Fluid, Electrolyte, and Acid-Base 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
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.
summarizes several causes and
consequences of electrolyte imbalance.
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
, a disorder entailing
deﬁcient mineralocorticoid hormone production by the adrenal
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
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
What change in plasma is most important for triggering
thirst? Where is that change sensed?
ADH, by itself, cannot reduce an increase in osmolality in
body ﬂuids. Why not? What other mechanism is required?
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
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,
loss of H
O to ECF
into cells by
osmosis; cells swell
Disturbances in water balance.