Cold Weather Adaptations and Physiological Responses

Human Adaptation to Cold

• Humans are generally well-adapted to different parts of the planet, colonizing most areas from a tropical origin in sub-Saharan Africa.

• However, bodies aren't naturally adapted to the cold; adaptations to cold climates are primarily behavioral.

• Behavioral adaptations include:

  • Using radiators and heating systems.

  • Wearing insulated clothing to retain body heat.

  • Staying indoors.

• Unlike heat adaptation, the body doesn't make significant physiological adjustments to tolerate cold.

Physiological Responses to Cold

• When behavioral strategies aren't enough, physiological changes are needed to maintain thermal balance in cold weather.

• One way to increase heat storage in the body is to increase metabolism.

• The body manipulates the heat balance equation (radiation, conduction, convection, and evaporation) to maintain heat storage.

Vascular Adjustments

• In cold weather, the body tries to conserve heat and keep the central part of the body warm.

• Vasoconstriction occurs, reducing blood flow to the periphery (hands, toes, ears, nose).

• Reduced blood flow can lead to cell death due to lack of oxygen, resulting in frostbite.

Muscle Activity

• Muscle activity increases to generate heat.

• This includes:

  • Increased energy metabolism (leading to increased food intake).

  • Shivering: involuntary muscle contractions to increase heat production.

• Shivering can exhaust energy reserves and stop after prolonged cold exposure.

  • Shivering starts involuntarly at around 10 degrees Celcius.

Hormonal Changes

• Hormones like adrenaline and noradrenaline are released to stimulate heat production.

Key Goals of Interventions

• Increase internal heat production.

• Prevent heat dissipation.

Initial Physiological Responses

• Skin vasoconstriction reduces blood flow, decreasing heat transfer between the core and the shell.

• Heat loss from exposed surfaces is faster than replacement, so skin temperature declines.

Plasma Volume Changes

• Cold exposure decreases plasma volume by about 10\%.

• This leads to cold-induced diuresis (increased urination) because the body centralizes blood volume and excretes excess fluid.

• Dehydration can occur quickly in the cold, reducing the amount of oxygen available to muscles, which impairs exercise performance.

• This increase in central blood volume is mediated by the inhibition of vessel presence secretion.

• Cold water exposure leads to a greater decrease in plasma volume than cold air exposure.

• Acclimation to cold doesn't prevent plasma volume decrease.

Metabolic Rate Changes

• As core body temperature decreases, metabolic rate increases, leading to an increased VO2 requirement during shivering.

• Shivering requires 15 to 20\% of maximum voluntary contraction, demanding significant energy.

• Lactate threshold may increase by up to 49\% in -2 degrees.

Problems Caused by Cold

• Reduced efficiency due to decreased nerve conduction velocity.

  • Nerve signals take longer to reach their destination.

• Consequences on muscle function:

  • Improper muscle recruitment.

  • Muscle damage due to poor coordination.

  • Decreased muscle efficiency.

  • Dehydration affecting muscle function.

  • Shivering interfering with motor control.

  • Decreased perfusion due to reduced plasma volume.

• Increased metabolic cost of movement on snow and ice, along with the weight of clothing and equipment.

Homeostatic Responses

• Cold stress leads to changes in adrenal response, altering energy use.

• Causes generic activation by catecholamines, stimulating various physiological responses, such as lipolysis.

• Catecholamines stimulates glycogenolysis, particularly in the liver, to generate more glucose and neo genesis within the muscle itself.

• Stress in the brain activates the hypothalamus to release, uh, a zero, uh, ACTH, which activates the pituitary gland, stimulates the release of a hormone which activates the adrenal cortex on the kidneys to secrete various different metabolites, such as cortical and cortical steroids.

• The adrenal medulla is also activated to release adrenaline or noradrenaline.

• A 3 to 4 degree centigrade decrease in mean body temperature can cause a significant increase in oxidation of carbon dioxide and increased fat oxidation to meet energy demands.

Nerve Conduction Velocity

• Nerve conduction velocity slows down as temperature decreases.

• This is due to decreased sodium channel gating, which reduces the spread of the action potential along the axonal membrane.

Cold Sensing Channels (TRP Channels)

• TRP channels are cold-sensing receptors located in high density in the oral cavity and sensory nerve endings in the periphery.

• They detect cold stimuli and send information to the brain.

• These channels are typically active between 10 and 25 degrees Celsius.

• Menthol activates these channels, creating a cooling sensation.

• Other TRP channels activate at different temperature ranges.

• Manipulating these channels with menthol can test how the body responds to cold.

Menthol and Performance

• Menthol can trick the brain into thinking it's cooler in hot environments.

• Studies show that people can exercise longer in hot environments when given menthol because they feel cooler.

• This is mediated by a decrease in thermal sensation.

Hypothermia and Cold Weather Injuries

• Normal core body temperature is around 37 degrees Celsius.

• Effects of hypothermia begin to occur when body temperature drops to 35 degrees Celsius (mild hypothermia).

• Shivering and vasoconstriction can maintain core body temperature for some time.

• It is very difficult to get somebody to decrease body temperature.
  * We get them the heat we exercise and body temperature increases to get people cold.
  * It's very difficult. Unless of course you put in a cold water bath and then you lose body heat quite quickly.

• A two degree drop, you see maximum shivering, increased blood pressure beyond that 34 degrees.

• Amnesia, poor judgement, behaviour changes starts at 34 degrees .

• Apthy starts at 33 degrees.

• Being a stupor starts at 32 degrees.

• Unconsciousness can occur around 29 degrees

• Symptoms of severe hypothermia include loss of reflexes, reduced cerebral blood flow, and diminished brain activity; beyond this point is typically death.

• There are documented cases of accidental hypothermia survival at very low body temperatures.

Wind Chill Index

• The Wind Chill Index accounts for wind speed and temperature to determine the effective temperature.

• The higher the wind, the more effective the cooling.

• Exposure times for athletes training in cold environments can be determined using this index.

Frostbite

• Frostbite results from vasoconstriction, reduced blood flow, and hypoxia in the periphery.

• This leads to:

  • Increased metabolic acidosis.

  • Capillary wall damage and oedema.

  • Increased blood viscosity.

  • Aggregation of blood cells, forming clots.

• Direct freezing of tissues leads to ice crystal formation, dehydration, and cell death.

• The combination of all these things leads to widespread cell death (frostbite).

• Risk factors include:

  • Low air temperature and wind chill.

  • Exposed skin.

  • Moisture in skin and clothing.

  • Contact with cold environments.

  • Tight clothing or boots.

  • Dehydration.

Cold Induced Vasodilation

• Cold induced vasodilation the hunting reaction, in response to cold and your specific constriction

• is a protective response that attempts to save the fingers from widespread vasoconstriction.

• Sudden, acute increases in vasodilation occur.

• The body briefly relaxes vasoconstriction every 5-10 minutes to allow blood flow to the periphery.

Degrees of Frostbite

• First-degree: redness and oedema, no tissue loss.

• Second-degree: blisters with clear fluid.

• Third-degree: blisters with bloody fluid, skin thickness, black masses; healing takes 3-4 months.

• Fourth-degree: damage penetrates all skin layers, leading to gangrene and amputation.

Heat Loss in Water

• Thermal conduction is 26 times greater in water than in air.

• Falling into cold water is dangerous; body temperature decreases rapidly.

• At 15 degrees water, body temperature decreases at a rate of 2 degrees per hour.

• At 4 degrees water, body temperature decreases at a rate of 3 degrees per hour.

• Moving water facilitates heat loss even more quickly.

Water Temperature and Survival

• Outdoor swimming events have temperature thresholds for safety.

• Based on scientific investigations, it has been determined that the the highest point at which wetsuits armed forces around about 22 degrees and the lowest, which you're allowed to swim is about 12 degrees.

• Channel swimmers increase body fat to insulate against cold water.

Acclimation to Cold

• Humans chronically exposed to cold may experience:

  • Habituation (getting used to the cold).

  • Increased metabolism.

  • Increased fat retention.

• These adaptations are subtle, slow, and vary among individuals.

• Unlike heat acclimation, cold adaptations primarily affect peripheral regions.

Deep Body Cooling

• For adaptations to occur, deep body cooling (reducing core body temperature) is needed.

• This can lead to:

  • Changes in metabolic production.

  • Increased shivering thermogenesis.

  • Non-shivering thermogenesis (increased metabolism without shivering).

  • Changes in vessel constriction and blood flow redistribution.

• Blunted shivering and vasoconstriction occur in response to cold environments.

Human Adaptation to Cold

• Humans adapt behaviorally to cold climates rather than physiologically.

• Behavioral adaptations: heating systems, insulated clothing, staying indoors.

Physiological Responses to Cold

• Physiological changes maintain thermal balance when behavioral strategies fail.

• Increased metabolism boosts heat storage.

• Body manipulates radiation, conduction, convection, and evaporation.

Vascular Adjustments

• Conserve heat by reducing blood flow to extremities (vasoconstriction).

• Can cause frostbite.

Muscle Activity

• Muscle activity generates heat.

• Increased energy metabolism and shivering (involuntary muscle contractions).

• Shivering exhausts energy reserves.

Hormonal Changes

• Adrenaline and noradrenaline stimulate heat production.

Key Goals of Interventions

• Increase internal heat production and prevent heat dissipation.

Initial Physiological Responses

• Skin vasoconstriction reduces heat transfer; skin temperature declines.

Plasma Volume Changes

• Cold exposure decreases plasma volume by ~10\%, leading to cold-induced diuresis.

• Dehydration impairs exercise performance.

• Cold water exposure causes greater plasma volume decrease.

Metabolic Rate Changes

• Metabolic rate increases as core temperature decreases, increasing VO2 during shivering.

• Shivering requires 15-20\% of maximum voluntary contraction.

• Lactate threshold may increase by up to 49\% in -2 degrees.

Problems Caused by Cold

• Reduced nerve conduction velocity.

• Muscle function consequences: improper recruitment, damage, decreased efficiency, dehydration, shivering interference, decreased perfusion.

• Increased metabolic cost of movement with heavy clothing and equipment.

Homeostatic Responses

• Cold stress changes adrenal response, altering energy use.

• Catecholamines stimulate lipolysis and glycogenolysis.

• Decreased body temperature increases carbon dioxide oxidation and fat oxidation.

Nerve Conduction Velocity

• Nerve conduction velocity slows due to decreased sodium channel gating.

Cold Sensing Channels (TRP Channels)

• TRP channels detect cold stimuli, especially between 10 and 25 degrees Celsius.

• Menthol activates these channels, creating a cooling sensation.

Menthol and Performance

• Menthol can trick the brain into feeling cooler in hot environments, improving exercise duration by decreasing thermal sensation.

Hypothermia and Cold Weather Injuries

• Hypothermia starts at 35 degrees Celsius (mild).

• Symptoms: shivering, vasoconstriction, amnesia, poor judgement, apathy, stupor, unconsciousness.

• Severe hypothermia: loss of reflexes, reduced cerebral blood flow, death.

Wind Chill Index

• Accounts for wind speed and temperature to determine effective temperature.

Frostbite

• Results from vasoconstriction, reduced blood flow, and hypoxia.

• Leads to metabolic acidosis, capillary damage, increased blood viscosity, and cell death.

• Risk factors: low temperature, wind chill, exposed skin, moisture, tight clothing, dehydration.

Cold Induced Vasodilation

• Protective response that briefly relaxes vasoconstriction to allow blood flow to the periphery.

Degrees of Frostbite

• First-degree: redness and oedema.

• Second-degree: blisters with clear fluid.

• Third-degree: blisters with bloody fluid, skin thickness, black masses.

• Fourth-degree: damage penetrates all skin layers, leading to gangrene and amputation.

Heat Loss in Water

• Thermal conduction is 26 times greater in water than in air; body temperature decreases rapidly.

• At 15 degrees water, body temperature decreases at a rate of 2 degrees per hour.

• At 4 degrees water, body temperature decreases at a rate of 3 degrees per hour.

Water Temperature and Survival

• Swimming events have temperature thresholds.

• Wetsuits highest at 22 degrees, lowest at 12 degrees.

Acclimation to Cold

• Chronic cold exposure leads to habituation, increased metabolism, and fat retention.

• Primarily affects peripheral regions.

Deep Body Cooling

• Required for adaptations: changes in metabolic production, increased shivering/non-shivering thermogenesis, and changes in vessel constriction and blood flow redistribution.