Thermoregulation and Exercise in Hot Environments

temp regulation

normal: 37 C

above 45C: can damage proteins

below 34C: impair metabolism and cardiac function

what does contracting skeletal muscle produce

heat

preoptic anterior hypothalamus (POAH) function

maintains constant core temp around set point

response to core temp increase

sweat gland stimulation and cutaneous vasodilation

what is cutaneous vasodilation

withdrawal of vasoconstrictor tone

response to core temp decrease

decreased skin blood flow via vasoconstriction

what are pyrogens

proteins and signaling molecules from bacteria or virus

thermal events during submaximal exercise (cool enviornment)

heat production increases, venous blood leaves working muscle, POAH thermoreceptors increase heat loss

what does blood flow do to lose heat

blood flow increases to skin

what does blood flow to do retain heat

blood flow decreases to skin

where is the heat that isn’t lost stored?

body tissues

4 types of heat loss

radiation, conduction, convection, evaporation

radiation

transfer of heat via infrared rays to objects not in direct contact with body (60% of heat loss at rest)

conduction

heat loss due to contact with another surface

example of conduction

sitting on a cold chair

convection

heat transferred from body to air or water molecules

example of convection

fan or wind blowing air

evaporation

when water (sweat) gains sufficient heat and is converted to gas (water vapor)

what is the most important type of heat loss during exercise?

evaporation is the most important during exercise

during exercise what results when body temp increases?

sympathetic stimulation of eccrine glands

what is responsible for anticipatory sweating

SNS

when does sweat increase?

hot enviornment, higher body temp, larger body mass

central nervous system dysfunction

decreased motivation, reduced voluntary activation of motor units

cardiovascular dysfunction

reduced stroke volume, cardiac output and muscle blood flow

accelerated muscle fatigue

increased radical production, decreased muscle pH, muscle glycogen depletion

accelerated muscle fatigue

muscle glycogen depletion, increased h+ production and lactate, increased free radical production

what is muscle glycogen depletion?

increased rate of glycolytic enzymes and muscle glycogen breakdown

what happens when there is an increased cardiovascular strain?

reduced SV, decreased Q and muscle blood flow

stages of heat injury/hyperthermia

heat syncope, heat exhaustion, heat stroke

heat syncope

fainting/dizziness, low BP

heat exhausation

fatigue, confusion, dehydration

heat stroke

rectal temp over 40.5C, disorientation, hot skin

responses to cold stress

cutaneous vasoconstriction, shivering, non-shivering thermogenesis

voluntary heat production

about 70-80% of energy is released as heat during exercise

involuntary heat production

shivering increases about 5%, hormone action

peripheral vasoconstriction

reduced convective heat loss due to decreased blood flow to skin

what happens when blood flow is constricted

increased tissue insulation, SNS control, protects core body temp

what activates POAH

cold sensing afferents in skin and spinal cord

what is the primary signal for shivering

skin temp

non shivering thermogenesis

brown adipose tissues, norepinephrine, thyroid hormones

other physiologic responses to cold

urine formation increase, exercise induced asthma, heart workload increases

hypothermia

less than 35C, can result in cardiac arrhythmias or cardiac arrest

which gender shows faster reduction in body temp?

women

at what age do you become less tolerant to the cold

older than 60 years

which age group experiences faster body temp falls?

children