Piokilotherms vs Homeotherms

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

3. 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

4. Temperature Regulation • Metabolism • Shivering • Nonshivering thermogenesis • Metabolic Rate • Heat loss

5. 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)

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

7. 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)

8. 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

9. 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)

10. 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

11. Heat Loss • Radiation • Conduction • Convection • Evaporation

12. 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

13. 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

14. 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?

15. 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)

16. 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

17. 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

18. 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

19. Hypothalamus and temperature regulation • Temperature regulatory center • 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

20. Ant. Hypothalamus stimulates the sweat glands, evaporative heat loss • “Hunting Reflex” primarily in hands and feet • Hypothalamus is the body’s thermostat • Set-point it tries to keep • Sweating normally occurs at 37°C • Set-point can change in response to dehydration, starvation, and fever

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

22. Clothing • Insulation value of clothing must be balanced with increased metabolic production of the exercise • Traps warm air next to the skin an decreases heat loss by conduction and convection • Should be worn in layers

23. Oxygen Consumption • Maximum uptake is unaffected by the cold • Submax. VO2 increases in the cold • Why? • Shivering can persist during exercise, which also can increase VO2

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

25. Heart • Cold exposure causes peripheral vasoconstriction, increase central BV • BP increase due to increased afterload and sometimes increased preload too • SV may increase, inc. EDV • Changes in cardiac performance more common in men • Incidence of arrhythmias increase in cold

26. Muscle Strength • Strength decreases with lower muscle temp. • Also lower enzyme activity • Increased MU recruitment may be necessary to compensate • Other factors: increased muscle viscosity, decreased ATP metabolism and contractility

27. 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

28. 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

29. 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

30. 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

31. 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

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

33. Exercise in the Heat

34. Cardiovascular Effects

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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