"You can survive for several weeks
without food, but only a few days without water"!
Water is extremely important for a
healthy life for the following reasons ....
65% of our body is water, its
in every cell and makes up most of the blood.
It is a versatile solvent -
nutrients like glucose, salts, amino acids are all soluble in
blood, carbon dioxide and oxygen are exchanged via blood and
other waste products like urea are dissolved and removed in
It is essential for enzyme
action - water is the medium for their substrates and products.
Water has a specific high
heat capacity, that is important for temperature regulation -
Typical daily human input and
output of volumes of water - which must be constantly kept in balance
Volume taken in cm3:
drink 1050; in food 600; from respiration 250; total 1900
Volume given out cm3:
urine 1000; exhaled air 350; skin 400; faeces 150;
total 1900 cm3
There is considerable
variation in these values from person to person and textbook to
If the water or
ion content of the body is wrong, too much water may move into or out of the
cells and damage them.
If your blood becomes too
concentrated in e.g. ions (salts), water is drawn out of the cells and tissues by osmotic
action and they become dehydrated. Under these circumstances enzymes
cease to work efficiently and so do the cells.
If the concentration of e.g.
sodium ions in the blood increases, the concentration of water
in the blood decreases too. The brain will detect the blood
needs more water and the pituitary gland releases more ADH, so
more water will be reabsorbed. from the collecting ducts in the
kidney, so the water content in the blood will rise.
ADH negative feedback
mechanism for water balance
If the blood becomes too dilute
the reverse osmotic action happens. Water will collect in the
tissues and the cells swell up - again a situation of imbalance.
The water content of the blood is
continually influenced by the temperature around us (how much we
sweat), our diet and the amount of water we lose (see below).
The water content of the body –
our skin keeps us waterproof but water leaves the body
(i) the lungs when we breathe out
(you see the condensed water from our breath in the winter),
(ii) the skin when we sweat to cool us
(iii) excess water is lost via the kidneys in the urine,
(iv) and in our faeces.
and this 'output' is balanced by
the 'input' of water we take in from food and drinks,
BUT, we can't control how much we
lose in the ways described above, so we need a balancing system
between the amount of water we consume and the amount of water
removed by the kidneys in urine.
We need to, and are continually
taking in water via drinks and food.
Any loss needs to be replaced,
more so in the summer when we sweat more than in the winter.
On cold days you sweat less and
pass more pale dilute urine, and on warm days you sweat more and pass more
darker coloured more concentrated urine (assuming a similar fluid intake
each day), either way the water balance is maintained.
After vigorous exercise (keeping
fit or sport) you will have lost water and ions through sweat as well as
burning up more glucose than usual.
Sports drinks contain water, ions and
sugar to replace those lost but there are many products on the market
competing for this lucrative revenue stream and each claiming to be just the
right drink to take!
I gather from a TV program using
a cheap, 'to dilute' fruit juice drink with a spoonful of sugar
dissolved in it,
is just as effective - but my doctorate is in chemistry!
The control of the body's water
balance via the ADH hormone negative feedback mechanism is described in the