RESPONSE TO HEAT STRESS

Thermoregulation

Definition: Mechanisms behind temperature regulation

- Humans, like all mammals (and birds) are homeotherms, they havea thermoregulatory control system

• Normal body temperature for a human being is 37° C

• The lethal core body temperature above this is 43° C

HEAT REGULATION

• Monitoring the body’s temperature is the job of thehypothalamus

• The hypothalamus promotes activities thateffect the rate of heat loss or heat production/heat retention

• Supplemented by behavioural changes suchas:

- clothing alteration - hot/cold drinks - moving to a different area

HEAT TRANSFER

Heat is transferred (lost) from one place to another byfour mechanisms:

1. Conduction: The transfer through direct contact, from molecule to molecule

2. Convection: The loss of heat to surrounding air due to movement of molecules from warm to cool areas “Hot air Rises”

3. Radiation: Heat energy in the form of infrared radiation. This energy can be given off by warm objects, and can be absorbed by cooler ones. “Feeling the sun’s heat”

4. Evaporation: Water absorbs heat until enough is present to change its state to a gas and then it carries heat energy away with it.

Rate is proportional to surface area and the temperature gradient, but inversely to the humidity!

Methods of Heat Transfer

Homeostatic Feedback Diagram

Temperature receptors sensory nerve in skin hypothalamus

detect heat motor nerve

promote heat loss effector to or carry out action decrease heat production and heat retention

Decreasing Heat Production

Only two ways:

• lowering rates of metabolic activity in cells• decreasing level of physical activity using

muscles

Cellular respiration supporting metabolism and muscle activity generates a lot of heat!

Promoting Heat Loss

1. Sweating Sweat is produced by special glands in the dermis Nervous stimulation can increase sweat production Sweat absorbs body heat through conduction and carries

it away through evaporation

water (liquid) + heat ---------> water (vapour) from body

SWEAT GLAND

2. HAIR LOWERING

• hundreds of hairs cover each square inch of skin• when raised, air is trapped around body by preventing air

flow over the body, keeping air still, allowing it to be heated• lowering the hair allows air flow over the body helping

heat transfer away from the body

3. VASODILATION

• Increase of blood flow to the skin as smooth musclerelaxes (“red-skin”)

• Diameter of vessel lumen increases, increasing blood flow • Heat is lost from the blood through conduction,

convection, and radiation

When thermoregulatory mechanisms fail…..

Causes:• Environmental heat exposure for prolonged periods• Physical activity in intense heat• High humidity levels• Consumption of alcohol, caffeine, stimulants, and

some medications may predispose individuals, asthey cause dehydration

Result: An increase in core body temperature above 37°C:

HYPERTHERMIA (hyper – above, therm – heat)

Stages of Hyperthermia

• Heat tetany – short periods of stress in intense heat(hyperventilation, respiratory problems, numbness, muscle spasms)

• Heat rash – skin irritation from excessive sweating• Heat cramps – muscle pains or spasms due to

exercise in hot weather• Heat exhaustion - heavy sweating, rapid breathing,

a fast, weak pulse• Heat stroke – body temp > 40.6°C dry skin, dizziness, confusion, ineffective sweating

PREVENTION OF HYPERTHERMIA

• Wear light, loose-fitting clothing• Wear wide-brimmed hats with vents• Avoid strenuous exercise in hot weather• Avoid enclosed spaces such as automobiles• Wear light-coloured clothing• Drink lots of liquids to replace fluids lost from sweating• Consume salt to replace amount lost from sweating• Avoid alcohol and caffeinated drinks• Use fans/air-conditioning• Rest regularly

ADAPTATIONS TO PREVENT HYPERTHERMIA

• STRUCTURAL: how the organism is genetically designed to allow it to successfully live in a particular hot environment, or to cope with periods of heat

• PHYSIOLOGICAL: internal adjustments made which will help dissipate heat, or lower heat production

• BEHAVIORAL: what the organism does to help cool itself and prevent over-heating

Behavioral, Physiological or Structural Adaptations?

1. Panting by a dog physiological2. Plants are up off the ground preventing

heat transfer by conductionstructural

3. Vasodilationphysiological

4. Pale-coloured fur and feathers reflect sunlight

structural5. Increase in transpirational rates from leaves

physiological6. Decrease physical activity levels

behavioural7. Bats urinate on themselves to promote

evaporative coolingbehavioural

8. Jack rabbits have very long ears that are rich in blood vessels to dissipate heat

structural9. Vasodilation of blood vessels near the skin

physiological10. Animals move into cooler areas

i.e. under tree canopy or into burrowsbehavioural

Cooling off………

Jack Rabbit

11. Migration of birds to cooler climatesbehavioural

12. Small size offers ability to hide under rocks or in burrows to escape the heat

structural13. Long-legged beetles/lizards absorb less

heat from the groundstructural

14. Estivation – become inactive and lower metabolism during the dryest times of year and to avoid sweltering heat

physiological

15.Leaves face directly up or down to decrease surface area hit by the Sun

structural16.Nocturnal Activity to avoid daytime heat

“do work when it is cooler”behavioural

SO… what if you’re experiencing heat stress and water loss??

Discussion: Which is best? Pros and Cons?