Chapter 12. Temperature Regulation. Temperature Regulation. Heat Balance. An Overview of Heat Balance. Temperature Regulation. Temperature Regulation

Chapter 12 Chapter 12

Temperature Regulation Temperature Regulation

Temperature Regulation Temperature Regulation

Body core temperature regulation Body core temperature regulation

– – Critical for: Critical for:

Cellular structures Cellular structures Metabolic pathways Metabolic pathways –

– Too high Too high

Protein structure of cells destroyed Protein structure of cells destroyed – – Too low Too low

Slowed metabolism Slowed metabolism Cardiac arrhythmias Cardiac arrhythmias

Temperature Regulation Temperature Regulation

Homeothermic Homeothermic

– – Internal body temperature is kept fairly constant Internal body temperature is kept fairly constant Humans are unable to tolerate wide changes Humans are unable to tolerate wide changes Hour to hour, day to day variations Hour to hour, day to day variations < < 1 1

C C Normal temperature

Normal temperature - - 37 37

C C Normal range 36.1

Normal range 36.1 – – 37.8 37.8

C (97 C (97 – – 100 100

F) F)

– – Heavy exercise, illness, extreme conditions of heat Heavy exercise, illness, extreme conditions of heat and cold

and cold – – outside this range outside this range

Heat Balance Heat Balance

Greater extremes Greater extremes

– – Early morning (<36 Early morning (<36

C) C) – – Heavy exercise (>40 Heavy exercise (>40

C) C)

Body temperature (core temperature) reflects Body temperature (core temperature) reflects the balance between heat production and heat the balance between heat production and heat loss

loss

An Overview of Heat Balance An Overview of Heat Balance

In order to In order to maintain a maintain a constant core constant core temperature, temperature, heat loss must heat loss must match heat gain match heat gain

Temperature Regulation Temperature Regulation

Body core temperature Body core temperature

– – 4 4 ^{o o} C C to 30 to 30 ^{o o} C C

– – Core temperature is independent of Core temperature is independent of environmental temperature

environmental temperature

Rises in proportion to exercise intensity Rises in proportion to exercise intensity –

– Skin temperature reflects environmental Skin temperature reflects environmental temperature

temperature Metabolic rate Metabolic rate Clothing Clothing

State of hydration (skin t

State of hydration (skin t ↑ ↑ with with ↓ ↓ sweating) sweating)

Temperature Regulation Temperature Regulation

Net gain of body heat Net gain of body heat

– – Heat loss is less than heat production Heat loss is less than heat production –

– Body temperature rises Body temperature rises –

– Positive thermal balance Positive thermal balance Net loss of body heat Net loss of body heat

– – Heat loss exceeds heat production Heat loss exceeds heat production – – Body temperature decreases Body temperature decreases –

– Negative thermal balance Negative thermal balance

Temperature Regulation Temperature Regulation

Core temperature is usually defined as the Core temperature is usually defined as the temperature of the hypothalamus

temperature of the hypothalamus – – Temperature regulatory center of the body. Temperature regulatory center of the body.

Thermal gradients Thermal gradients

–

– Determine rate and direction of heat transfer Determine rate and direction of heat transfer –

– Heat transfer from higher to lower Heat transfer from higher to lower temperature

temperature –

– Core to shell (skin) Core to shell (skin) – – 4 4 ^{o o} C C

Heat Balance Heat Balance

People live and work in extreme People live and work in extreme temperatures

temperatures –

– Russia and Canada (< 0 Russia and Canada (< 0 ^{o o} C) C)

Exposed skin can freeze within one minute Exposed skin can freeze within one minute –

– Australia, Texas, Southwest states Australia, Texas, Southwest states 43 43- -49 49

C (+90 C (+90

F) F)

Behavioral means Behavioral means Physiological means Physiological means

Heat Balance Heat Balance

Physiological adjustments have limitations Physiological adjustments have limitations

– – > 41 > 41 ^{o o} C, cell organelles begin to deteriorate C, cell organelles begin to deteriorate Heat stroke and permanent brain damage occur if Heat stroke and permanent brain damage occur if high temperature prolonged

high temperature prolonged – – < 34 < 34 ^{o o} C, cellular metabolism slows C, cellular metabolism slows

Leads to unconsciousness, cardiac arrhythmias Leads to unconsciousness, cardiac arrhythmias

Temperature Extremes Temperature Extremes

Heat Balance Heat Balance

Body temperature is a Body temperature is a balance between heat balance between heat gain:

gain:

– – Metabolism Metabolism Exercise Exercise Shivering Shivering Thyroxine Thyroxine

Sympathetic stimulation Sympathetic stimulation Q

Q

Effect Effect –

– Environment Environment

Radiation

Radiation

Conduction

Conduction

Convection

Convection

Heat Balance Heat Balance

–

– Heat loss Heat loss Radiation Radiation Conduction Conduction Convection Convection Evaporation Evaporation

Heat Gain Heat Gain

Heat is kinetic activity of molecules, not a Heat is kinetic activity of molecules, not a substance

substance Heat Production

Heat Production- -metabolism metabolism

– – Heat is a byproduct of biochemical reactions Heat is a byproduct of biochemical reactions – – Metabolic reactions are not 100% efficient Metabolic reactions are not 100% efficient – – A portion of the energy is lost as heat (60- A portion of the energy is lost as heat (60 -70%) 70%) Heat Production

Heat Production- -shivering shivering –

– Main mechanism for producing heat when body is in Main mechanism for producing heat when body is in negative heat balance

negative heat balance

Heat Gain Heat Gain

Shivering Shivering

– – Involuntary contraction of muscles Involuntary contraction of muscles –

– Can increase heat production up to 5 x Can increase heat production up to 5 x’ ’s s. . –

– Effective increase in temperature- Effective increase in temperature -no work no work done by shivering muscles

done by shivering muscles –

– Energy expended as heat Energy expended as heat

Heat Gain Heat Gain

Shivering Shivering

– – Increases cardiac output Increases cardiac output –

– Muscle pump stimulates increased VR Muscle pump stimulates increased VR –

– Increased VR increases SV Increased VR increases SV – – Increased SV increases Q Increased SV increases Q

Heat Gain Heat Gain

Shivering Limitations Shivering Limitations – – Glycogen depletion Glycogen depletion –

– Hypoglycemia Hypoglycemia – – Fatigue Fatigue – – Exercise Exercise –

– Hypoxia Hypoxia

– – Drugs- Drugs -alcohol, barbiturates alcohol, barbiturates

Heat Gain Heat Gain

Nonshivering

Nonshivering thermogenesis thermogenesis – – Thyroid releases thyroxin Thyroid releases thyroxin – – Adrenals release catecholamines Adrenals release catecholamines

– – Both increase metabolic rate of all cells in Both increase metabolic rate of all cells in body body

Sympathetic stimulation Sympathetic stimulation

–

– Causes peripheral vasoconstriction to Causes peripheral vasoconstriction to arterioles in skin

arterioles in skin –

– Decreased blood flow to skin, reduces heat Decreased blood flow to skin, reduces heat loss

loss

Heat Gain Heat Gain

Q Q _{10 10} effect effect

– – Metabolic activity of cells doubles for every 10 Metabolic activity of cells doubles for every 10

C C increase in temperature

increase in temperature

– – At high temperatures, hypothalamus loses ability to At high temperatures, hypothalamus loses ability to cool body

cool body –

– Rate of temperature increase is faster at higher Rate of temperature increase is faster at higher temperature

temperature –

– Core temperatures > 41.5 Core temperatures > 41.5

C external cooling C external cooling necessary

necessary- -hypothalamus may not be functioning hypothalamus may not be functioning

Heat Gain Heat Gain

Radiation Radiation Conduction Conduction Convection Convection

–

– When environmental temperature is greater When environmental temperature is greater than skin temperature

than skin temperature

Heat Loss Heat Loss

Conduction (3%) Conduction (3%)

– – Transfer of heat from the warmer (usually the Transfer of heat from the warmer (usually the body) to a cooler object

body) to a cooler object

– – In the body, down a thermal gradient In the body, down a thermal gradient From muscle to skin across tissues From muscle to skin across tissues

Heat Loss Heat Loss

Convection Convection- -12% 12%

– – Conduction of heat to or from air or water Conduction of heat to or from air or water – – More rapid in water or in wind More rapid in water or in wind

– – Air/water around skin, warmed, moves, warm Air/water around skin, warmed, moves, warm what replaces

what replaces

– – Heat flux depends directly on the temperature Heat flux depends directly on the temperature gradient between body and water

gradient between body and water

– – > 40 mph no increase in heat loss as heat > 40 mph no increase in heat loss as heat transfer to skin does not occur fast enough transfer to skin does not occur fast enough

Heat Loss Heat Loss

Radiation

Radiation- -60% total heat loss 60% total heat loss

– – Loss (or gain) of heat in the form of electromagnetic Loss (or gain) of heat in the form of electromagnetic waves

waves –

– Sun is greatest source of radiant energy Sun is greatest source of radiant energy – – Radiant heat loss varies with body position and Radiant heat loss varies with body position and

clothing clothing –

– Body radiates and receives heat at same time Body radiates and receives heat at same time

Heat Loss Heat Loss

Radiation (cont) Radiation (cont)

– – Body temperature > environment Body temperature > environment Heat radiates away from body Heat radiates away from body –

– Body temperature < environment Body temperature < environment Heat radiates towards body Heat radiates towards body

Human skin, regardless of color, absorbs 97% of Human skin, regardless of color, absorbs 97% of radiant energy that strikes it

radiant energy that strikes it

Heat Loss Heat Loss

Evaporation Evaporation

– – At rest in comfortable environment, 25% of At rest in comfortable environment, 25% of heat loss due to evaporation

heat loss due to evaporation

– – Only means of cooling at high environmental Only means of cooling at high environmental temperature

temperature

– – Critically important during exercise when heat Critically important during exercise when heat production may be 100

production may be 100 x x’ ’s s rest rest

Heat Loss Heat Loss

Evaporation Evaporation

– – Sweating is critical Sweating is critical –

– Inability to lose heat by evaporation, raises Inability to lose heat by evaporation, raises body temperature

body temperature – – Moderate exercise Moderate exercise

Body temperature increases 0.2 Body temperature increases 0.2

C/min C/min Thermal injury can occur in 15 Thermal injury can occur in 15- -20 min 20 min

Heat Loss Heat Loss

Evaporation Evaporation

– – Sweat is only effective for cooling if it Sweat is only effective for cooling if it evaporates

evaporates

– – High humidity reduces evaporation rate High humidity reduces evaporation rate Air already contains water molecules Air already contains water molecules Less capacity to accept more Less capacity to accept more Sweat runs off, doesn

Sweat runs off, doesn’ ’t evaporation t evaporation No cooling benefit

No cooling benefit

Heat Loss Heat Loss

Heat produced is Heat produced is moved from muscles moved from muscles to skin

to skin

– – Through circulation Through circulation –

– By conduction through By conduction through tissues

tissues To environment To environment

–

– Conduction, Conduction, convection, radiation convection, radiation and sweat evaporation and sweat evaporation

Heat Loss Heat Loss

Sweat glands controlled by stimulation of Sweat glands controlled by stimulation of hypothalamus

hypothalamus- - –

– Increased blood temperature Increased blood temperature – – Impulses through sym nerve fibers Impulses through sym nerve fibers – – Sweat glands to form sweat- Sweat glands to form sweat -eccrine eccrine

Glands are tubular structures from dermis of skin Glands are tubular structures from dermis of skin to epidermis

to epidermis

Light sweat, sodium and chloride reabsorbed Light sweat, sodium and chloride reabsorbed Heavy sweating, sodium and chloride are lost Heavy sweating, sodium and chloride are lost

Heat Exchange During Exercise

Heat Exchange During Exercise

Temperature Regulation Temperature Regulation

Body temperature and house thermostat Body temperature and house thermostat – – Body temperature more complicated and Body temperature more complicated and

precise precise

– – Set point=normal body temperature Set point=normal body temperature

– – Deviations of more than 1 Deviations of more than 1 ^{o o} C send signals to C send signals to thermoregulatory center

thermoregulatory center

Temperature Regulation Temperature Regulation

Three major thermal receptor areas Three major thermal receptor areas

– – 1. Anterior hypothalamus- 1. Anterior hypothalamus -central receptors central receptors Contains hot and cold sensitive neurons Contains hot and cold sensitive neurons Sense temperature of arterial blood Sense temperature of arterial blood More hot than cold neurons (3 to 1) More hot than cold neurons (3 to 1) Temperature fluctuations of 0.2 to 0.5 Temperature fluctuations of 0.2 to 0.5

F F

Temperature Regulation Temperature Regulation

2. Thermal receptors or sensors 2. Thermal receptors or sensors

– – Skin receptors- Skin receptors -environmental changes environmental changes Cold and warm beneath surface of skin Cold and warm beneath surface of skin Send input to coordinating center along with Send input to coordinating center along with central receptors in anterior hypothalamus central receptors in anterior hypothalamus

3. Thermal effectors 3. Thermal effectors

–

– Effector Effector organs are muscles organs are muscles

– – Signals sent from coordinating center to elicit Signals sent from coordinating center to elicit change to correct situation

change to correct situation

Responses to Heat Stress Responses to Heat Stress

Fig 12.4

Responses to Cold Stress Responses to Cold Stress

Fig 12.5

Heat Loss Heat Loss

Various temperatures Various temperatures M=heat production M=heat production THL=total heat loss THL=total heat loss – – Both are constant Both are constant R=radiation R=radiation C=conduction C=conduction E evaporation E evaporation

Exercise in the Heat Exercise in the Heat

Sets up a competition between active Sets up a competition between active muscles and skin for the limited blood muscles and skin for the limited blood supply

supply –

– Muscles need oxygen Muscles need oxygen

– – Skin needs blood to help with heat loss Skin needs blood to help with heat loss – – Blood volume = 5 to 6 L Blood volume = 5 to 6 L

–

– Q must be shared Q must be shared

Exercise in the Heat Exercise in the Heat

Cardiovascular Demands Cardiovascular Demands

– – Blood flow needs increase for Blood flow needs increase for Muscle

Muscle Skin Skin May exceed Q May exceed Q

– – VD in skin decreases VR, decreases SV VD in skin decreases VR, decreases SV – – Decrease in plasma volume, contributes to Decrease in plasma volume, contributes to

decrease in SV decrease in SV

-Body must maintain blood pressure - Body must maintain blood pressure

Exercise in the Heat Exercise in the Heat

Short term, light ex Short term, light ex

–

– Q increases similarly Q increases similarly –

– Hot environment- Hot environment -Q Q achieved by achieved by ↑ ↑HR, HR, ↓SV ↓ SV – – VD skin VD skin → → ↓ ↓SV SV – – MAP maintained MAP maintained –

– VC kidneys, GI tract VC kidneys, GI tract

Exercise in the Heat Exercise in the Heat

Prolonged, heavy, Prolonged, heavy, heat

heat

– – Q increases less than Q increases less than in neutral environment in neutral environment –

– Q fails to reach same Q fails to reach same level

level –

– SV progressive SV progressive decline decline

– – HR higher, unable to HR higher, unable to

compensate and equal

compensate and equal

neutral environment

neutral environment

Exercise in the Heat Exercise in the Heat

–

– Severe VC may cause Severe VC may cause tissue damage tissue damage MAP difficult to maintain MAP difficult to maintain

–

– Excessive water loss will Excessive water loss will – – ↓ ↓ SV, Q, and SV, Q, and

– – ↓ ↓ MAP will occur if fluid not MAP will occur if fluid not replaced

replaced –

– Performance will suffer Performance will suffer –

– Heat illness is likely Heat illness is likely

Distribution of Cardiac Output Distribution of Cardiac Output

Thermoneutral Thermoneutral environment vs hot environment vs hot environment environment

Exercise in the Heat Exercise in the Heat

Compromised BF to Compromised BF to muscles

muscles

Increased use of Increased use of glycogen glycogen

Increased production Increased production of LA

of LA

Hastens fatigue Hastens fatigue

Sweat Rates Sweat Rates

Maximal daily sweat rates

Maximal daily sweat rates- -10 10- -15 L 15 L Highest sweat rates 2 to 3 L/hr Highest sweat rates 2 to 3 L/hr

– – Can be sustained for only a few hours Can be sustained for only a few hours 1 to 3% loss in body weight through 1 to 3% loss in body weight through dehydration can adversely affect dehydration can adversely affect performance and body

performance and body’ ’s physiologic s physiologic function

function

Heat Exchange During Exercise Heat Exchange During Exercise

Fig 12.6

Exercise in Hot/Humid Exercise in Hot/Humid

Environments Environments

Inability to lose heat Inability to lose heat

– – Higher core temperature Higher core temperature –

– Higher sweat rate Higher sweat rate Can result in:

Can result in:

–

– Impaired performance Impaired performance

– – Hyperthermia Hyperthermia

Core Core Temperature Temperature and Sweat and Sweat Rate During Rate During Exercise in Exercise in Heat/Humidity Heat/Humidity

Fig 12.10

Acclimatization Acclimatization

Repeated exercise in the heat improves Repeated exercise in the heat improves body

body’ ’s ability to eliminate excess body s ability to eliminate excess body heat heat

5 to 10 days progressive exercise program 5 to 10 days progressive exercise program Improvements

Improvements

– – Increased peripheral heat conductance Increased peripheral heat conductance Less BF needed to move heat to skin Less BF needed to move heat to skin

Acclimatization Acclimatization

– – Increased plasma volume Increased plasma volume

3 to 27% increase in plasma volume with training 3 to 27% increase in plasma volume with training in heat

in heat

Increased levels of ADH,

Increased levels of ADH, renin renin and and aldosterone aldosterone early in process, these levels decrease as training early in process, these levels decrease as training continues

continues

Increased PV helps maintain Increased PV helps maintain

– – CBV CBV – – SV SV

– – Sweating capacity Sweating capacity

Plasma Volume Change Plasma Volume Change

Acclimatization Acclimatization

– – Increased sweating capacity Increased sweating capacity Almost 3

Almost 3 x x’ ’s s as great as great –

– Decreased core temperature at onset of Decreased core temperature at onset of sweating

sweating

Sweating begins earlier Sweating begins earlier

Greater distribution of sweat over body Greater distribution of sweat over body –

– Decreased NaCl Decreased NaCl losses in sweat and urine losses in sweat and urine due to increased secretion of

due to increased secretion of aldosterone aldosterone

Acclimatization Acclimatization

– – Lower skin temperature Lower skin temperature

improves temperature gradient from core to skin improves temperature gradient from core to skin and environment

and environment

– – Fluid replacement (Fox 19.3) Fluid replacement (Fox 19.3)

Water still best choice if activity less than one hour Water still best choice if activity less than one hour More than one hour sports drinks helpful More than one hour sports drinks helpful Cool and hypotonic drinks leave stomach faster Cool and hypotonic drinks leave stomach faster

–

– Better fluid replacement Better fluid replacement

Acclimatization

Acclimatization