Maintenance of the Body
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
Whenever the pH of arterial blood rises above 7.45, a person
is said to have
. A drop
in arterial pH below 7.35 results in
. Because pH 7.0 is neutral, chemically speaking 7.35
is not acidic. However, it is a higher-than-optimal H
tration for most cells, so any arterial pH between 7.0 and 7.35 is
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
into the ECF.
Anaerobic respiration of glucose produces
Fat metabolism yields other organic acids, such as fatty acids
Loading and transport of carbon dioxide in the blood as
liberates hydrogen ions.
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
, and that the acidity of a solution
reﬂects only the
hydrogen ions, not those bound to anions.
dissociate completely and liberate all their H
. Tey can dramatically change a solution’s pH.
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.
. Strong bases are those that dis-
sociate easily in water and quickly tie up H
. Conversely, weak
bases are less likely to accept protons.
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
whenever the pH drops and re-
leasing them when pH rises.
Te three major chemical buﬀer systems in the body are the
protein buﬀer systems
that causes a shi± in H
concentration in one ﬂuid compart-
ment simultaneously causes a change in the others. As a result,
membrane, while Ca
pumps and antiporters export it at the
basolateral membrane. Under normal circumstances about 98%
of the ﬁltered Ca
is reabsorbed owing to the action of P²H.
As a rule, 75% of the ﬁltered phosphate ions (including
, and PO
) 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
from bone is inhibited, more Ca
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
in the ECF
and, like sodium, Cl
helps maintain the osmotic pressure of
the blood. When blood pH is within normal limits or slightly
alkaline, about 99% of ﬁltered Cl
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
transport are coupled.
When acidosis occurs, less Cl
reabsorption is stepped up to restore blood pH to its
normal range. Tus, the choice between Cl
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
Jacob has Addison’s disease (insufﬁcient aldosterone release).
How does this affect his plasma Na
does this affect his blood pressure? Explain.
Renal handling of Na
can be summed up as “The kidneys
reabsorb almost all of the Na
as ﬁltrate passes through its
tubules.” Make a similar summary for K
Which hormone is the major regulator of Ca
in the blood?
What are the effects of hypercalcemia? Hypocalcemia?
For answers, see Appendix H.
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
concentration. It follows then that nearly all
biochemical reactions are inﬂuenced by the pH of their ﬂuid en-
vironment, and the
of body ﬂuids is closely
regulated. (For a review of the basic principles of acid-base reac-
tions and pH, see Chapter 2.)