the body
and describe how they resist pH changes.
Recall that acids are
proton donors
, and that the acidity of a solution
reflects only the
free
hydrogen ions, not those bound to anions.
Strong acids
dissociate completely and liberate all their H
1
in water
(Figure 26.11a)
. Tey can dramatically change a solution’s pH.
By contrast,
weak acids
dissociate only partially (Figure 26.11b).
Accordingly, they have a much smaller effect on pH. However,
weak acids are efficient at preventing pH changes, and this feature
allows them to play important roles in chemical buffer systems.
Bases are
proton acceptors
. Strong bases are those that dis-
sociate easily in water and quickly tie up H
1
. Conversely, weak
bases are less likely to accept protons.
A
chemical
buffer
is a system of one or more compounds
that resists changes in pH when a strong acid or base is added.
Tey do this by binding to H
1
whenever the pH drops and re-
leasing them when pH rises.
Te three major chemical buffer systems in the body are the
bicarbonate, phosphate
, and
protein buffer systems
. Anything
that causes a shi± in H
1
concentration in one fluid compart-
ment simultaneously causes a change in the others. As a result,
membrane, while Ca
2
1
pumps and antiporters export it at the
basolateral membrane. Under normal circumstances about 98%
of the filtered Ca
2
1
is reabsorbed owing to the action of P²H.
As a rule, 75% of the filtered phosphate ions (including
H
2
PO
4
2
, HPO
4
2
2
, and PO
4
3
2
) are reabsorbed in the PC² by
secondary active transport. Phosphate reabsorption is set by its
transport maximum. Amounts over that maximum simply flow
out in urine. P²H inhibits active transport of phosphate by de-
creasing its transport maximum.
When ECF calcium levels are within normal limits (9–11
mg/100 ml of blood) or higher, P²H secretion is inhibited. Con-
sequently, release of Ca
2
1
from bone is inhibited, more Ca
2
1
is
lost in feces and urine, and more phosphate is retained. Hor-
mones other than P²H alter phosphate reabsorption. For exam-
ple, insulin increases it while glucagon decreases it.
Regulation of Anions
Chloride is the major anion accompanying Na
1
in the ECF
and, like sodium, Cl
2
helps maintain the osmotic pressure of
the blood. When blood pH is within normal limits or slightly
alkaline, about 99% of filtered Cl
2
is reabsorbed. In the PC²,
it moves passively and simply follows sodium ions out of the
filtrate and into the peritubular capillary blood. In most other
tubule segments, Na
1
and Cl
2
transport are coupled.
When acidosis occurs, less Cl
2
accompanies Na
1
because
HCO
3
2
reabsorption is stepped up to restore blood pH to its
normal range. Tus, the choice between Cl
2
and HCO
3
2
serves
acid-base regulation. Most other anions, such as sulfates and
nitrates, have transport maximums, and when their concentra-
tions in the filtrate exceed the amount that can be reabsorbed,
the excess spills into urine.
Check Your Understanding
7.
Jacob has Addison’s disease (insufficient aldosterone release).
How does this affect his plasma Na
1
and K
1
levels? How
does this affect his blood pressure? Explain.
8.
Renal handling of Na
1
can be summed up as “The kidneys
reabsorb almost all of the Na
1
as filtrate passes through its
tubules.” Make a similar summary for K
1
.
9.
Which hormone is the major regulator of Ca
2
1
in the blood?
What are the effects of hypercalcemia? Hypocalcemia?
For answers, see Appendix H.
Acid-Base Balance
List important sources of acids in the body.
Because of their abundant hydrogen bonds, all functional pro-
teins (enzymes, hemoglobin, cytochromes, and others) are in-
fluenced by H
1
concentration. It follows then that nearly all
biochemical reactions are influenced by the pH of their fluid en-
vironment, and the
acid-base balance
of body fluids is closely
regulated. (For a review of the basic principles of acid-base reac-
tions and pH, see Chapter 2.)
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