1

Exercise and Environment

Brooks Chapter 23

2

Introduction

• People have a remarkable ability to live and work in very hot and very cold environments.

• Humans tolerate these hot and cold climates due to a well developed ability to regulate core body temperature.

• Humans regulate core temperature through behavioral and physiological means.

3

• Poikilotherms: body temperature varies with the environment (lizards, insects).

• Homeotherms: maintain constant body core temperature (humans, birds, bears).

• In homeotherms, various physiological processes depend on normal body temperature to function properly.– at temps above 41oC, the interior of many cells begin

to deteriorate.

– at temps below 34oC, cellular metabolism slows greatly, leading to unconsciousness and cardiac arrhythmias.

4

Normal Body Temperature

• Normal resting core temperature in humans is 36.5oC to 37.5oC.

• During exercise body temp can exceed 40oC.

• There is considerable variation throughout the body. The core is relatively constant while skin temp is influenced by the environment.

5

Core Temperature• Usually defined as the temperature of the

hypothalamus.

• The hypothalamus is the temperature regulatory center of the body.

• Oral temp is influenced by breathing cool air.

• Rectal temp is a commonly used in research.

• Tympanic temp is close to the hypothalamus but it is influenced by head skin temp.

• Esophagus and stomach temp has also been used to measure core temp.

6

Hypothalamus• Works like a thermostat.

• Controls rates of heat gain and heat loss.

• Uses feedback from peripheral heat and cold receptors.

• Thermal receptors in the hypothalamus itself.

• Initiates the rate of heat production when body temp falls, and the rate of heat dissipation when temp rises.

7

• The assessment of mean body temp (MBT) takes into consideration skin and core temp.

MBT = (0.33 x SkinTemp) + (0.67 x RectalTemp)

• Thermal gradients are temp differences from one point to another that lead to the movement of heat.

• Thermal gradients determine the rate and direction of heat transfer.

8

Heat Transfer• The temperature of an object is a measure of

the kinetic activity of its molecules. If an object is hot, its molecules move rapidly.

• The temperature of a body is directly proportional to the amount of heat it stores.

• Core temperature is regulated by controlling the rate of heat gain and heat loss.

9

Energy Balance Equation

S = M + Cv + Cd + R - E

where:

S = heat storage (kcal/min or watts)

M = metabolic heat production

Cv= convective heat loss or gain

Cd= conductive heat loss or gain

R = radiant heat loss or gain

E = evaporative heat loss

10

Body Temperature, Environment and Exercise Intensity

• Core temp rises in direct proportion to relative exercise intensity when ambient temp is between 4oC and 30oC.

• Skin temp is a function of environmental temp, metabolic rate, clothing, and hydration state.

• Clothing and air movement across the skin affect skin temp by influencing the capacity for heat loss.

11

Heat Production

• Heat is a by-product of all biochemical reactions, as these reactions are not 100% efficient.

• Metabolism is the body’s source of internal heat production.

• At BMR the average body generates approximately 100kcal of heat per hour.

• During exercise heat production can be elevated 10 - 15 times resting values.

12

Shivering• Shivering is an involuntary contraction of muscle.

• The main mechanism for heat production during negative heat balance is shivering.

• Shivering can the body’s heat production 5x.

• Preshivering tone can increase heat production by 50 to 100%.

• Shivering is an effective way to body temp as all of the expended energy appears as heat (no work).

13

Nonshivering thermogenesis

thyroxin secretion from the thyroid gland and catecholamine secretion from the adrenals metabolic rate.

• Thyroxin increases the metabolic rate of all cells in the body.

• Catecholamines (norep) cause the release of fatty acids, which will metabolic heat production.

14

Q10 Effect

body temp can be dangerous because metabolic rate with rising temperature.

• By increasing the temp by 10oC you will double the rate of enzymatic reaction.

• At high temperatures (41.5oC) the hypothalamus loses its ability to cool itself.

15

Heat Balance

Heat GainHeat Loss

Metabolic heat prod Radiation

-exercise Conduction

-shivering Convection

-thyroxine Evaporation

-symp stim

-Q10 effect

Radiation

Conduction

Convection

16

Heat Loss

• When skin temp is greater than ambient temp, the body losses heat by radiation, conduction, convection and evaporation

17

Radiation• The body loses and gains heat by radiation

to its surroundings by transmitting electromagnetic waves.

• Radiation accounts for 60 % of the body's total heat loss at room temperature while at rest.

• The sun is the greatest source of radiant energy.

• Human skin absorbs 97% of the radient energy that strikes it.

18

Conduction

• Conduction of heat occurs between 2 surfaces in contact with each other.

• Only 3% of the body's heat loss at room temperature occurs through conduction.

• The rate depends on the temperature gradient between the skin and a surrounding surface, as well as the conductive property of the surface

19

Convection• The conduction of heat to or from air or water is

called convection.

• Convection accounts for 12% of heat loss at normal room temp.

• Convective heat flow depends on the extent of the body surface area exposed to the surrounding medium, the rate of flow of the medium, and the difference in temp between

the skin the medium.

• Heat loss by convection is greater in water (~25x) than air.

• Heat loss by convection is greater in the wind as warm air is replaced with cool air (windchill).

20

Evaporation• At rest about 25% of heat loss is due to

evaporation.

• The body loses .58 kcal of heat for each gram of water that evaporates (latent heat of vaporization).

• Evaporation is the body’s only cooling mechanism at high temps.

• Sweating is critical during exercise to keep core temperature at a safe level.

• Sweat is only effective for cooling if it evaporates.

21

Evaporation• There is insensible water loss (600 ml/day)

through vent and diffusion through the skin.

• Sweat rates are near 0 when skin temp is low.

• Max sweat rates: 1.5 L/hr sedentary unacclim– 4 L/hr acclimatized, 5L/hr acclim athlete

• Eccrine sweat glands tubular structures that cover most of the body (clear odorless sweat).

• Apocrine glands: under arms, genital areas

• Sweat contains NaCl, urea, KCl, lactic acid.

22

Temperature Regulation

• The hypothalamus acts as a thermostat and keeps the body’s core temp within a normal range.

• Thermoreceptors are located in different regions of the body and transmit nerve impulses to the spinal cord and then up to the hypothalamus.

• The greatest density of thermoreceptors are in the skin and hypothalamus, but there are also some in the blood vessels and abdominal cavity.

• There are more cold than warm receptors in the skin.

• When core temp goes above or below its set-point, the hypothalamus initiates processes to heat production or heat loss.

23

Temperature Regulation• The anterior hypothalamus stimulates sweat glands.

• Normally sweating begins at precisely 37oC.

• The set-point can change temporarily in response to dehydration, starvation, or fever.

• When cold receptors in the skin and hypothalamus are stimulated, various processes will heat production.

• The hunting reflex maintains BF to the hands and feet.

• Vasoconstriction can the effective core insulation.

• Stimulation of the shivering center causes shivering.

• The post hypo initiates release of norep and thyroxin.

24

Behaviour

• Human behaviour is an important component in temperature regulation.

• When the anterior hypothalamus is overheated a person will have the sensation of being hot.

• If a person feels warm they can drink water, remove some clothing, move to a cool area.

• A cold person can put on more clothes, go indoors, turn up the heat.

25

Exercise in the Cold

• Cold exposure can limit physical performance.

• Exposed skin will lose sensation and is susceptible to frostbite.

• Movement is restricted by heavy clothing.

• Shivering may decrease muscle efficiency.

26

Clothing• The insulation value of clothing must be balanced with

the metabolic heat production of exercise.

• Clothes the body’s insulation by trapping warm air next to the skin. This reduces heat loss by conduction and convection.

• The clo unit is a measure of the thermal insulation provided by clothing.

• Optimal cold weather clothing allows for evap of sweat while providing insulation.

• Wear clothes in layers so that it may be removed when metabolic and environmental conditions dictate.

27

Cardiopulmonary Responses to the Cold

• Maximal O2 consumption is unaffected in the cold.

• Submaximal VO2 increases in the cold due to greater heat loss through conduction and convection.

• The skin and muscle blood flow during exercise means a greater potential for heat loss in the cold.

• Shivering can persist during exercise and VO2 .

• Nonshivering thermogenesis may also VO2 .

28

• Swimming in cold water can significantly exercise capacity.

• Body fat is an important factor in determining heat loss in cold water.

• It is unclear if increased body fat in distance swimmers is a physiological or psychological adaptation.

• During exercise the peripheral blood vessels vasodilate during physical activity, which effectively the body’s insulation to the cold.

• Exercise followed by cold exposure raises post exercise thresholds for vasoconstriction and shivering.

29

Ventilation

• Ventilation in the cold, particularly when cold exposure is sudden.

• Abrupt exposure causes a gasping reflex, which is accompanied by hyperventilation, tachycardia, peripheral vasoconstriction and BP.

ventilation will blood CO2 vasoconstrict brain blood vessels confusion and unconsciousness drowning.

30

Heart• Cold exposure causes peripheral vasoconstriction, which

central blood volume.

• BP due to an afterload and preload. This may HR and SV.

• The incidence of arrhythmias in the cold.

• Cold secretion of epi which can the incidence of fatal arrhythmias.

• Ventricular fibrilation is the leading cause of death in people with hypothermia.

31

Muscle Strength

• Muscle strength and peak power output as muscle temperature .

• Enzymes for muscle contractions are not as efficient at lower temperatures.

• Other factors impairing muscle performance in the cold include muscle viscosity and ATP metabolism and muscle contractile velocity.

32

Metabolic Changes

• Cold exposure the use of CHO as a substrate.

• Muscle glycogen faster during light exercise in the cold.

• Prolonged cold exposure hypoglycemia, which shivering core temp.

• Fat metabolism is due to a in blood flow to subcutaneous fat cells.

• Cold exposure protein degradation.

33

Acclimatization to Cold

• Acclimatization: physiological compensation to environmental stress occurring over a period of time.

• Habituation: is the lessening of the sensation associated with a particular environmental stressor.

• Cold acclimatized people maintain heat production with less shivering through in thyroxine and sensitivity to norepi.

• Chronic exposure to cold may skinfold thickness.

• Physical conditioning results in an body temp during sleep tests in the cold.

34

Hypothermia• The hypothalamus ceases to control body

temperature at very low core temps (< 30oC).

• Hypothermia: central blood volume, peripheral resistance, blood viscosity, HR, more susceptible to arrhythmia.

• Hypothermia occurs when heat loss is greater than heat production (cold water, high wind chill, lack of clothing etc.).

• Frostbite occurs when ice crystals form within tissues. Risk when temp is below -6oC)

35

Rewarming from Hypothermia• Proper management of patients with hypothermia is crucial due

to high mortality rates.

• Afterdrop is the continued fall of deep body temp during active external rewarming from hypothermia.

• Afterdrop is thought to endanger the heart by further cooling from cold blood returning from the periphery.

• Vasodilation and the way heat moves through tissue causes afterdrop during rewarming .

• Afterdrop depends on the method of rewarming.

• Central rewarming with heated air (45oC) will heat the core and gradually heat the peripheral tissues.

• Passive rewarming is quick, easy and gradual.

36

Exercise in the Heat

• Exercise in the heat may produce a heat gain.

• Core temp proportionally with in exercise intensity.

• High environmental temp adds to the metabolic heat stress of exercise.

37

Cardiovascular Effects

• During intense exercise in the heat the combined circulatory demands of muscle and skin can impair O2 transport.

• Plasma volume during exercise, due to BP which the rate of plasma filtration from the vascular space.

• The in plasma volume is compounded by fluid loss through sweating.

• The in blood volume will preload and HR.

38

Cardiovascular Effects

• During submax exercise HR is .

• During maximal exercise skin vessels vasoconstrict and help maintain cardiac output.

• Circulatory regulation takes precedence over temperature regulation.

• VO2max is not impaired in the heat, unless the subject started with a thermal imbalance.

39

Sweating Response• Sweating is the primary means of heat dissipation

during exercise.

• During exercise sweat rate is more related to exercise intensity than environmental temp.

• The sweating rate differs in various parts of the body during intense exercise.

• The onset of sweating occurs at a lower core temperature in bright light.

• Pre-cooling the body prior to endurance exercise may improve performance.

40

Acclimatization to Heat

Physiological Change Effect

sweating threshold prevents early in Tcore

sweat rate evaporative heat loss

plasma volume HR, VO2max

better control of SV and BP

skin blood flow heat loss by radiation and convection

• These occur after 10 to 14 days of heat exposure.

41

Acclimatization to Heat• 2/3 to 3/4 of acclimatization occur in 4-6 days.

• Acclimatization to heat must be accompanied by exercise for optimal results.

• Athletes acclimatize quicker than sedentary individuals.

sweat concentrations of Na and Cl take 10 days.

in sweat rates are not fully evident until 14 days.

42

Thermal DistressDehydration: loss of body fluid.

• Moderate levels of dehydration (2%) will impair CV and temperature regulation and performance.

• A fluid loss of 5% of body weight will cause irritability, fatigue and discomfort. This level of dehydration is common in football and distance running.

• Dehydration levels > 7% are extremely dangerous. At this level salivating and swallowing are difficult.

• Dehydration levels > 10% there are coordination problems and spasticity.

• At 15% the person experiences delirium and shriveled skin. Above 20% death will occur.

43

Dehydration Strategies

• Aggressive fluid replacement during ex in the heat.

• Regular fluid breaks.

• Drink fluids in proportion to sweat loss.

• Drink cold fluids (8 - 13oC) with moderate amounts of carbohydrate (~7%) and electrolytes.

• Physical fitness helps prevent dehydration ( BV).

44

Heat Cramps: involuntary cramping and spasm in muscles used during exercise.

• Originally thought to be due to electrolyte imbalance, may be due a spinal neural mechanism.

Heat Exhaustion: hypotension and weakness caused by an inability of the circulation to compensate for acute plasma volume loss and vasodilation.

• It is characterized by rapid weak pulse, hypotension, faintness and profuse sweating.

• Occurs at core temperature below 39.5 oC.

• Treat individual with fluids and move to a cool location.

45

Heat Syncope: a person faints due to hypotension. Occurs when blood pools in the legs following exercise.

Heat Stroke: occurs when there is a failure of the temperature regulatory function of the hypothalamus, and represents a medical emergency.

• Results in an explosive in core temperature (> 41oC) and is characterized by hot dry skin, confusion and convulsions.

• The patient should be treated in a hospital with a spray of tepid water and a cool air stream.

• Outside of the hospital the patient can be cooled with a fan and ice packs on the neck, axillae, and groin.

46

Preventing Thermal Stress• Ensure athletes are in good physical condition.

• Ensure athlete does not become overheated before exercising in the heat.

• Athletes should not train above normal intensities.

• Schedule training during cooler times in the day.

• Cancel training if the temperature is too high.

• Implement regular fluid breaks.

47

Preventing Thermal Stress• Supply cold fluids (8 - 13oC) with moderate amounts of

carbohydrate (~7%) and electrolytes.

• Drink 600ml of fluid 2 hours before, and 400ml 15 minutes prior to exercise.

• Fluid replacement should be encouraged during the early stages of a training session.

• Weigh athletes before each practice to ensure they are not dehydrated at the start of training.

• Do not use salt tablets.