1. Exercise Thermoregulation, Fluid Balance, and Rehydration
Chapter 10
Exercise Thermoregulation, Fluid Balance, and Rehydration

2. Simple Facts
A reduction in body water results in a decrease in performance
Hydration status is determined by water intake and loss
Lack of water consumption impairs the body and after several days, can result in death
Over consumption of water can also result in problems
50-60% of the body’s mass is water

3. Simple facts (cont.) Lean body mass (LBM) is comprised of 75% water
Fat contains 5% water
154lb male=42L
154lb female=35L
Sweating: uses water to cool body and prevent hyperthermia

4. Body Temperature Hyperthermia is an increase in body temperature by
5°C or more.
Core body temperature: deep tissues
Shell body temperature: peripheral
Core temperature rises quickly when heat gain exceeds heat loss during vigorous exercise in a warm environment.
The relative stress of exercise determines the magnitude of the temperature increase.

5. Hypothalamic Regulation
Hypothalamus is the central coordinating center for temperature regulation.
Acts as a thermostat
Initiates responses to protect the body from heat gain or heat loss

6. Exercise Generates Heat
Core temperature normally increases during exercise.
The relative stress of exercise determines the magnitude of the increase.
A well-regulated temperature increase creates a more favorable environment for physiologic and metabolic functions.

7. Cutaneous and muscle blood flow increase during exercise in the heat, while other tissues temporarily compromise their blood supply.
For every liter of oxygen (O2), 4 calories of heat are produced, with only 1 calorie used for energy
An athlete using 4L O2/minute, heat production is about 917 calories for hour

8. Factors that contribute to heat gain and heat loss to regulate core temperature at about 37˚C

9. Core temperature
Humans tolerate relatively small variations in core temperature.
Exposure to heat or cold initiates thermoregulatory mechanisms that generate and conserve heat at low ambient temperatures and dissipate heat at high temperatures.

10. Heat-Regulating Mechanisms
Become activated in two ways:
Temperature changes in blood perfusing the hypothalamus directly stimulate this thermoregulatory control center.
Thermal receptors in the skin provide input to modulate hypothalamic activity.
Structures in the skin and subcutaneous tissue help to regulate temperature.

12. Thermoregulation
Thermoregulation is the body’s mechanism to regulate body temperature and prevent overheating
The hypothalamus serves as the “thermostat” for temperature regulation.
The hypothalamus initiates adjustments from:
Thermal receptors in the skin.
Changes in hypothalamic blood temperature.
Nervous signals from osmoreceptors and pressure receptors of blood volume
Hormones

13. Heat Storage
Heat production is directly proportional to exercise intensity, so both hot and cool environments can increase body temperatures
If body temp> 103°F, it will cause the CNS (brain) to begin shutting down and fatiguing
Normal body temp= °F

14. Heat Loss Can occur due to: Radiation Conduction Convection
Evaporation
The body’s thermoregulatory mechanisms primarily protect against overheating.

15. Heat Loss
Increased body core temperature is sensed by the thermoregulatory center in the hypothalamus and increases blood flow to the skin to induce sweating
Warm blood diverts from the body’s core to the skin in response to heat stress.
Heat loss occurs by radiation, conduction, convection, and evaporation.
Evaporation provides the major physiologic defense against overheating at high ambient temperatures and during exercise.

16. Radiation Objects emit electromagnetic heat waves.
Body temperature is warmer than the environment.
Radiant heat energy leaves the body through air to solid cooler objects around us.
The body absorbs radiant heat energy when the temperature of objects in the environment exceeds skin temperature.
This form of heat transfer, similar to how the sun’s rays warm the earth, does not require molecular contact between objects.

17. Conduction
Transfers heat directly through a liquid, solid, or gas from one molecule to another
The circulation transports most of the body heat to the shell.
A small amount continually moves by conduction directly through the deep tissues to the cooler surface.
Conductive heat loss then involves the warming of air molecules and cooler surfaces in contact with the skin.
The rate of conductive heat loss depends on the temperature gradient between the skin and surrounding surfaces and their thermal qualities.

18. Convection Air movement
Warm air next to the skin acts as a zone of insulation.
If cool air continuously replaces the warmer air surrounding the body, heat loss increases.
For example, air currents at 4 miles per hour cool twice as effectively as air moving at 1 mile per hour.

19. Evaporation Major physiologic defense against overheating
Water vaporization from the respiratory passages and skin surface continually transfers heat to the environment.
In response to heat stress, 2-4 million sweat (eccrine) glands secrete large quantities of hypotonic saline solution.
Cooling occurs when sweat evaporates from the skin surface.
Along with heat loss through sweating, approximately 350 mL of water seeps through the skin (called insensible perspiration) each day and evaporates to the environment. Also, approximately 300 mL of water vaporizes daily from the respiratory passages’ moist mucous membranes.

20. Heat production within active muscle and its subsequent transfer from the core to the skin. Under appropriate environmental conditions, excess body heat dissipates to the environment and core temperature stabilizes within a narrow range.

21. Environmental Temperature
Increased ambient temperature reduces the effectiveness of heat loss by conduction, convection, and radiation.
Sweat evaporation from the skin depends on:
Surface exposed to the environment
Temperature and relative humidity of ambient air
Convective air currents around the body
For someone relaxing in a hot, humid environment, the normal 2-L daily fluid requirement doubles or even triples from evaporative fluid loss.
Relative humidity exerts the greatest impact on the effectiveness of evaporative heat loss.
Relative humidity refers to the percentage of water in ambient air at a particular temperature compared with the total quantity of moisture that the air could carry.

22. Environmental Heat Stress and Heat Loss Via Sweat Evaporation
Determined by:
ambient temperature
Humidity
Wind velocity
Solar radiation
During exercise, heat production and loss from the skin occurs via evaporation (convection) and conduction
If the environment is hotter, heat is gained
If the environment is humid, decreased evaporation of heat occurs
One L of water takes 573 calories to evaporate

23. Approximate hourly sweating rates related to environmental conditions and exercise intensity.

24. Heat-Dissipating Mechanisms
Circulation
“Workhorse” to maintain thermal balance
Evaporation
Hormones
Antidiuretic hormone
Aldosterone
At rest in hot weather, heart rate and blood flow from the heart (cardiac output) increase while superficial arterial and venous blood vessels dilate to divert warm blood to the body’s shell.
A large cutaneous blood flow coupled with evaporative cooling generally produces an effective thermal defense.
ADH increases water reabsorption from the kidney tubules, causing urine to become more concentrated.
Aldosterone increases the renal tubules’ reabsorption of sodium and decreases sodium concentration in sweat, which aids in additional electrolyte conservation.

25. Exercise Clothing Cottons and linens readily absorb moisture.
Heavy “sweatshirts” and rubber or plastic garments produce high relative humidity close to the skin.
Dark colors absorb light rays and add to radiant heat gain.
Light colors reflect heat rays away from the body.
Moisture-wicking fabrics provide optimal transfer of heat and moisture from the skin to the environment.
Moisture-wicking fabrics include polypropylene, Coolmax, and Drylite.
Football uniforms and equipment present a considerable barrier to heat dissipation during environmental heat exposure.
Cycling helmets do not impede heat loss from the head.

26. Water Loss Dehydration
Considerable water loss occurs during several hours of intense exercise in a hot environment.
Both intracellular and extracellular compartments contribute to fluid deficit.
The risk of heat illness greatly increases when a person begins exercising in a dehydrated state.
Sweat is hypotonic with other body fluids.
Dehydration refers to an imbalance in fluid dynamics when fluid intake does not replenish water loss from either hyperhydrated or normally hydrated states.

27. Sweating and Exercise
Increased sweating strains fluid reserves, creating a relative state of dehydration.
Excessive sweating without fluid replacement decreases plasma volume and causes core temperature to rise precipitously.

28. Dehydration and Exercise
Just about any degree of dehydration impairs the capacity of circulatory and temperature-regulating mechanisms to adjust to exercise demands.
Dehydration of as little as 2% body mass impairs physical work capacity and physiologic function and predisposes to heat injury when exercising in a hot environment.
The risk for dehydration increases during vigorous cold- weather exercise.
Colder air contains less moisture than air at warmer temperature, particularly at higher altitudes. Greater fluid volumes leave the respiratory passages as the incoming cold, dry air becomes fully humidified and warmed to body temperature. Cold stress also increases urine production, which adds to total-body fluid loss.

29. Factors that increase the potential for dehydration during cold-weather exercise. The illustration depicts a back-mounted hydration system to provide ready access to water during continuous exercise in the outdoor environment.

30. Dangerous Conditions: Physiological Effects of Dehydration
Decreased blood volume
Decreased blood flow to skin
Decreased sweat rate (due to increased blood osmolarity)
Decreased heat dissipation
Increased core temperature
Increased muscle glycogen use
Decreased cardiac output
Decreased gastric emptying 20-25% dehydration)
Heat stress from the environment alone can decrease VO2max by 7%

31. Dangerous Conditions: Effects of Dehydration on Exercise Performance
2% dehydration impairs performance
5% dehydration decreases work capacity by about 30% (impedes heat dissipation, compromises cardiovascular function, and diminishes exercise capacity)
2.5% dehydration decreases sprinting capacity (can occur with acute dehydration)
7% dehydration will increase fatigue due to heat strain

32. Prevention Factors to be aware of: Hydration status
Environmental temperature
Consume fluids prior to exercise (hyperhydrate)
Consume fluid during early stages (minimize dehydration and maximize availability)
Acclimate slowly
Be aware of heat-related illness signs

33. Effects of Fluid Intake on Performance
Pre-exercise hyperhydration
Helps to decrease thermal and cardiovascular strain
May help to decrease HR, core temp., and increase sweating
Consume large volume of water or water+electrolytes 1-3 hours prior to exercise
Increased fluid retention with glycerol added to fluids
May help to decrease heart and core temperatures and increase performance
Should consume 500 ml 2 hours pre-exercise and 500ml 15’ prior to event

34. Hydration during exercise
One can avoid dehydration if intake=output
Difficulties:
Sweat rates =2-3L/hr, volume consumed > 1L/hr can be uncomfortable
Sweat rates vary
Thirst is not a good indicator (occurs after2% loss/mild dehydration)
Limited ability to intake water during competition (soccer, tennis)

35. During exercise
Every 15-20’ consume ml (~1/4 cup)
Only need water if activity is 30-60’
Glucose and electrolyte beverages if longer (offer at 30’ mark of exercise to prevent an insulin response)
Concentration: glucose=20-60g/L, sodium=20- 60mmol/L, and osmolality<290
Most sports drinks: glucose=60-80g/L, Na+=20-25
Have athletes practice drinking during training (estimate how much they will/can consume)

36. Rehydration
Properly scheduling fluid replacement maintains plasma volume, so circulation and sweating progress optimally
A well-hydrated individual always functions at a higher physiologic and performance level than a dehydrated - person.
Achieving hyperhydration before exercising in a hot environment protects against heat stress because it:
Delays dehydration
Increases sweating during exercise
Diminishes the rise in core temperature
In addition to increasing fluid intake 24 hours before strenuous exercise in the heat, it is recommend to consume 400 to 600 mL (13-20 oz) of cool water about 20 minutes before exercise.
Pre-exercise hyperhydration does not replace the need to continually replace fluid during exercise.

37. Adequacy of Rehydration
Body weight changes indicate the extent of water loss from exercise and adequacy of rehydration during and after exercise or athletic competition.
Urine and hydration:
Dark yellow urine with a strong odor = inadequate hydration
Large volume, light color, without a strong odor = adequate hydration
Drink at least % of the existing fluid loss (body weight loss) as soon as possible after exercising. The 25-50% “extra” water accounts for that portion of ingested water lost in urine.

38. Computing the magnitude of sweat loss and rate of sweating in exercise
Computing the magnitude of sweat loss and rate of sweating in exercise. In this example, Mackinzie should drink about 1000 mL of fluid during each hour of activity to remain well hydrated (250 mL every 15 minutes).

39. Sodium and Rehydration
A moderate amount of sodium added to a rehydration beverage provides more complete rehydration.
Maintaining a relatively high plasma concentration of sodium helps:
Sustain the thirst drive
Promote retention of ingested fluids
More rapidly restore lost plasma volume during rehydration
Restoring water and electrolyte balance in recovery occurs most effectively by adding moderate to high amounts of sodium (100 mmol·L-1, an amount exceeding that in commercial beverages) to the rehydration drink or combining solid food (with appropriate sodium content) with plain water.

40. 1-1.5 L fluid for every kg lost
Should consume beverages over 6 hours to rehydrate
Avoid caffeine and alcoholic beverages

41. Cumulative urine output during recovery from exercise-induced dehydration. The oral rehydration beverage consisted of four test drinks containing sodium in a concentration of 2 (A), 26 (B), 52 (C), or 100 (D) mmol/L.

42. Hyponatremia Low blood level of sodium (<135 mEq/L)
Can occur due to excessive water intake
A sustained low plasma sodium concentration creates an osmotic imbalance across the blood–brain barrier that causes rapid water influx into the brain.
The resulting swelling of brain tissue produces a cascade of symptoms that range from mild to severe.
Mild: headache, confusion malaise, nausea, cramping
Severe: seizures, coma, pulmonary edema, cardiac arrest, death

43. Factors that contribute to the development of hyponatremia.

44. Physiologic consequences of hyponatremia. CNS = central nervous system.

45. Acclimatization
Heat acclimatization refers to the physiologic adaptations that improve heat tolerance.
The acclimatized individual:
Has larger quantities of blood shunt to cutaneous vessels
Has more effective cardiac output
Has an earlier onset of sweating
After 10 days of heat exposure, sweating capacity nearly doubles, and sweat becomes dilute (less salt lost) and more evenly distributed on the skin surface. Optimal acclimatization necessitates adequate hydration.

46. Repeated heat stress initiates thermoregulatory adjustments that improve exercise capacity and reduce discomfort on subsequent heat exposure.
Acclimatization triggers favorable redistribution of cardiac output and increases sweating capacity.
Exercise improves temperature regulation, but exercise intensity must be increased (70-100%) in order to improve a response
Full acclimatization generally occurs in about 10 days of heat exposure.

47. Increased size of sweat glands
Increased total blood volume and cardiac output
Marathoners have a lower body temperature at rest and a lower threshold to sweating (lower BMR and skin temp)
Aging affects thermoregulatory function, yet acclimatization to moderate heat stress does not appreciably deteriorate with age.

48. When controlling for fitness and acclimatization levels, women and men show equal thermoregulatory efficiency during exercise.
Women produce less sweat than men do at the same core temperature.

49. Average rectal temperature, heart rate, and sweat loss during 100 minutes of daily heat-exercise exposure for 9 consecutive days.

50. Age Differences in Acclimatization
Older individuals have:
A decreased sensitivity of thermoreceptors
Limited sweat gland output
Dehydration-limited sweat output with insufficient fluid replacement
Altered structure and function of the skin and its vasculature
A decreased recovery from dehydration

51. Other Factors Affecting Acclimatization
Children
Gender
Men sweat more.
Women show heat tolerance similar to men.
Body fat
Children exposed to environmental heat stress should exercise at reduced intensity and receive more time to acclimatize than more mature competitors.
Females tolerate the physiologic and thermal stress of exercise as well as men of comparable fitness and level of acclimatization; both sexes acclimatize to a similar degree.
Excess body fat negatively affects exercise performance in hot environments.

52. Wet Bulb-Globe Temperature
Used to evaluate the environment for its potential thermal challenge
Index of environmental heat stress
Incorporates ambient temperature, relative humidity, and radiant heat
DBT = dry-bulb (air) temperature in the shade recorded by an ordinary mercury thermometer that measures air temperature.
WBT (accounts for 70% of the index) = temperature recorded by an ordinary mercury thermometer and a thermometer with a wet wick that surrounds the mercury bulb (wet bulb) exposed to rapid air movement in direct sunlight.
GT = globe temperature in direct sunlight recorded by a thermometer with a black metal sphere surrounding the bulb.

53. Wet bulb-globe temperature for outdoor activities and wet-bulb temperature guide.

54. Heat Illnesses Heat cramps
Involuntary muscle spasms that occur after intense physical activity
Heat exhaustion
Most common heat illness
Heat stroke
Most serious and requires immediate medical attention
If left untreated, the disability becomes fatal from circulatory collapse, oxidative damage, a systemic inflammatory response, and damage to the central nervous system and other organs.
Immediate treatment includes alcohol rubs and application of ice packs. Whole-body cold or ice water immersion remains the most effective treatment for a collapsed hyperthermic individual.

55. Heat Illnesses
Increases if athlete is dehydrated, less trained, and less acclimatized
Initial stages:
Exercise increase energy production & increases heat which will increase core temperature
Increased blood flow to the skin, decreased blood to organs
Increased toxins occur, causing blood poisoning, decreased BP, and fainting

56. The Heat Index