Lecture 14 - Temperature and fluid balance during exercise Flashcards
__% of oxygen consumption ends up as heat
80%
What are means by which we lose heat?
radiation, conduction, convection and evaporation of sweat (the primary mechanism)
convection is the ..
movement of air across the skin
the water vapour pressure in the atmosphere will influence…
the evaporation of sweat,
would much rather exercise in dry heat not humid
What determines the core temperature and what is the major determinant?
The balance of heat production and heat loss
major determinant is exercise intensity
we do regulate our body core temperature, but because our heat loss mechanisms lag, there is a ‘‘temperature deficit’’ which is determined by the …
how intense the exercise is
rectal temperature has been shown to be a bit higher experimentally, why?
heat from the legs skews the results
What can happen if the muscle temperature gets really high?
can start to break down membranes/proteins - damage the muscle
people can die from this
at 20 degrees what proportion of heat loss is from sweat?
about half
and increasingly more as the temperature is raised above 20 degrees
If you were exercising in water, relative to air, what would be the differences in the proportion of heat lost by conduction and convection?
it would be a greater proportion
Sweating is regulated by not just core temperature, also ___ temperature
skin temperature
What makes up sweat?
Na, Cl, K+, Mg2+
of decreasing proportions
Is sweat hypertonic or hypotonic?
hypotonic (the major substance you lose with sweat is water)
sweat is an ultrafiltrate of plasma,
If your sweating at a higher rate, you’re Na concentration will be…
a bit higher as there isn’t enough time to reabsorb it along the sweat duct
Trained indivuduals will be better able to maintain their plasma Na, why?
aldosterone not only acts on the kidney but also the sweat duct
after training, aldosterone is stimulated to absorb Na to a greater extent - this helps maintain a higher plasma volume
At higher temperature more CO and oxygen uptake is going to…
the skin
raises the question of whether you should warm up on a hot day - could warm up the muscle temp, but will also redistribute the CO
If you pre-expose to heat, and send alot of blood to the skin, the skin circulation means you will lose a bit of blood volume, meaning..
CO goes down, VO2 max goes down
heat stress impairs exercise capability due to…
cardiovascular limitations, not glycogen or other fuel substrates
True or False
Hyperthermia limits exercise performance?
true
What are the observations which show Exercise-heat stress and its relationship to ‘‘central fatigue’’
Increased rating of perceived exertion
Reduced voluntary activation of muscle
Reduced Cerebral blood flow, but increased VO2 (probably not a major factor)
Hyperprolactinaemia - hormone released from pituitary (prolactin), if you get an increase in this its thought to reflect changes in the brain that are associated with fatigue
- there are high levels of this in the blood during exercise
True or False
There is reduced neuromuscular performance with hyperthermia
true
If the brain is getting hot, why is there reduced meuromuscular performance?
With hyperthermia, the brain needs to shut down the muscle as it is the producer of the most heat
a good protective reflex
What are some strategies to enhance exercise performance in the heat?
Heat acclimatisation - get used to it
Pre-cooling
Fluid ingestion (to a lesser extent)
What are the Physiological and metabolic adaptations to heat acclimatisation?
Increased BV
reduced HR
lower core and skin temps
Increased sweat rate and earlier onset
more dilute sweat
reduced muscle glycogen use
plasma adrenaline is lower
Fluid ingestion does what?
blunts the rise in core temperature
prior dehydration does what to exercise performance?
decreases performance
what are the physiological effects of fluid replacement?
you blunt the CV drift because you prevent the loss of SV (CO)
What are the benefits of fluid ingestion?
Increased BV
Decreased HR
Increased SV and CO
lower core temp
lower plasma Na+ and osmolality
reduced muscle glycogen use
enhanced exercise performance
what is responsible for promoting fluid and salt retention?
Increases in plasma renin and aldosterone
Is there good evidence that IV fluid replacement is better than oral?
no - over the longer term
if it’s an immediate emergency
