RESPONSE TO HEAT STRESS

Thermoregulation Definition: Mechanisms behind temperature regulation -Humans, like all mammals (and birds) are homeotherms, they have a 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 the hypothalamus

The hypothalamus promotes activities that effect the rate of heat loss or heat production/ heat retention Supplemented by behavioural changes such as: - clothing alteration - hot/cold drinks - moving to a different area

HEAT TRANSFER Heat is transferred (lost) from one place to another by four 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 muscle relaxes (“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, as they 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 physiological 2.Plants are up off the ground preventing heat transfer by conduction structural 3.Vasodilation physiological

4.Pale-coloured fur and feathers reflect sunlight structural 5.Increase in transpirational rates from leaves physiological 6. Decrease physical activity levels behavioural 7.Bats urinate on themselves to promote evaporative cooling behavioural

8.Jack rabbits have very long ears that are rich in blood vessels to dissipate heat structural 9.Vasodilation of blood vessels near the skin physiological 10. Animals move into cooler areas i.e. under tree canopy or into burrows behavioural

Cooling off………

Jack Rabbit

11. Migration of birds to cooler climates behavioural 12. Small size offers ability to hide under rocks or in burrows to escape the heat structural 13.Long-legged beetles/lizards absorb less heat from the ground structural 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 structural 16.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?