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

Transcription

1 Chapter 12 Body core temperature regulation Critical for: Cellular structures Metabolic pathways Too high Protein structure of cells destroyed Too low Slowed metabolism Cardiac arrhythmias Homeothermic 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 o C ( o F) Heavy exercise, illness, extreme conditions of heat and cold outside this range Heat Balance Greater extremes Early morning (<36 o C) Heavy exercise (>40 o C) Body temperature (core temperature) reflects the balance between heat production and heat loss An Overview of Heat Balance In order to maintain a constant core temperature, heat loss must match heat gain Body core temperature 4 o C to 30 o C Core temperature is independent of environmental temperature Rises in proportion to exercise intensity Skin temperature reflects environmental temperature Metabolic rate Clothing State of hydration (skin t with sweating) Fig

2 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 Core temperature is usually defined as the temperature of the hypothalamus Temperature regulatory center of the body. Thermal gradients Determine rate and direction of heat transfer Heat transfer from higher to lower temperature Core to shell (skin) 4 o C Heat Balance People live and work in extreme temperatures Russia and Canada (< 0 o C) Exposed skin can freeze within one minute Australia, Texas, Southwest states o C (+90 o F) Behavioral means Physiological means Heat Balance Physiological adjustments have limitations > 41 o C, cell organelles begin to deteriorate Heat stroke and permanent brain damage occur if high temperature prolonged < 34 o C, cellular metabolism slows Leads to unconsciousness, cardiac arrhythmias Temperature Extremes Heat Balance Body temperature is a balance between heat gain: Metabolism Exercise Shivering Thyroxine Sympathetic stimulation Q 10 Effect Environment Radiation Conduction Convection 2

3 Heat loss Radiation Conduction Convection Evaporation Heat Balance Heat is kinetic activity of molecules, not a substance Heat Production-metabolism Heat is a byproduct of biochemical reactions Metabolic reactions are not 100% efficient A portion of the energy is lost as heat (60-70%) Heat Production-shivering Main mechanism for producing heat when body is in negative heat balance Shivering Involuntary contraction of muscles Can increase heat production up to 5 x s. Effective increase in temperature-no work done by shivering muscles Energy expended as heat Shivering Increases cardiac output Muscle pump stimulates increased VR Increased VR increases SV Increased SV increases Q Shivering Limitations Glycogen depletion Hypoglycemia Fatigue Exercise Hypoxia Drugs-alcohol, barbiturates Nonshivering thermogenesis Thyroid releases thyroxin Adrenals release catecholamines Both increase metabolic rate of all cells in body Sympathetic stimulation Causes peripheral vasoconstriction to arterioles in skin Decreased blood flow to skin, reduces heat loss 3

4 Q 10 effect Metabolic activity of cells doubles for every 10 o C increase in temperature At high temperatures, hypothalamus loses ability to cool body Rate of temperature increase is faster at higher temperature Core temperatures > 41.5 o C external cooling necessary-hypothalamus may not be functioning Radiation Conduction Convection When environmental temperature is greater than skin temperature Conduction (3%) Transfer of heat from the warmer (usually the body) to a cooler object In the body, down a thermal gradient From muscle to skin across tissues Convection-12% Conduction of heat to or from air or water More rapid in water or in wind Air/water around skin, warmed, moves, warm what replaces Heat flux depends directly on the temperature gradient between body and water > 40 mph no increase in heat loss as heat transfer to skin does not occur fast enough Radiation-60% total heat loss Loss (or gain) of heat in the form of electromagnetic waves Sun is greatest source of radiant energy Radiant heat loss varies with body position and clothing Body radiates and receives heat at same time Radiation (cont) Body temperature > environment Heat radiates away from body Body temperature < environment Heat radiates towards body Human skin, regardless of color, absorbs 97% of radiant energy that strikes it 4

5 Evaporation At rest in comfortable environment, 25% of heat loss due to evaporation Only means of cooling at high environmental temperature Critically important during exercise when heat production may be 100 x s rest Evaporation Sweating is critical Inability to lose heat by evaporation, raises body temperature Moderate exercise Body temperature increases 0.2 o C/min Thermal injury can occur in min Evaporation Sweat is only effective for cooling if it evaporates High humidity reduces evaporation rate Air already contains water molecules Less capacity to accept more Sweat runs off, doesn t t evaporation No cooling benefit Heat produced is moved from muscles to skin Through circulation By conduction through tissues To environment Conduction, convection, radiation and sweat evaporation Heat Exchange During Exercise Sweat glands controlled by stimulation of hypothalamus- Increased blood temperature Impulses through sym nerve fibers Sweat glands to form sweat-eccrine eccrine Glands are tubular structures from dermis of skin to epidermis Light sweat, sodium and chloride reabsorbed Heavy sweating, sodium and chloride are lost Fig

6 Body temperature and house thermostat Body temperature more complicated and precise Set point=normal body temperature Deviations of more than 1 o C send signals to thermoregulatory center Three major thermal receptor areas 1. Anterior hypothalamus-central receptors Contains hot and cold sensitive neurons Sense temperature of arterial blood More hot than cold neurons (3 to 1) Temperature fluctuations of 0.2 to 0.5 o F Responses to Heat Stress 2. Thermal receptors or sensors Skin receptors-environmental environmental changes Cold and warm beneath surface of skin Send input to coordinating center along with central receptors in anterior hypothalamus 3. Thermal effectors Effector organs are muscles Signals sent from coordinating center to elicit change to correct situation Fig 12.4 Responses to Cold Stress Fig

7 Various temperatures M=heat production THL=total heat loss Both are constant R=radiation C=conduction E evaporation Exercise in the Heat Sets up a competition between active muscles and skin for the limited blood supply Muscles need oxygen Skin needs blood to help with heat loss Blood volume = 5 to 6 L Q must be shared Exercise in the Heat Cardiovascular Demands Blood flow needs increase for Muscle Skin May exceed Q VD in skin decreases VR, decreases SV Decrease in plasma volume, contributes to decrease in SV -Body must maintain blood pressure Exercise in the Heat Short term, light ex Q increases similarly Hot environment-q achieved by HR, HR, SV VD skin SV MAP maintained VC kidneys, GI tract Exercise in the Heat Prolonged, heavy, heat Q increases less than in neutral environment Q fails to reach same level SV progressive decline HR higher, unable to compensate and equal neutral environment 7

8 Exercise in the Heat Severe VC may cause tissue damage MAP difficult to maintain Excessive water loss will SV, Q, and MAP will occur if fluid not replaced Performance will suffer Heat illness is likely Distribution of Cardiac Output Thermoneutral environment vs hot environment Exercise in the Heat Compromised BF to muscles Increased use of glycogen Increased production of LA Hastens fatigue Sweat Rates Maximal daily sweat rates L Highest sweat rates 2 to 3 L/hr Can be sustained for only a few hours 1 to 3% loss in body weight through dehydration can adversely affect performance and body s s physiologic function Heat Exchange During Exercise Exercise in Hot/Humid Environments Inability to lose heat Higher core temperature Higher sweat rate Can result in: Impaired performance Hyperthermia Fig

9 Core Temperature and Sweat Rate During Exercise in Heat/Humidity Acclimatization Repeated exercise in the heat improves body s s ability to eliminate excess body heat 5 to 10 days progressive exercise program Improvements Increased peripheral heat conductance Less BF needed to move heat to skin Fig Acclimatization Plasma Volume Change Increased plasma volume 3 to 27% increase in plasma volume with training in heat Increased levels of ADH, renin and aldosterone early in process, these levels decrease as training continues Increased PV helps maintain CBV SV Sweating capacity Acclimatization Increased sweating capacity Almost 3 x s as great Decreased core temperature at onset of sweating Sweating begins earlier Greater distribution of sweat over body Decreased NaCl losses in sweat and urine due to increased secretion of aldosterone Acclimatization Lower skin temperature improves temperature gradient from core to skin and environment Fluid replacement (Fox 19.3) Water still best choice if activity less than one hour More than one hour sports drinks helpful Cool and hypotonic drinks leave stomach faster Better fluid replacement 9

10 Acclimatization 10