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WATER LOSS IN HORSES
Peter (Doc) Waller B.App.Sc.
EQUINE EDUCATION SERVICES
flyingm@broad.net.au
The majority of horse owners would not or could not be expected to
know the amount of water lost from a horse's body through sweat and
metabolic processes during hard work, however Endurance riders would
almost be an exception to this observation.
Sure, we all see our horse sweating, and sometimes running down the
legs and dripping off the belly, but have you ever stopped to think
just how much heat the horse has generated during the exercise it has
been required to perform and how much fluid loss this horse is
experiencing?
Huge amounts of heat are generated during exercise with about 80% of
muscle energy being aerobically metabolised and released as heat. This
release of heat will then raise the horse's core body temperature from
39o C to around 42o C in a matter of minutes.
Imagine the Melbourne Cup being run. Every single horse competing in
this race will generate enough body heat by producing something in the
vicinity of 4 million watts of energy each. If this heat could be
harnessed, it would be enough to boil about 30 litres of water in
around 3.5 minutes. Compare this with electric jugs or kettles that
use about 5000 watts of power to boil about 1.5 litres in the same
time.
It is during this heat release period we must preserve and maintain
fluid and electrolyte balances, which are vital in the prevention of
dehydration. These elements also help provide efficient temperature
regulation and heart and blood vessel (erythrocyte1.)stability and
function.
By maintaining these balances it will also assist in delaying the
effects and onset of fatigue in rapidly contracting muscles. (After
hard exercise the muscle fibres, after having been stretched must then
return to their normal position.)
Up to 350 litres of blood per minute is pumped to the working muscles
throughout the horses body, and this blood carries along with it
between 70-90 litres of oxygen. In fact, horses will circulate their
own blood volume 5-6 times through the major muscle pathways for every
minute they exercise at the gallop.
Sweating is the mechanism of the evaporative cooling process, and is
the most efficient method of removing heat from the muscles and
lowering the body core temperature. The red cells (haemoglobin) in the
blood also act as couriers to transport generated heat away from the
muscles to the lungs and extremities nearer the skin surface where
there is a greater overall surface area, and whereby more effective
cooling takes place.
At the same time, blood flow to the kidneys is cut to 20% of its
resting flow (when the horse is not working or exercising). This is to
maintain adequate blood volume in the body and to reduce urinary fluid
losses. This in turn then conserves more fluid for muscle activity,
sweat loss and cooling. The increase in blood volume in the body also
assists in the removal of lactic acid and other waste products of
energy metabolism during exercise.
Yet another effect of this blood flow drop to the kidneys is the
triggered release of the hormone, aldosterone. Once this is released
into the blood it acts on the kidney tubules and causes the
reabsorption of sodium salts in order to correct the deficiency in the
blood cells that first triggered the response (through osmotic
pressure). This increased sodium-salt reabsorption means that more
water is osmotically reabsorbed as well, and helps in restoring blood
volumes for further use in the muscles, and maintains muscle tissue
cell integrity.
As exercise continues, further fluids are then lost through sweat.
This then provides evaporative cooling in an attempt to lower the core
temperature of the horse.
Continued exercise can also lead to the onset of dehydration unless
the horse is allowed to rehydrate at intervals, and unless the horse
is allowed to rehydrate, the cellular osmotic pressure 2 can quickly
become compromised. When this occurs, the horse's metabolic system
will begin to shut down. Substantial loss of electrolytes though sweat
also causes fatigue, muscle weakness and 'tying up'. It also decreases
thirst response and may lead to dehydration. Severe cases of
dehydration bought about through loss of electrolytes may require the
horse be transfused with an Isotonic (saline) solution.
Thirst is stimulated when electrolyte concentrations in the blood
rise, if a large proportion of salt is lost in sweat, blood
concentrations remain static and though the horse is dehydrated he
does not have the physiological signal to drink. If you are familiar
with the term - 'you can lead a horse to water, but you can't make it
drink', this then become a literal term.
When homeostatic balance eventually returns and the horse then again
shows an interest in having a drink, it does not mean you allow the
horse to drink uncontrollably and take in huge volumes of water. It
means you supervise the amount of water being ingested, and only allow
smallish amounts until the core temperature begins to cool, then allow
more liberal access to water. It is also advisable to load the water
with electrolytes to replenish the body loss.
INDEX:
1. - Red blood cells (or erythrocytes) are small biconcave shaped
discs with a large surface area that specialise in oxygen exchange
across cell membranes. It is the haemoglobin within the erythrocyte
that is responsible for this oxygen transfer, and which gives the red
colour to blood.
2.- Cells have a requirement for pressure equilibrium on both sides of
the membrane, and the Osmotic balance refers to the movement of water
across these cell membranes. For instance, a cell has a higher
concentration of solute particles (protein or other dissolved
substances that are contained within the cell and cannot cross the
membrane) than the interstitial fluid that bathes the cell, then water
will move into the cell from the interstitial fluid until the pressure
is again equalised on both sides of the membrane.
Very quickly, this is what is termed as osmotic pressure, and it is
this osmotic pressure that is important in determining what will enter
or leave the cell in terms of fluids.
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