54 | New Scientist | 31 August 2019
Seven litres a day
How does water hydrate us?
If we drink a lot of it we only
pass it as excess waste.Eleanor Horton,
Canterbury, Kent, UK
Breaking down food – and our
metabolism in general – generates
waste. This needs to be removed
or it would damage the body.
Water is the solvent for these
waste products – it dissolves them
and allows them to pass out of the
body as urine, as well as in sweat.
Urine is produced in the kidneys
and consists of urea and other
waste products dissolved in water.
We need to drink more water to
replenish the fluid that leaves the
body this way.
When you are dehydrated, your
urine will be dark yellow. This is
because there isn’t enough water
in your system to dilute the urea
sufficiently. Water itself is not a
waste product – it is a mechanism
by which the body removes waste.Andrew Sanderson
Spennymoor, County Durham, UK
We lose water in four principal
ways: in urine, sweat, breath
and faeces. This doesn’t include
minor losses such as in tears
and spitting. We gain water by
drinking, and by breaking down
food during metabolism into
carbon dioxide and water.
Sweat, breath and faeces stay at
the same concentration, so the
main control of body fluid content
is via our kidneys. Their activity
is controlled by a molecule known
as vasopressin or antidiuretic
hormone, which is secreted by
the pituitary gland. This is
regulated by an area of the brain
called the hypothalamus, which
contains receptors sensitive to
the blood’s concentration of
sodium and other substances.
The kidneys are the main way
for us to excrete salts. If you eat
a lot of salt, your kidneys will
increase the concentration of
the urine up to their maximum
ability. Past this, the volume will
increase. If you drink a lot of water,urine concentration falls and
volume increases. The more
you exercise and sweat, the
more salts you lose, because
sweat can’t be concentrated,
hence the marketable value of
sports drinks. The amount you
need to drink is unique to you.
Linked to the hydration sensors
are the thirst parts of your brain.
If you feel thirsty, then drink
water. Otherwise, keep someone
else happy and rich by buying
their fluid replacement and by
all means carry a bottle with you
to prove that selective advertising
has an effect on you.Brian Pollard
North Hill, Cornwall, UK
The human body is made up
mainly of water, and our
physiology operates to keep
within about a litre of the 45 or so
litres in an average-sized person.
The way it does this is by
regulating the feeling of thirst.
When the fluid level starts to get
low, your body makes you feelthirsty. You drink, and the body’s
regulatory system works out how
much you should drink to restore
the balance to be within required
limits. When you have drunk
enough, you feel sated and stop
drinking. This system is
remarkably efficient.
It takes several hours for the
fluid levels in the body to respond
to the liquid you have drunk, but
the regulatory system works well
enough most of the time to keep
the body’s fluid content within
its typical parameters.
If you lose a lot of water
quickly, on a very hot day for
example, you may lose too
much, and then you become
dehydrated, and it feels
unpleasant. The unpleasant
feeling is your body requesting
an urgent ingestion of liquid.Roll of honour
Why do dogs and horses roll on
their backs when happy?David Muir
Edinburgh, UK
Horses roll and writhe on their
backs not because they are happy
but because they want to get rid
of an itchy irritation. They could
be trying to get rid of their winter
coat, which makes them sweaty
in the summer. If they are being
bothered by biting insects, then
rolling in mud, or even dust,
affords some protection.
Dogs are different. A very
relaxed dog will lie on its back with
its vulnerable abdomen exposed.
On the other hand, a dog that is
frightened may roll over as a sign
of submission and thus avoid
attack by another dog.
Some dogs retain their
evolutionary urge to roll in other
animals’ excreta, such as fox
faeces, to disguise their own scent.
This seems to make dogs happy
and their owners very unhappy.Tony Holkham
Boncath, Pembrokeshire, UK
Horses, and many other animals,
roll to rid themselves of irritation
or parasites that they can’t reach
with their mouths or feet. It is
necessary, but leaves the animal
vulnerable for a short time.
Dogs roll for this reason too,
especially because many modern
breeds are unable to groom
themselves effectively. They
also do it to submit to another
member of the pack and, in
domesticated dogs at least,
because they love to have their
belly rubbed. My Jack Russell
Sparky would probably put this
last reason at the top of the list. ❚This week’s new questions
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