APK 3110C Lecture Notes - Lecture 14: Cerebral Circulation, Thermal Conduction, Altered Level Of Consciousness
6 May 2018
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Thermal Stress- Chapter 25
Methods of heat exchange
• Heat conservation
o Blood redirects to deep cranial, thoracic and abdominal cavities and parts of
muscles
o Optimizes insulation from subcutaneous fat and body “shell”
o Shivering in severe cold can increase oxygen consumption to 1200 mL/min
• Heat dissipation
o Sweating rate of 2 L/hr during exercise in the heat
• Activation of heat regulating mechanism
• Skin thermoreceptors
• Early warning system about environmental temperature changes à appropriate
physiological adjustments then made
• Changes in temperature of blood that perfuse hypothalamus
Heat Stress
• Primary thermoregulatory mechanisms defend against overheating
• 4 processes of body heat loss
o Radiation
o Conduction
o Convection
o Evaporation
Heat Loss: Radiation
• Human body is normally warmer than surrounding environment à radiant energy travels
to other cooler objects in the environment
• Examples
o Sunlight
o Reflection off of snow, sand, water even if ambient temp is low
o Surfaces do not have to be touching
Heat Loss: Conduction
• Heat exchange via direct heat transfer from one molecule to another through a solid,
liquid or gas
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• Air molecules or surfaces that contact the skin get warmed
• Must be a temperature gradient between the skin and surrounding surface
• Water vs. Air
o Water absorbs heat faster and pulls it away from the body à water feels cooler
than air of same temperature
Heat Loss by Convection
• Air or water flowing by/over skin causing heat loss
• More air flow (breeze) = more heat loss
• In water: body loses heat faster when moving (swimming) than if it was stationary
Heat Loss: Evaporation
• Water vaporizing from the respiratory passages and skin surface transfers heat to the
environment
• Each vaporized liter = 580 kcal transferred to environment
• Sweating
• Hypotonic saline solution
• Evaporation from skin has cooling effect
• Blood from warmer interior then goes to skin and gets cooled
• Lose about 300 mL of water each day from respiratory passage
• Evaporation is major defense against overheating
• When ambient temperature > Body temperature:
• Body gains heat via conduction, convection, radiation
• Sweating/evaporation is only means of heat dissipation
• Sweat rate usually increases with temperature
• Amount of sweat vaporized from skin and pulmonary system depends on:
o Surface exposed to the environment
o Temperature and relative humidity of ambient air
o Convective air currents across the body
o Effectiveness of evaporative heat loss depends on relative humidity
o Humidity: Percentage of water in ambient air compared with its maximal
saturation
• In high humidity evaporation from skin decreases or stops à no heat dissipated, only
water lost à dehydration and overheating
• Core temp can increase to dangerous levels within 30 min of moderate to high intensity
activity in > 95°F and > 60% relative humidity
• Drying sweat off with a towel prevents evaporative cooling (sweating does not cool you
then)
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Document Summary
Heat stress: primary thermoregulatory mechanisms defend against overheating, 4 processes of body heat loss, radiation, conduction, convection, evaporation. Heat loss by convection: air or water flowing by/over skin causing heat loss, more air flow (breeze) = more heat loss. In water: body loses heat faster when moving (swimming) than if it was stationary. Heat dissipating mechanisms: note, heat can originate internally or externally, circulatory system. Physical activity in the heat: competition for arterial blood between the muscles (energy metabolism) and the periphery for heat dissipation, constriction of splanchnic vascular bed and renal tissues to decrease blood flow to these sites. Heat tolerance: acclimatization, physiological adaptations that improve heat tolerance, 2-4 hr of daily heat exposure, start with 15-20min of light intensity exercise. Complications from heat stress: heat illnesses, heat cramps, heat exhaustion, heat stroke, most commonly occur in overweight, acclimatized, unfit individuals who exercise when they are dehydrated.
