Environmental Physiology: Cold

Heat Balance

Humans are homeotherms

  • Use behavioural and physiological mechanisms to regulate our core temp
    • Resting Tc is 36.5-27.5
    • Core is defined as temp of hypothalamus which is the site of temp regulation
  • Experimentally the temperature of the core is measuring using
    • Rectal and esophageal probes
    • Pills that are swallowed and send a temp signal from the gastrointestinal system
  • Skin temperature is measured with dermal patches
    • Skin temp is influenced by environmental, metabolic rate, clothing and hydration state

During exercise Tc can rise above 40 degree Celsius, its increase is proportional to exercise intensity

Hypothermia

  • Tc is below 35 degrees Celsius

Heat Exhaustion

  • Tc is 38.5 - 40 degree Celsius

Heat stroke

  • Tc is 40 degree Celsius

Human body can gain heat through (Mechanism for heat gain)

  • Basal metabolic rate
  • Thermic effect of food
  • Shivering
  • Environment
    • Radiation
    • Conduction
    • Convection

Mechanism of Heat Loss

  1. Radiation
  2. Conduction
  3. Convection
  4. Evaporation

Radiation

  • Exchange of electromagnetic energy waves emitted from one object and absorbed by another

Conduction

  • Occurs whenever two surfaces with differing temperatures are in direct contact
    • Putting your hand on a
  • Insulators
    • Don't conduct heat readily
    • Still air is an excellent insulator, while water is an excellent conductor. Heat conduction in water is about 25 times greater than in air
  • Rate of Conductive heat exchange is inversely related to thickness of the insulating substance
    • Air trapped in goes down clothing, body fat, layer principle of dressing for the cold

Convection

  • Requires that one of the media be moving as occurs with a fluid or gaseous medium
    • Heat transfer from skin to moving air or water
  • Wind Chill Index
    • Gives the equivalent still air temperature for a particular ambient temperature at different wind velocities
  • Temperature gradient between the skin and air/water velocity are important factors in determining convective heat loss
  • Convective heat loss/gain during swimming is more than twice that during rest in still water

Evaporation

  • Transfer of heat from the body surface through the change of liquid water on the skin to a gaseous water vapour in the environment

Physiological Responses to a Cold Environment

2 primary physiological responses used by humans to defend against a cold environment:

  1. Increase in metabolic Rate
  2. Increased tissue insulation

Increase in Metabolic Rate

  • Voluntary (exercise)
    • Can increase heat production 10-20times the basal rate
  • Involuntary (shivering)
    • Can increase basal rate 3-4

Increase Tissue Insulation

  • Vasoconstriction of peripheral blood vessels and shunting of blood flow into deeper vessels
    • Blow flow stays deeper so it's not affected from the cold and retains heat
    • When its too hot, blood flow reaches skin surface to give off heat
  • Superficial blood vessels in the heat do not constrict in the cold, the heat is an important source of heat loss. Accounts for 30-35% of total heat loss at rest

Behavioural responses to the cold are important

Clothing, seeking shelter, starting a fire

Factors Affecting Response to Cold

Skinfold Thickness

  • The thicker the fat layer, the greater the insulation

Gender

  • Average female has more subcutaneous fat than the average male
    • More insulation
  • Women have larger surface area to mass ratio then men. Children have higher surface area to mass ratios
  • When men and women with similar skinfold thickness aer compared, women lose heat more rapidly than men during immersion in cold water

Clothing

  • Effective insulation given by clothing is a function of the air layer next to the skin, the thickness of the clothing and the air trapper between the layers of clothing
  • More clothing is needed at rest than exercise
  • Clothing insulation is reduced if the clothing becomes wet
  • Multiple layers of clothing are advisable during exercise in cold weather.
    • Outer layer - water repellent and wind resistant
    • Middle layers -provide insulation
    • Innermost layer - provide insulation and wick moisture away from the skin. Reduced evaporative heat loss
      • Polyester or polypropylene thermal underwear work, Merina wool is natural and works best
  • Clothing insulation is reduced if wind penetrates the outer garments

Cold Injuries

Hypothermia

  • Body core temp below 35 degrees Celsius
    • Body core is brain, heart, lungs, blood, liver kidneys
  • Critical areas for heat loss
    • Heat, neck, sides of chest, groin
  • Shivering will cease when core temp is below 32-34 degrees
  • Death occurs when Tc drops to 24-28 degrees

Factors associated with hypothermia

  • Immersion in cold water or wet clothing
  • Wind
  • Physical exhaustion
  • Inadequate clothing for conditions
  • Lower percent body fat
  • Hypoglycemia
  • Alcohol consumption
    • Decreases in shivering, increase blood flow to skin and impairs judgement

Hypothermia causes the oxyhemoglobin dissociation curve to shift to the left

Increased hemoglobin affinity for O2 > less O2 is released to tissues > body tissue shift to anaerobic metabolism >hypoxia of heat and brain plus metabolic acidosis > depressed brain function and cardiac output > ventricular fibrillation and death

Frostbite

  • Freezing of superficial tissues that occurs when skin temp reaches -2 to -6 degrees C
  • Many are unaware of frostbite because the sensory nerves are blocked and skin is numb

Cold Exposure and Respiratory Tract

  • Inhaled air is conditioned as it enters the upper respiratory passageways
    • Warmed to 37 degrees C and saturated with water vapour
  • Can't freeze your lungs when exercising in very cold weather
  • Cold air is very dry
    • Cells lining the respiratory passageway becomes dry
      • Possibility of throat irritation

Effects of Cold on Performance

Strength, Power and Flexibility

  • These all decreased when muscle and nerve temp decreases:
    • Strength and power
    • Nerve conduction velocity
    • Reaction time
    • Manual dexterity
    • Flexibility

Cardiovascular Endurance

  • Optimal marathon performances occurs at an ambient tempo of 14 degrees C
    • More circulation is directed to working muscles and less is required for heat dissipation
  • Due to increase metabolism and heat production with this exercise - a drop in Tc during exercise is rare
  • A drop can occur with very low intensity duration walking or trekking
  • If Tc does drop, there will be a reduction in aerobic performance

Cold Exposure

  • Cold-water immersion can result in a more dramatic and rapid heat loss
  • Water is 25* more conductive than air
  • Survival for more than a few hours in unlikely even in water that is 10 degrees C