both the osmotic pressure of the body fluids and
the central venous volume. The same mecha-
nisms are involved in water and solute reabsorp-
tion in the kidneys and in the control of blood
pressure. The thirst control centres are located in
the hypothalamus and forebrain, and appear to
play a key role in the regulation of both thirst and
diuresis. Receptors in the thirst control centres
respond directly to changes in osmolality,
volaemia and blood pressure, while others are
stimulated by the fluid balance hormones which
also regulate renal excretion (Phillips et al.1985).
These regions of the brain also receive afferent
input from systemic receptors monitoring osmo-
lality and circulating sodium concentration, and
from alterations in blood volume and pressure.
There may also be a direct neural link from the
thirst control centres to the kidneys which would
allow a greater degree of integration between the
control of fluid intake and excretion. Changes in
the balance of neural activity in the thirst control
centres regulated by the different monitoring
inputs determine the relative sensations of thirst
and satiety, and influence the degree of diuresis.
Input from the higher centres of the brain,
however, can override the basic biological need
for water to some extent and cause inappropriate
drinking responses.
A rise of between 2% and 3% in plasma osmo-
lality is sufficient to evoke a profound sensation
of thirst coupled with an increase in the circulat-
ing concentration of ADH (Hubbard et al.1990).
The mechanisms that respond to changes in
intravascular volume and pressure appear to be
less sensitive than those that monitor plasma
osmolality, with hypovolaemic thirst being
evident only following a 10% decrease in blood
volume (Fitzsimons 1990). As fairly large varia-
tions in blood volume and pressure occur during
normal daily activity, this lack of sensitivity
presumably prevents excessive activity of the
volaemic control mechanisms. Prolonged exer-
cise, especially in the heat, is associated with a
decrease in plasma volume and a tendency for an
increase in plasma osmolality, but fluid intake
during and immediately following exercise is
often less than that required to restore normal
212 nutrition and exercise
hydration status (Ramsay 1989). This appears
not to be due to a lack of initiation of the drinking
response but rather to a premature termination
of the drinking response (Rolls et al.1980).
When a water deficit is present and volunteers
are allowed free access to fluids, the normal
drinking response involves an initial period of
avid drinking during which more than 50% of
the total volume is consumed; this is followed by
a longer period of intermittent consumption of
relatively small volumes (Verbalis 1990). The
initial alleviation of thirst occurs before sig-
nificant amounts of the beverage have been
absorbed and entered the body pools. Therefore,
although decreasing osmolality and increasing
extracellular volume promote a reduction in the
perception of thirst, other preabsorptive factors
also affect the volume of fluid ingested. Recep-
tors in the mouth, oesophagus and stomach are
thought to meter the volume of fluid ingested,
while distension of the stomach tends to reduce
the perception of thirst. These preabsorptive
signals appear to be behavioural, learned
responses and may be subject to disruption in
situations which are essentially novel to the indi-
vidual. This may partly explain the inappropri-
ate voluntary fluid intake in individuals exposed
to an acute increase in environmental tempera-
ture or to exercise-induced dehydration.
In addition to the water consumed in the form
of drinks, some water is obtained from solid
foods, and water is also available as a result of the
oxidation of nutrients. The amount of water
available from these sources will depend on the
amount and type of food eaten and on the total
metabolic rate. Oxidation of the components of a
mixed diet, with an energy content of 12.6 MJ
(3000 kcal) per day, will give about 400 ml water ·
day–1. The contribution of this water of oxidation
to water requirements is appreciable when water
turnover is low, but becomes rather insignificant
when water losses are high.Role of the kidney
The excretion of some of the waste products of
metabolism and the regulation of the body’s