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(5 o C) Protein structure of cells destroyed –Too low – hypothermia (10 o C) 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 < 1 o C Normal temperature: 37 o C Normal range 36.1 – 37.8 o C (97 – 100 o F) –Conversion: Tf = (Tc * 1.8) + 32 –So, if it is 37 o C, Tf = (37 * 1.8) + 32 – = 98.6 o F –Heavy exercise, illness, extreme conditions of heat and cold – can push an individual outside this range

Heat Balance Greater extremes –Early morning (<36 o C) –Heavy exercise >40 o C Muscles >42 o C or o F 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  1 o C)  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 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

 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