Exercise in the Cold and Hot

Piokilotherms vs Homeotherms Piokilotherms: at mercy of the elements Homeotherms: can function independent of the environment Ability to maintain constant temperature

Normal body temperature: 36.5-37.5°C During exercise can increase to 40°C with no ill effects Core temp: temperature of the hypothalamus, temperature regulator of the body Thermal gradients: temperature differences from one point to another the lead to movement of heat Temperature always equilibrates from hot to cold

Temperature Regulation Metabolism Shivering Nonshivering thermogenesis Metabolic Rate Heat loss

Metabolism Heat is produced naturally during normal metabolic reactions Most reactions lose ~75% of energy as heat At BMR heat loss is ~100 kcal/hr BMR is porportional to ¾ power of body weight (surface rule)

Shivering Main mechanism for increasing heat during negative heat balance Involuntary muscle contraction Maximum shivering can increase body’s heat production 5X

Preshivering tone can increase heat production 50-100% An effective way to increase body temperature b/c no work is done by the muscles and most of the energy is expended is heat Increases Q by increasing SV via increased venous return Limits: glycogen depletion, hypoglycemia, fatigue, ex., hypoxia, drugs (alcohol and barbiturates)

Nonshivering thermogenesis Increased thyroxin secretion (thyroid) and catecholamine secretion (adrenals) increase metabolic rate Thyroxin increases the rate of all cells Cats., esp norepi. release FFA, increasing metabolic rate

Metabolic Rate (Q10 and food) Q10 is rate of a physiologic process at a particular temperature to the rate at a temperature 10°C lower Increased metabolic rate can be self-perpetuating (dangerous)

At high temperatures, hypothalamus loses ability to cool the body Rate of temperature increases faster at higher temperatures Metabolic rate temporarily increases following food intake, esp. proteins

Heat Loss Radiation Conduction Convection Evaporation

Radiation Loss or gain in the form of electromagnetic waves At rest, in a comfortable environment, radiation accounts for 60% of total heat loss Varies with body position and clothes Human skin, regardless of color, absorbs ~97% of radiant energy that strikes it

Conduction Transfer of heat from a body to an object Or, heat transfer within an organism down a thermal gradient ~3% of total heat loss at room temperature occurs this way

Convection Conduction of heat to air or water Amounts to ~12% of all heat lost at room temperature Heat is conducted to water or air, moves so that other molecules can be heated Faster in water or air?

Greater in the wind (air movement) Wind chill effect Can also occur in the circulatory system Heat moves with the blood from the core to the periphery As skin temp. incr., heat loss to environment increases Rate is affected by blood flow and temperature gradient (core/periphery)

Evaporation ~25% of heat is lost this way in a comfortable environment Quantity of heat absorbed by sweat as it evaporates: latent heat of vaporization Body loses 0.58 kcal/gm H2O evaporates Only means of cooling at high environmental temperatures Critical for exercise

If body cannot lose heat this way, body temp. increases rapidly Sweat is only effective if it evaporates High humidity: evaporation reduced or prevented Effective evaporation also hampered by little air movement

Women have a lower sweating capacity than men do Process occurs by sweating and insensible water loss: ventilation, diffusion through skin, does not include sweat, urine, and feces Sweat rates are at 0 when temperature is low

Hypothalamus and temperature regulation Temperature regulatory center Set-point it tries to keep Sweating normally occurs at 37°C Set-point can change in response to dehydration, starvation, and fever Responds to heat primarily through heat-sensitive neurons in the preoptic area of anterior portion

More cold than heat receptors in the skin Thermoregulators transmit impulses to the spinal cord, to hypothalamus, initiates response Ant. Hypothalamus stimulates the sweat glands, evaporative heat loss “Hunting Reflex” primarily in hands and feet

Exercise in the Cold Clothing Oxygen Consumption Ventilation Heart Muscle Strength Metabolic Changes

Oxygen Consumption Maximum uptake is unaffected by the cold Submax. VO2 increases in the cold Why?

Ventilation Increases in the cold, particularly if exposure is sudden Abrupt exposure can lead to gasping reflex

Heart Changes in cardiac performance more common in men Incidence of arrhythmias increase in cold Muscle Strength Strength decreases with lower muscle temp.

Metabolic Changes Increase use of CHO as substrate Light ex: glycogen depletes faster Max ex: depletion is independent of temp. Prolonged exposure to cold: hypoglycemia, suppresses shivering, core temp drops, lactate higher Fat metabolism suppressed, even though cat. response is higher

Acclimatization and Habituation to Cold Acclimatize: physiologic compensation to environmental stress over time Habituation: lessening of the sensation associated with an environment Shivering threshold First test of acclimatization Cold-acclimatized people maintain heat production with less shivering, more nonshivering thermogenesis

Hand and Feet temperature Second test for acclimatization Acclimatized: maintain almost normal temperature Habituation also plays a role Ability to sleep in the cold Third and final test Seems to depend on extent of nonshivering thermogenesis induced by increased secretion of norepi

Hypothermia Depresses the CNS, lose ability to shiver, sleepiness, coma, death Lower temp: lower cellular met. rate, further lowering temp. Profound effects on the CV system Central BV decreases; plasma sequestration, inadequate fluid intake, cold diuresis

Risk factors cold exposure Lack of protective clothing Leanness Inadequate fluid intake High wind chill Use of alcohol &/or drugs Use of snow to relieve thirst Glycogen depletion

Frostbite Caused by ice crystal formation within the tissue Occurs in exposed skin Can lead to tissue death

Exercise in the Heat

Plasma volume decreases during exercise in the heat becomes acute at intensity increases decrease of plasma volume is made worse by loss of body fluids through sweating may not be enough blood to adequately perfuse all areas during exercise in the heat

central blood volume may decrease and cause a decrease in cardiac filling pressure results in increased HR in attempt to compensate for lower SV submax. HR increases also @ max levels, skin vessels vasoconstrict to help maintain blood pressure and Q

negative response on heat transfer circulatory regulation takes precedence over temperature regulation in this case VO2 max is not impaired in the heat unless the person was experiencing thermal imbalance before beginning the exercise

Sweating Response primary means of heat dissipation during exercise (evaporation) in heat, sweating is very important b/c body tends to gain rather than lose heat by radiation, conduction, and convection during exercise, sweating is related more to intensity than environmental temperature

Acclimatization to heat first two weeks of heat exposure results in lower heart rate, core temperature, RPE, and skin temperature at rest and during exercise primary physiological adjustments are increased peripheral heat conductance, plasma volume, and sweating, decreased core temperature at onset of sweating, and improved distribution of sweat over the skin

CV Adaptations acclimatization induces a 3-27% increase in plasma volume, if acclimatization is done with exercise training increased plasma volume helps to maintain SV, central blood volume, and sweating capacity also increases in vasopressin, renin, and aldosterone in early days

blood flow to the skin decreases decreased skin blood flow helps to maintain central blood volume, which is vital for maintaining BP, SV, and muscle blood flow during exercise core temperature is lower during exercise decrease in skin blood flow is accompanied by a large increase in sweating and evaporative cooling capacity

Sweating Response acclimatization increases this response almost 3X, from ~1.5 l/hr to 4 l/hr accompanied by a more even distribution of sweating sweat losses of sodium chloride decrease b/c of increased secretion of aldosterone

Thermal Distress includes dehydration, heat cramps, heat exhaustion, heat syncope, and heat stroke hyperthermia is caused by an imbalance between heat gain and heat loss

Dehydration loss of fluid from the body can decrease sweat rate, plasma volume, Q, VO2 max, work capacity, muscle strength, and liver glycogen at fluid deficit of 5% of BW, symptoms include discomfort, and alternating states of lethargy and nervousness level >7% is extremely dangerous

at levels >10%, ability to walk is impaired, and is accompanied by discoordination and spasticity as 15% is neared, the person experiences delirium, shriveled skin, along with decreased urine volume, loss of ability to swallow food, and difficulty swallowing water >20% the skin bleeds and cracks

thirst does not keep up with fluid requirements physical fitness helps prevent this during prolonged exercise, can develop hyponatremia, caused by excessive sodium loss in the sweat without adequate replacement

Heat cramps characterized by involuntary cramping and spasm in muscle groups used during exercise it occurs in people who have exercised and sweated heavily often the individual is conditioned and acclimatized fluid and electrolyte replacement, rest, some say magnesium helps

Heat exhaustion rapid, weak pulse, hypotension, faintness, profuse sweating and psychological disorientation results from acute plasma volume loss and inability of the circulation to compensate for the concurrent vasodilation in the skin and the active working muscles

Treatment have person lie down in cool area administer fluids rest and drink plenty of fluids for next 24 hr. not allowed to participate for rest of the day

Heat syncope related to heat exhaustion, but can occur without major sweat loss typically, it occurs after exercise when the individual stops moving and blood pools can occur secondary to heat exhaustion or independently

Heat stroke failure of hypothalamic temperature regulatory center; represents a major medical emergency principally caused by failure of the temperature regulatory center in hypothalamus, which causes failure of the body’s heat loss mechanisms

characterized by a high core temperature, hot, dry skin, and extreme CNS dysfunction associated with increases in plasma norepinehprine, epi, and endotoxin levels that have been associated with vascular collapse risk is greatest in high temperatures, humidity, hottest time of the day, and on days with little wind

Treatment cool person with tepid water ice packs on the groin, neck, and axilla send to hospital ASAP

Prevention ensure athletes are well-conditioned avoid overheating be aware of early symptoms of heat stress: thirst, fatigue, lethargy, and visual disturbances athletes should not train harder than normal intensity should not compete if have an illness accompanied by a fever

schedule practice during the cooler times of the day modify or cancel sessions when the wet bulb temperature is 25.5°C or greater plan regular fluid breaks supply a drink that is cold and contains some CHO and electrolytes

hyperhydrate before activity fluid replacement should be encouraged during the early stages of practice and competition athletes should be weighed every day before and after practice. If a 2-3% decrease in weight, should consume more fluid, if 4-6%, should decrease activity levels, 7% loss, consult a physician

Rhabdomyolysis Myoglobin excretion is increased Myoglobin is cleared by the kidneys Kidneys shut down, especially with heat Can lead to death Occurs with excessive exercise Rare May be a predisposition, latent metabolic disorder Diuretics contribute to this CHO as well, but a rare possibility