HA - hot macroenvironments

Conduction
Convection
Radiation
Evaporation

Transfer of heat between 2 solid objects in physical contact

Very minor role in human thermoregulation
Heat loss / gain proportionate to surface area

Gain/loss of heat through air/water contact with body

Convection currents: warm air rises and cool air moves in to replace it
Exchange rapid if skin warm and air cold

Exchange of electromagnetic energy between objects close to each other

High ambient temperature: radiant heat may be gained from the sun, ground and surrounding surface
Rate of thermal exchange affected by temperature, reflectance and surface area

Heat loss through conversion of liquid to a gas on the body surface eg. sweating

Surface area
Vapour pressure of surroundings
Surface temperature
Amount of liquid present on surface

Evaporation - especially when exercising

Deserts and savannahs
Extremely high temp - air temp up to 58C, ground temp up to 93C
High rates of heat gain from solar radiation during day
Heat causes rapid movement as hot air rises, increasing convective heat gain
Limited water restricts plant growth and natural shade
Wide temperature fluctuations, heat radiates away at night, leading to rapid cooling

Tropical rainforests
Moist air and clouds restrict solar radiative heat gain during the day
Abundant water and shade, lower daily temp variation
Less convective and radiative heat gain
Humidity slows evaporation, limits cooling
Higher humidity increases how hot it feels and increases risk of heat injury

36-38C for bodily functions

Elevated skin and core body temp
Inadequate sweating
Dehydration
Dizziness and mental confusion
Nausea and collapse

42C

Body temp above 41C

Untreated = brain damage followed by death
Treated = mortality rate still at around 30%

Therefore, the dangers of heat stroke acts as a selective pressure, leading to adaptations

Behaviour and culture
Biological

Activity scheduling
Housing
Clothing

Vascular responses
Sweat glands
Body mass

Limit activity during hottest time to reduce heat generation

Thick walls delay heat absorption (day) and loss (night)
High heat capacity building materials
Minimal SA:V
Whitewashed to reflect sun

Air circulation
Air vents

Passive cooling of structures through evaporation
Very effective

Courtyard houses have pools/fountains
Water evaporates, removing heat and cooling air
Window screens keep sun out and wetting screens cools house as water evaporates

Inhibits convective heating from hot winds
Permeable to water to allow for evaporative cooling
Light, reflective colours reduce radiative heat gain
Dark colours protect better against sunburn
Loose clothes promote convection

As little clothing as possible

Peripheral vasodilation
Heat transferred away from body (core to shell to environment)

- Strain on cardiovascular system
- Increased HR and BP
- Blood pools in peripheral veins, slowing return to heart
- If air temp > body temp, heat absorbed into body

Apocrine
Eccrine

Associated with hair follicles
Armpits and groin
Secrete fluid rick in lipids and proteins
Rich substrate for bacteria, leading to smell
Not very involved in cooling response

All over body
Secrete water and salt

2,400kJ of stored heat energy
Up to 2-3 litres of sweat/hr

> 75% humidity
Dehydration still a risk

Vasodilation
High SA:V

- BUT overall cooling mechanisms ineffective

Increased sweating - initiated at lower temp and starts in limbs
Increased plasma volume (haemodilution) - reservoir for sweat

Reduced sodium ion losses (lower sweat salt content)
Reduced skin and core temp
Reduced HR and skin blood flow

Occurs within 10-14 days
Lost in few days/weeks

Individuals who inhabit warmer regions will have smaller body mass than individuals in colder regions

Pre 1950:
- Humans show evidence of Bergmann's rule

After 1953:
- Reduced association of human body mass with annual temp

Shorter extremeties (eg. limbs, ears) = reduced SA:V = more heat retention

1981
Neanderthal limb proportions associated with mean annual temp
Warmer = longer tibia/femur

1969
Pigs from same litters
Raised at 5/20/35 C
Cold group:
- More body hair
- Greater body mass
- Shorter limbs
- Shorter tails, ears

Intermediate group = intermediate morphology

Suggests adaptations plasticity > genetics

Shows climate related changes
Especially in infancy
Plasticity: infants are smaller, more affected by heat stress so have to adapt faster
Not necessarily plasticity - genetics may also play a role

Human body can be modelled as a cylinder
SA:V changes little with height
Pelvic breadth key to heat dissipation

Correlate latitude with stature and pelvic breadth
Far stronger postitive relationship between latitude and pelvic breadth than with stature
Higher latitude = colder

1. Dramatic reduction in direct solar radiation exposure
2. Higher distribution of body surfaces (higher wind speeds and lower air temp further from the ground) - increases rate of heat dissipation via convection
3. Greater airflow and lower relative humidity above surface vegetation increases sweat evaporation - also as a result of hair loss?

Macroenvironment: deserts, savannahs (both = intense midday heat, cold nights)
Leads to behavioural/cultural buffer: shading, housing, activity scheduling, evaporative cooling (essential resources include water, clothing)
Potentially progresses to biological buffer: SA:V, vasodilation, perspiration
Impact of demographic effects: infants, elderly, ppl with cardiovascular problems (less able to physically adapt)

+ Simplifies, allows focus of one stressor
- Environments often multi-stress

Macroenvironments: rainforests (little diurnal change)
Leads to behavioural/cultural buffer: ventilation, minimise clothing (essential resources include raised houses)
Potentially progresses to biological buffer: vasodilation, perspiration only somewhat effective
Impact of demographic effects: infants, elderly, ppl with cardiovascular problems