the glomerular filtrate. Most of the reabsorption
occurs in the proximal tubule, but active absorp-
tion also occurs in the distal tubules and collect-
ing ducts. A number of factors influence the
extent to which reabsorption occurs, and among
these is the action of aldosterone, which pro-
motes sodium reabsorption in the distal tubules
and enhances the excretion of potassium and
hydrogen ions. Aldosterone is released from the
kidney in response to a fall in the circulating
sodium concentration or a rise in plasma potas-
sium: aldosterone release is also stimulated by
angiotensin which is produced by the renin-
angiotensin system in response to a decrease in
the plasma sodium concentration. Angiotensin
thus has a two-fold action, on the release of
aldosterone as well as ADH. Atrial natriuretic
factor (ANF) is a peptide synthesized in and
released from the atria of the heart in response to
atrial distension. It increases the glomerular
filtration rate and decreases sodium and water
reabsorption leading to an increased loss: this
may be important in the regulation of extracellu-
lar volume, but it seems unlikely that ANF plays
a significant role during exercise. Regulation of
the body’s sodium balance has profound impli-
cations for fluid balance, as sodium salts account
for more than 90% of the osmotic pressure of the
extracellular fluid.
Loss of hypotonic fluid as sweat during pro-
longed exercise usually results in a fall in blood
volume and an increased plasma osmolality:
both these changes act as stimuli for the release of
ADH (Castenfors 1977). The plasma ADH con-
centration during exercise has been reported to
increase as a function of the exercise intensity
(Wade & Claybaugh 1980). Renal blood flow is
also reduced in proportion to the exercise inten-
sity and may be as low as 25% of the resting level
during strenuous exercise (Poortmans 1984).
These factors combine to result in a decreased
urine flow during, and usually for some time
after, exercise (Poortmans 1984). It has been
pointed out, however, that the volume of water
conserved by this decreased urine flow during
exercise is small, probably amounting to no more
than 12–45 ml · h–1(Zambraski 1990).
The effect of exercise is normally to decrease
the renal excretion of sodium and to increase the
excretion of potassium, although the effect
on potassium excretion is rather variable
(Zambraski 1990). These effects appear to be
largely due to an increased rate of aldosterone
production during exercise (Poortmans 1984).
Although the concentrations of sodium and
more especially of potassium in the urine are
generally high relative to the concentrations in
extracellular fluid, the extent of total urinary
losses in most exercise situations is small.
The daily water intake in the form of food and
drink is usually in excess of obligatory water
loss, with the kidneys being responsible for
excretion of any excess and the regulation of
body water content. The kidneys can only func-
tion effectively, however, if the fluid intake is in
excess of the requirement. Drinking is a complex
behaviour which is influenced by a number of
physiological, psychological and social events.
The sensation of thirst is only one of the factors
involved, and short-term studies suggest that it
is a poor indicator of acute hydration status in
man (Adolph et al.1947). The overall stability of
the total body water content, however, indicates
that the desire to drink is a powerful regulatory
factor over the long term (Ramsay 1989).
The urge to drink, which is perceived as thirst,
may not be directly involved with a physiologi-
cal need for water intake, but can be initiated by
habit, ritual, taste or desire for nutrients, stimu-
lants, or a warm or cooling effect. A number of
the sensations associated with thirst are learned,
with signals such as dryness of the mouth or
throat inducing drinking, while distension of the
stomach can stop ingestion before a fluid deficit
has been restored. There are clearly changes in
the sensitivity of the thirst mechanism associated
with the ageing process, with older individuals
showing a reduced response to mild levels of
dehydration (Kenney 1995).
Notwithstanding the various factors that
modulate the subjective perception of thirst,
there is an underlying physiological basis involv-
ing both chemical and pressure sensors. The
sensation of thirst is controlled separately by