components will occur where large volumes of
sweat are produced. The electrolyte composition
of sweat is variable, and the concentration of
individual electrolytes as well as the total sweat
volume will influence the extent of losses. The
normal concentration ranges for the main ionic
components of sweat are shown in Table 15.2,
along with their plasma and intracellular concen-
trations for comparison. A number of factors con-
tribute to the variability in the composition of
sweat: methodological problems in the collection
procedure, including evaporative loss, incom-
plete collection and contamination with skin
cells account for at least part of the variability,
but there is also a large biological variability
(Shirreffs & Maughan 1997).
The sweat composition undoubtedly varies
between individuals, but can also vary within
the same individual depending on the rate of
secretion, the state of training and the state of
heat acclimation (Leithead & Lind 1964), and
there seem also to be some differences between
different sites on the body. In response to a stan-
dard heat stress, there is an earlier onset of sweat-
ing and an increased sweat rate with training and
acclimation, but the electrolyte content decreases
although it would normally be expected to
increase with increasing sweat rate, at least for
sodium. These adaptations allow improved
thermoregulation by increasing the evaporative
capacity while conserving electrolytes. The con-
servation of sodium in particular may be impor-
tant in maintaining the plasma volume and thus
maintaining the cardiovascular capacity.
The major electrolytes in sweat, as in the extra-
cellular fluid, are sodium and chloride (Table
15.2), although the sweat concentrations of these
ions are invariably substantially lower than those
in plasma, indicating a selective reabsorption
process in the sweat duct. Contrary to what
might be expected, Costill (1977) reported an
increased sodium and chloride sweat content
with increased flow, but Verde et al.(1982) found
that the sweat concentration of these ions was
unrelated to the sweat flow rate. Acclimation
studies have shown that elevated sweating rates
are accompanied by a decrease in the concentra-
tion of sodium and chloride in sweat (Allan &
Wilson 1971). The potassium content of sweat
appears to be relatively unaffected by the sweat
rate, and the magnesium content is also
unchanged or perhaps decreases slightly. These
apparently conflicting results demonstrate some
of the difficulties in interpreting the literature in
this area. Differences between studies may be
due to differences in the training status and
degree of acclimation of the subjects used as well
as difference in methodology: some studies
have used whole-body washdown techniques to
collect sweat, whereas others have examined
local sweating responses using ventilated cap-
sules or collection bags.
Because sweat is hypotonic with respect to
body fluids, the effect of prolonged sweating is to
increase the plasma osmolality, which may have
a significant effect on the ability to maintain
body temperature. A direct relationship between
plasma osmolality and body temperature has
been demonstrated during exercise (Greenleaf
et al.1974; Harrison et al.1978). Hyperosmolality
of plasma, induced prior to exercise, has been
shown to result in a decreased thermoregulatory
effector response; the threshold for sweating is
elevated and the cutaneous vasodilator response
is reduced (Fortney et al.1984). In short-term
(30 min) exercise, however, the cardiovascular
and thermoregulatory response appears to be
independent of changes in osmolality inducedthermoregulation and fluid balance 209
Table 15.2Normal concentration ranges of the major
electrolytes in sweat, plasma and intracellular water.
From Maughan (1994b).
Intracellular
Sweat Plasma water
(mmol · l-^1 ) (mmol · l-^1 ) (mmol · l-^1 )Sodium 20–80 130–155 10
Potassium 4–8 3.2–5.5 150
Calcium 0–1 2.1–2.9 0
Magnesium <0.2 0.7–1.5 15
Chloride 20–60 96–110 8
Bicarbonate 0–35 23–28 10
Phosphate 0.1–0.2 0.7–1.6 65
Sulphate 0.1–2.0 0.3–0.9 10