Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
997
26
Electrolyte Balance
Indicate routes of electrolyte entry and loss from the body.
Describe the importance of sodium in the body’s fluid and
electrolyte balance, and indicate its relationship to normal
cardiovascular system functioning.
Describe mechanisms involved in regulating sodium
balance, blood volume, and blood pressure.
Explain how potassium, calcium, and anion balances in
plasma are regulated.
Electrolytes include salts, acids, bases, and some proteins, but
the term
electrolyte balance
usually refers to salt balance in the
body. Salts are important in controlling fluid movements and
provide minerals essential for excitability, secretory activity, and
membrane permeability. Although many electrolytes are crucial
for cellular activity, here we will specifically examine the regula-
tion of sodium, potassium, and calcium. In the next section we
will consider acids and bases, which are intimately involved in
determining the pH of body fluids.
Salts enter the body in foods and fluids, and small amounts are
generated during metabolic activity. For example, phosphates are
liberated during catabolism of nucleic acids and bone matrix. Ob-
taining enough electrolytes is usually not a problem. Indeed, most
of us have a far greater taste than need for salt. We shake table salt
(NaCl) on our food even though natural foods contain ample
amounts and processed foods contain exorbitant quantities. Te
taste for very salty foods is learned, but some liking for salt may be
innate to ensure adequate intake of these two vital ions.
We lose salts from the body in perspiration, feces, urine, and
vomit. Even though sweat is normally hypotonic, large amounts
of salt can be lost on a hot day simply because more sweat is
produced. Gastrointestinal disorders can also lead to large salt
losses in feces or vomitus. Consequently, the flexibility of renal
mechanisms that regulate the electrolyte balance of the blood
is a critical asset.
Table 26.1
summarizes several causes and
consequences of electrolyte imbalance.
Homeostatic Imbalance
26.1
Severe electrolyte deficiencies may prompt a craving for salty
or sour foods, such as smoked meats or pickled eggs. Tis is
common in those with
Addison’s disease
, a disorder entailing
deficient mineralocorticoid hormone production by the adrenal
cortex.
When minerals such as iron are deficient, a person may even
eat substances not usually considered foods, like chalk, clay,
starch, and burnt match tips. Tis appetite for abnormal sub-
stances is called
pica
.
The Central Role of Sodium
in Fluid and Electrolyte Balance
Sodium holds a central position in fluid and electrolyte balance
and overall body homeostasis. Indeed, regulating the balance
between sodium input and output is one of the most important
which plasma proteins pass through “leaky” renal filtration
membranes and are lost in urine).
Edema can also result when lymphatic vessels are blocked
or have been surgically removed. Te small amounts of plasma
proteins that seep out of the bloodstream do not return to the
blood as usual. As the leaked proteins accumulate in the IF, they
exert an ever-increasing colloid osmotic pressure, which draws
fluid from the blood and holds it in the interstitial space.
Edema can impair tissue function because excess fluid in the
interstitial space increases the distance nutrients and oxygen
must diffuse between the blood and the cells. However, the most
serious problems resulting from rapid onset of edema affect the
cardiovascular system. When fluid leaves the bloodstream and
accumulates in the interstitial space, both blood volume and
blood pressure decline and the efficiency of the circulation can
be severely impaired.
Check Your Understanding
4.
What change in plasma is most important for triggering
thirst? Where is that change sensed?
5.
ADH, by itself, cannot reduce an increase in osmolality in
body fluids. Why not? What other mechanism is required?
6.
For each of the following, state whether it might result in
dehydration, hypotonic hydration, or edema: (a) decreased
synthesis of plasma proteins due to liver failure; (b) copious
sweating; (c) using ecstasy (MDMA), which promotes ADH
secretion.
For answers, see Appendix H.
(b) Consequences of hypotonic hydration (water gain).
If more water
than solutes is gained, cells swell.
(a) Consequences of dehydration.
If more water than solutes is lost,
cells shrink.
1
2
3
Excessive
loss of H
2
O
from ECF
ECF osmotic
pressure rises
Cells lose
H
2
O to ECF
by osmosis;
cells shrink
1
2
3
Excessive
H
2
O enters
the ECF
ECF osmotic
pressure falls
H
2
O moves
into cells by
osmosis; cells swell
Figure 26.7
Disturbances in water balance.
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