Lecture 33: Thermal Stress Flashcards
What is the normal range of body temperature?
36.5 –> 37.5
What are the consequences of excessive heat stress?
Heat stroke and brain damage
What area of the body is core temperature taken from and why?
The hypothalamus because it is the temperature control centre of the body
There are also no heat changes in this part of the body
Give a brief description of the four ways in which body temperature can be measured.
Oral (affected by hot drinks and exercise)
Rectal (affected by blood temperature of exercising lower limbs)
Tympanic (measures brain blood temperature but can be affected by heat loss due to sweating)
Stomach (most effective and can move through GI tract to get an overall picture of body temperature)
What is the average skin temperature?
32.5 –> 35
What is the determinant of an object’s temperature?
Its kinetic energy
How do we equate heat storage?
S = M +/- Cv +/- Cd +/- R - E
What are our heat loss mechanisms?
Conductance
Convection
Radiation
Evaporation
What are our heat gain mechanisms?
Metabolism (BMR) Convection Conductance Radiation Postural changes Thermic effect of food Muscle activity Hormones Environment
Why does our metabolic heat production rise with exercise intensity?
Because we perform more aerobic/anaerobic energy conversion which produces ATP and 75% heat
What is the key reason we cannot store heat?
Denaturation of key enzymes
Describe conductance.
Travel of heat down a temperature gradient when two objects are in direct contact with one another
3% of heat loss
How heat is passed between cells
Dependent on: a) conductivity of an object b) temperature gradient
Describe convection.
Travel of heat between an object and moving fluid or air
12% of heat loss
Depends on: a) temperature gradient b) velocity of moving substance c) conductance of object d) surface area of substance
As described by the wind-chill index, what is the effect of increased wind speed on internal temperature?
Increased wind-speed lowers internal temperature (increased velocity of flow)
Describe radiation.
Heat loss/gain due to EM waves
60 % of heat loss
Depends on: a) temperature gradient b) area of exposed surface c) colour d) reflectivity
What is latent heat of evaporation?
The quantity of heat lost via sweat evaporation
Describe evaporation.
The loss of heat via sweating
25% of heat loss
Depends on: a) temperature gradient b) surface area exposed c) convection currents d) humidity of the room
Which areas of the body does the hypothalamus stimulate to control temperature?
Sympathetic stimulation to: adrenal medulla (epinephrine)/ sweat glands/ skin arterioles
Involuntary motor response: skeletal muscles
What is the effect of body temperature on cardiac functions and blood volume during VO2 max?
Increased body temperature increases heart rate and lowers blood volume and therefore stroke volume
Describe the integrated CV response to exercise and how it is affected by increased heat.
Increased blood flow to cutaneous system due to requirement to lose heat
This lowers the central blood volume as blood volume is lost as sweat
This lowers venous return to the right ventricle and therefore amount of blood that can be oxygenated by lungs
This causes vasoconstriction of non-exercising regions and increased heart rate
Why do we have greater vasodilation to the skin at rest?
Because there is less competition between the skeletal muscle and the cutaneous system for blood flow
What is the effect of dehydration on the body?
It decreases: plasma volume, cardiac output, sweat loss, work output by muscles, liver glycogen
It increases: heat storage and tolerance to heat stress
Why does sweating increase despite its inability to cool the body in high humidity and the negative consequences it has on blood volume?
The body can regulate heat loss via the hypothalamus, but there is no regulatory system that has the means to regulate the amount of sweat loss, therefore as the body gets hotter the hypothalamus just continues to increase sweat output
What are the physiological response to heat acclimatization?
Sweat loss: early stage of exercise; covers more body areas; more sweat loss
Decreased HR, increased cutaneous blood flow, more effective distribution of cardiac output
Describe how heat illness can develop.
Heat syncope –> heat cramps –> heat exhaustion –> heat stroke
What is the best treatment for decreasing body temperature?
Cold water immersion
Describe the factors that affect heat injury.
Fitness (better acclimisation/ more sweat loss) Hydration Humidity Wind Clothing Environment (convection/radiation) Metabolic Rate (exercise intensity)
What temperature is the body during hypothermia and what are the stages decline due to hypothermia?
Below 35
Two degree drop: maximum shivering
Four degree drop: ataxia and apathy
6 degree drop: unconsciousness
What has been found to be the effect of the cold on oxygen consumption during sub-maximal and maximal exercise?
Sub-maximal exercise in the cold leads to an increased VO2 due to shivering
What are the physiological responses to decreased temperature?
Decreased lipid breakdown Decreased skin blood flow Decreased heart rate Increased central blood volume Increased submaximal VO2
Describe cold acclimitisation.
Takes one week
Shivering threshold occurs earlier on
Improved peripheral blood flow
Improved ability to sleep
