Exercise Thermoregulation, Fluid Balance, and Rehydration

Chapter 10

Part 1

Temperature Regulation

• Body core temperature regulation– Critical for:

• Cellular structures• Metabolic pathways

– Too high - hyperthermia(5oC)• Protein structure of cells destroyed

– Too low – hypothermia (10oC)• Slowed metabolism• Cardiac arrhythmias

Temperature Regulation

• Thermal gradients– Determine rate and direction of heat transfer– Heat transfer from higher to lower

temperature– Core to shell (skin)

• Typical gradient when ambient temp is less than body temp

– Skin to core• If ambient temperature is Greater than body temp

Temperature Regulation

• Net gain of body heat– Heat loss is less than heat production– Body temperature rises– Positive thermal balance

• Net loss of body heat– Heat loss exceeds heat production– Body temperature decreases– Negative thermal balance

Temperature Regulation

• Homeotherms (mammals)– Internal body temperature is kept fairly constant

• Humans are unable to tolerate wide changes • Hour to hour, day to day variations < 1o C• Normal temperature: 37o C • Normal range 36.1 – 37.8o C (97 – 100o F)

– Conversion: Tf = (Tc * 1.8) + 32– So, if it is 37o C, Tf = (37 * 1.8) + 32– 66.6 + 32 = 98.6o F

– Heavy exercise, illness, extreme conditions of heat and cold – can push an individual outside this range

Heat Balance

• Greater extremes – Early morning (<36oC)– Heavy exercise >40oC

• Muscles >42oC or 107.6oF

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

Body Temperature

Core body temperature: deep tissues Skeletal muscles and organs

Shell body temperature: peripheral Skin Initiate response to external temperature Thermoreceptors relay information to

Hypothalamus Cerebral cortex

Hypothalamic Regulation

Hypothalamus (37 1oC) Central coordinating center

Acts as a thermostat Heat gain Heat loss

Thermal Balance

Heat-Regulating Mechanisms

Activated by:• Temperature changes in blood

• Skin heats up/cools down changes temperature of blood

• Directly stimulate hypothalamus

• Thermal receptors in the skin • Provide input to modulate hypothalamic activity.

Receptors (temperature regulation) Skin Subcutaneous tissue.

Heat Transfer

Thermal Regulation

Radiation

Objects “radiate” heat Body temperature is warmer than the

environment. Radiant heat energy transfers to solid cooler

objects around us. Body temperature is cooler than the

environment. Radiant heat energy transfers to us.

Conduction

Transfers heat directly through a liquid, solid, or gas from one molecule to another We conduct heat to cooler air, water or objects Warmer air, water or objects conduct heat to us

Convection

Air, water movement So, if cool/hot air is travelling over the

body (e.g. wind) or cool/hot water, we will be cooled/heated

Evaporation

Major physiologic defense against overheating Respiratory water loss Sweat

Heat stress 2-4 million sweat (eccrine) glands

Secrete large quantities of hypotonic saline solution.

*Cooling occurs when sweat evaporates from the skin surface. Has to evaporate

Environmental Temperature

Increased ambient temperature Reduces heat loss by conduction, convection,

and radiation. This heat loss stops when ambient temp

exceeds body temp At that point, the only way to dissipate heat is

Sweat evaporation from the skin depends upon:

• Surface exposed to the environment • relative humidity of ambient air • Convective air currents around the body

High Humidity

Heat-Dissipating Mechanisms Circulation

• Cardiac output (HR x SV)• Rest – 15-20% to skeletal muscles• Heavy exercise – 80-85% to skeletal muscles

• “Workhorse” to maintain thermal balance• 15-25% of cardiac output – periphery• If cardiac output falls, skin blood flow falls and

ability to dissipate heat is reduced• So, when exercising in the heat

• Cardiac output is directed more to periphery• Less to muscle• Cause of fatigue

Cardiac Drift

Heat-Dissipating Mechanisms

Sweating: rate dependent upon: Climate conditions Exercise intensity Clothing Conditioning

Evaporation Major cooling mechanism

Fluid conserving Mechanisms

Hormones• Antidiuretic hormone - ADH (Vasopressin)

• Posterior Pituitary • Increases water reabsorption from kidneys• Conserves water and maintains BP

• Aldosterone• Adrenal cortex• Increases reabsorption of sodium

Heat-related Deaths

• Since 1936 (National Safety Council) – 30,000 people have died from heat related illnesses.

• On the average, 384 people die each year from heat stroke.– Associated with normal hot temperatures.  – In the event of a heat wave, the number of deaths increase

tremendously. • Occurrence of heat related injuries

– Often with the elderly;  – People who are not in good physical condition; – or not acclimatized to the heat– Or heat waves

Exercise in the Heat

• Increased heat load• Increased sweating• Increased Fluid loss

– Decreased plasma volume– Decreased cardiac output– Decreased cutaneous circulation

Distribution of Cardiac Output

• Thermoneutral environment vs hot environment

Water Loss

Dehydration Sweat loss 0.5 – 1.5 L/h – moderate exercise– Maximal daily sweat rates-10-15 L– Highest sweat rates 2 to 3 L/hr

• Can be sustained for only a few hours

Considerable water loss Both intracellular and extracellular compartments

The risk of heat illness Increases in a dehydrated state.

Sweat Rates

• 1 to 3% loss in body weight through dehydration can adversely affect performance and body’s physiologic function

• Marathon runners– Loss of 5 L– 6-10% of body weight

Core Temperature and Sweat

Rate During Exercise in

Heat/Humidity

Fig 12.10

Exercise in the Heat

• Compromised BF to muscles

• Increased use of glycogen

• Increased production of LA

• Hastens fatigue

Dehydration and Exercise Performance

Dehydration and Exercise Performance

Dehydration and Exercise Performance

Any degree of dehydration impairs Capacity of circulatory system Temperature-regulating mechanisms

Dehydration - 2% body mass Impairs physical work capacity and physiologic

function Predisposes to heat injury when exercising in a hot

environment.

Dehydration and Exercise Performance

The risk for dehydration increases during vigorous cold-weather exercise. Less moisture in cold Respiratory fluid loss is increased Excessive clothing

Creates a heated microclimate under the clothing