Environment Brooks Ch 22 OUTLINE Environmental Heat Environmental Cold Body temp and Heat Transfer Exercise in Heat Acclimatization to Heat Environmental Cold Exercise in Cold Heart, muscle and metabolic responses to cold hypothermia

Environmental Temperature People have an ability to live and work in very hot and very cold environments Able to tolerate these variable environments by tightly regulating our internal (core) temperature - homeotherms We utilize behavioral and physiological means to regulate our core temperature normal core temp 36.5-37.5oC core - defined as temperature of the hypothalamus Experimentally - rectal and esophageal temperatures are used for core temperature Oral temperature is influenced by breathing cold air Tympanic temperature is influenced by head skin temperature

Core Temperature Heat balance - fig 22-2 At temperatures above 41oC the interior of many cells begin to deteriorate At temperatures below 34oC cellular metabolism slows greatly, leading to unconsciousness and cardiac arrhythmias During exercise core temp can exceed 40oC - rise is proportional to intensity While the core remains relatively constant skin temperature is influenced by the environment, metabolic rate, clothing and hydration state - fig 22-1 Heat balance - fig 22-2 directed by hypothalamus - thermostat peripheral receptors in skin and hypothalamus heat and cold receptors Initiates heat production (shivering) or dissipation (evaporation) to regulate core temperature - fig 22-6 Clothing and air movement across the skin affect capacity for heat loss

Heat Transfer Radiation Conduction heat in form of electromagnetic radiation - 60% of heat loss at room temperature at rest Sun is largest source of radiant heat Conduction transfer through direct contact Rate of transfer depends on temperature gradient and conductive properties of surface Convection - conduction to/from air or water Depends on body surface area exposed to surrounding medium and the flow of that medium - wet suit ? more rapid in water (~25 times) heat loss is much greater in wind and moving water Wind-chill index fig 22-3

Heat Loss Evaporation quantity of heat absorbed by sweat as it evaporates from the skin 1 gm sweat = 2411.3 Joules (.58kcal) sweat is only effective for cooling if it evaporates max sweat rate is ~ 1.5 L/hr in sedentary untrained individuals Can improve to 4 L/hr with exercise acclimatization to hot humid environment Sweating results in loss of H2O, Na+, Cl-, urea, lactate and K+ Important to replace these following exercise in the heat which involves excessive sweating Sweating still occurs during swimming

Exercise in Heat Core temp inc with exercise intensity Fig 22-8 - high environ temp adds to metabolic heat stress of exercise Cardiovascular effects depends on bodies ability to dissipate heat and maintain bld flow to active ms during exercise (acute) plasma vol dec Due to inc BP and loss of fluid from sweat dec central bl vol results in dec filling pressure and a compensatory inc HR HR inc not viable at higher intensities near max - vasoconstrict periphery To maintain BP and CO - triage No change in VO2 max unless subject started with a thermal imbalance

Exercise in Heat Sweating Response Primary means of heat dissipation during exercise Sweat rate related more to exercise intensity than environmental temp Pre-cooling body prior to exercise may improve performance Ventilation may be affected by temperature during rest and exercise Dr. Matt White (SFU) Sedentary exposure to warm bath, increasing core temperature by 2 oC Observe inc HR, Ventilation and R value with no change in VO2 - indicate thermal rather than metabolic stimulus for these changes Aaron Chu - compare values during exercise with/without rise in core temp

Acclimatization Acclimatization Acclimation adaptations produced by a change in the natural environment Acclimation adaptation to laboratory environment heat and exercise are both required for optimal adaptational responses Acclimation in first 2 weeks dec HR, core temp, perceived exertion, skin temp at rest and submax exercise Table 22-2 inc plasma vol (range 3 - 27 %) inc sweat capacity (1.5 - 4 L per hour) dec core temp at onset of sweating dec skin blood flow - improve distribution of sweat over skin reduce losses of minerals (sweat and urine)

Individual Variation Majority of physiological adjustment in 4-6 days does not occur without exercise intense exercise most effective humidity specific adaptation atheltes a little faster in acclimatizing sweat rate and mineral changes take up to two weeks to adapt Loss of Acclimatization occurs in absence of heat and physical fitness physically fit retain benefits longer than sedentary individuals

Dehydration Dehydration - loss of body fluid Moderate levels of dehydration will impair CV and temperature regulation - impact performance - fig 22-9 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% dehydration death will occur.

Exercise In Cold Rare except when survival is at stake for people to exercise with cold core temp Protective clothing and high metabolism of exercise usually prevents drop in temp Work, however, has lower metabolic output, long hours and may create an increased risk in cold environments Movement in cold Numbing of exposed flesh Cumbersome protective clothing Manipulation with hands difficult Anaesthetizes sensory receptors in hands Clothing-layers and breathability important Must balance insulation value with heat production of exercise Additional clothing after exercise important Metabolism drops, heat loss remains high Shivering inc metabolic costs Increases perception of effort May also impair movement patterns Agonist and antagonist contract

CV responses in Cold O2 consumption VO2 max - unaffected by cold Submax VO2 increases at lower intensities (*work*) Due to higher heat loss Inc skin and ms blood flow during exercise Greater thermal gradient and heat loss Wet clothing in wind - 15-20% higher VO2 requirements Table 22-1 - exercise in cold Shivering - Inc VO2 utilization May also be inc in non-shivering thermogenesis - due to inc catecholamines (stress) and leptin Swimming in cold water Reduced VO2 peak at higher intensities- fig 22-7

Exercise in Cold Exercise can partially replace heat production of shivering during cold Peripheral vasodilation with exercise - reduces insulation in body Exercise followed by cold exposure - higher threshold for vasocontriction and shivering Ventilation Inc ventilation - especially with sudden exposure - gasping reflex Hyperventilation, tachycardia, peripheral vasocontriction, hypertension Reduced blood CO2 - vasocontriction in brain - confusion, unconsciousness Heart Cold - peripheral vasocontriction - Inc central blood volume - inc BP Arrythmias - inc in cold Inc afferent impulses to hypothalamus and cardiovascular control center Inc adrenal epinephrine Ventricular fibrillation - leading cause of death in people with hypothermia

Cold Exposure Muscle strength Ms strength and peak power dec as ms temp decreases - dec enzyme activity May require inc motor unit recruitment to compensate for reduced output May also see reduced ms blood flow Combine to inc lactate production and reduce clearance - early fatigue Metabolic Changes Inc use of carbohydrates Ms glycogen reduced faster in light exercise - augments inc in lactate Prolonged exposure - hypoglycemia Suppresses shivering, core temperature drops Fat metabolism depressed even with catecholamine rise may be due to reduced subcutaneous circulation These problems are compounded by fatigue, sleeplessness and underfeeding

Acclimatization to cold Shivering threshold Maintain temperature without shivering, or with less shivering Experiment - three week exposure - inc thyroid hormone - tissues more sensitive to Nor epinephrine and Epinephrine Uncoupled ox phosphorylation Heat released without ATP formed Leptin - released from Adipose Stimulates Sympathetic NS Temperature of hands and Feet Unacclimatized - temp dec progressively Acclimatization - maintained Intermittent vasodilation in periphery Habituation as well, become more tolerant Sleeping Ability in the cold Depends on non shivering thermogensis Aborigines (Australia) - vasocontriction of periphery - sleep in cold without covering

Hypothermia Hypothalamus ceases to control body temp at extremely low temperatures (< 30 oC) CNS depressed Lose ability to shiver Sleepiness - - -> coma Reduced metabolic rate --> dec temperature Cardiovascular Central blood volume decrease Exacerbated by inadequate fluid intake Plasma sequestration Cold diuresis Hypothermia is possible during endurance exercise events Heat loss greater than production Glycogen depletion - blood glucose declines, CNS functioning declines