ACUTE PHYSIOLOGICAL
RESPONSES TO HEAT STRESS
Basic Mechanisms of Thermoregulation
• Core temperature maintained between 35 to
41o C despite environmental extremes which
fluctuate between -88 to 58o C via:
1. Behavioral temperature regulation
such as choice of clothing,
shelter, ventilation, air conditioning,
heating, humidifiers, and
dehumidifiers.
2. Physiological temperature regulation controlled
by rate of metabolic heat production, heat flow
from core to skin, and sweating.
Basic Mechanisms of Thermoregulation
• Physiological control systems operate on a graded or
proportional response in which changes in controlled
variables (e.g., sweating and skin blood flow) are
proportional to displacements of the regulated
variable (e.g., core temperature) from a threshold
value.
• Note: each physiological response has a core
temperature at which the responses start to increase
and the actual response is
dependent on mean skin temperature; the lower the
skin temperature, the higher the increase in core
temperature before the response is initiated.
Basic Mechanisms of Thermoregulation
• Thus, thermoregulatory responses are related to
both core and mean skin temperature and hence,
(1) at any given skin temperature each response
is proportional to core temperature and (2) an
increase in skin temperature will decrease the
core temperature threshold and increase the
response at any given core temperature.
CORE TEMPERATURE
• Temperatures within body regions are
dependent on:
1. The metabolic rate of surrounding
tissues.
2. The source and magnitude of
blood flow.
3. The temperature gradients
between contiguous body regions.
SKIN TEMPERATURE
• Determination of skin temperature is useful
for:
1. Estimating the input of skin temperature
receptors into the hypothalamus for
thermoregulatory control.
2. Predicting core temperature.
3. Calculating mean body temperature for
heat storage determination.
4. Calculating sensible (radiation and
convection) heat exchange.
Exercise Intensity and Core Temperature
• At rest, 70% of metabolic heat comes from
internal organs and viscera within the body
core. During dynamic exercise, metabolic rate
increases rapidly by 5-15 fold with 70-90% of
metabolic rate released as heat (humans are
least efficient at slow and fast speeds of
movement). Thermoregulatory effectors for
heat dissipation respond more slowly.
• Core temperature increases rapidly, then
gradually leveling off at a steady-state value
when heat production equals heat loss.
Exercise Intensity and Core Temperature
• Magnitude of core temperature at steadystate is largely independent of
environmental conditions within a fairly
wide prescription zone. Increases in
core temperature are proportional to
increases in metabolic rate. 1 watt = 6
kgm/min = .01433 kcal/min.
• The prescription zone is smaller at
higher exercise intensities.
Exercise Intensity and Core Temperature
• If exercise intensity is expressed in absolute
terms (L/min), large individual differences exist
in steady-state core temperature; however, if
exercise intensity is expressed in relative
terms (%VO2max),the inter-individual
differences disappear.
Exercise Intensity and Core Temperature
• There is a curvilinear relation between steadystate core temperature and relative workload.
The prescription zone is smaller at higher
relative exercise intensities and it is more
difficult to reach steady-state core temperature
as core temperature and relative exercise
intensity increases.
• Note: An increase in VO2max from training
would decrease relative workload and hence,
reduce core temperature, particularly in a heat
acclimated person.
Exercise Intensity and Core Temperature
• An increase in ambient air temperature
increases the metabolic rate (energy
requirements) and hence relative workload of
an absolute exercise task as well as the
relative contribution of anaerobic energy
metabolism to the total energy requirements.
Effects of Mode of Exercise on Relative
Workload
• Although VO2max of upper body exercise (e.g.,
arm cranking) is lower than lower body exercise
(e.g., leg cycling) and consequently relative work
intensity is much higher when performing the
same absolute exercise task during arm
cranking, core temperature responses appear to
be quite similar when working at the same
absolute metabolic workload.
Acute Metabolic and Muscular Effects
of Heat
1. Decreased VO2max due to diversion of
blood to the skin (cutaneous)
vasculature.
- Decreased blood flow (% Q) to
muscle.
- Increased peripheral pooling of
blood, which decreases central
blood volume and hence enddiastolic volume (EDV).
SV = EDV - ESV, Q = SV X HR,
and VO2 = Q X [A - V O2 Diff]
Acute Metabolic and Muscular Effects
of Heat
2. The decreased muscle blood flow
increases tissue hypoxia and the
relative contribution of anaerobic
metabolism to total energy
requirements.
3. Increased carbohydrate utilization,
primarily from blood glucose.
Acute Metabolic and Muscular Effects
of Heat
4. Increased blood lactate levels, due to
increased lactate production as
well as reduced lactate removal due to
splanchnic vasoconstriction and hence,
decreased hepatic clearance of lactate.
5. Decreased free fatty acid utilization as an
energy source.
Acute Metabolic and Muscular Effects
of Heat
6. Increased blood glucose levels, as
glucose is released from the liver.
7. Decreased blood triglyceride levels, due
to decreased mobilization of fat.
Acute Metabolic and Muscular Effects
of Heat
8. Increased reliance on fast-twitch motor
units.
9. Decreased efficiency of skeletal muscle
contraction as fast-twitch motor units
expend greater energy than ST motor
units to develop the same tension.
Acute Metabolic and Muscular Effects
of Heat
10. Increased expired ventilation
rate, primarily due to an
increase respiratory rate
(i.e.,breathing frequency) with
minimal changes in
respiratory (i.e.,tidal) volume.
Effector Mechanisms of Acute Heat
Exposure
• Vasodilation of Cutaneous Vasculature
(Increased Skin Blood Flow)
• Increased Sweat Rate
Vasodilation of Cutaneous Vasculature
(Increased Skin Blood Flow)
• There is threshold above which skin
blood flow will increase. Skin blood
flow carries heat by convection from
the deep body tissues to the skin,
which may lead to sensible and/or
insensible heat loss.
Vasodilation of Cutaneous Vasculature
(Increased Skin Blood Flow)
• Skin blood flow is dependent on both core
temperature and skin temperature as:
- Increased skin blood flow is
proportional to core temperature at any
given skin temperature.
-
Increased skin temperature will
decrease the core temperature
threshold.
Heat Transfer by Skin Blood Flow
• Dependent on the rate of blood flow and
on the differences in temperature between
arterial blood leaving the core on its way
to the skin and venous blood returning to
the core from the skin. Increases in the
rate of blood flow, conduction of tissues,
and the difference between core and skin
temperatures will increase the heat
transfer.
• As ambient temperature increases, there
is an increased dependence on insensible
(evaporative) heat loss mechanisms and a
decreased dependence on sensible
(convection and radiation) heat loss
mechanisms to minimize exercise-induced
increases in core temperature.
• Evaporative heat loss is dependent on
skin blood flow (provides latent heat for
the evaporation of sweat) and on secretion
of perspiration from the sweat glands.
Types of Sweat Glands
1. Apocrine glands - nervous or
emotional sweat due to
neurochemical stimuli; secretion is a
watery substance that contains
lipids, trace of color, and odor;
most commonly found on the
palms of the hands, soles of
the feet, arm pits, groin area, face,
and upper lip.
Types of Sweat Glands
2. Eccrine glands - respond to
thermal stress by secreting a
watery substance which contains
electrolytes and is generally
colorless and odorless; there are 1.6 to
4 million eccrine glands and are most
numerous on the sole of the feet and
least numerous on the back, although
the back eccrine glands are the first to
respond to increases in core
temperature.
Sweat Rate
• People who sweat more (1) have larger
sweat glands, (2) have a greater amount of
sweating per gland, (3) have a higher
sweating rate per unit of tubular length or
unit volume of secretory coil, (4) have a
reduced hidromeiotic effect, and (5) the
sweat glands have a greater cholinergic
(AcH) sensitivity.
• Hidromeiotic Effect - increased skin
wettedness decreases sweating.
Sweat Rate
• Sweat rate is dependent on both core
temperature and skin temperature as:
-
Increased sweat rate is proportional to
core temperature at any given skin
temperature.
- Increased skin temperature will
decrease the core temperature
threshold.
• Generally, the heat dissipating
responses are sufficient to meet the
rise in core temperature during
exercise; if not however,
hyperthermia may occur leading to
heat strain and/or other
heat-related illnesses.
Heat Strain
• Heat strain results in a decrease in enddiastolic volume as blood pools in the
peripherally dilated veins and/or plasma
volume decreases, which leads to a decrease
in stroke volume and thus heart rate must
increase to maintain cardiac output. Plasma
volume decreases due to an increased
movement of fluid from the plasma to tissue
(affected by temperature, exercise intensity
and mode, hydration level, and status of heat
acclimation) and an increased fluid loss
through sweating.
Compensation of Heat Strain
1. Decreased
splanchnic
(visceral) and
renal blood
flow, which is
proportional to
relative
exercise
intensity and
skin
temperature.
Compensation of Heat Strain
2. Decreased skin blood flow at high intensities
as cardiovascular strain is increased (i.e., skin
blood flow is proportionally lower than
expected for a given skin or core
temperature); this response occurs more
quickly in the upright position as compared to
the supine position and is probably controlled
by cardiopulmonary baroreceptors.
Compensation of Heat Strain
3. Venoconstriction of cutaneous (i.e., skin)
veins during intense exercise.
4. Increased water and sodium retention by the
kidneys.
Heat Illnesses
•
•
•
•
•
•
Heat Cramps
Heat Syncope
Water Depletion Heat Exhaustion
Salt Depletion Leading to Heat Exhaustion
Heat Hyperpyrexia Leading to Heat Stroke
Skin Lesions
HYPOHYDRATION AND
HYPERHYDRATION
Hypohydration and Body Fluids
• SWEAT LOSS OF 5% BODY WEIGHT
WILL DECREASE TOTAL BODY
WATER (TBW) BY AS MUCH AS 8%.
Hypohydration and Body Fluids
•
•
•
•
Body Weight = 75 kg
TBW = 45 kg (60% of BW)
Sweat Loss of 5% BW = 3.75 kg
3.75 kg/45 kg = 8% Decrease in TBW
Hypohydration and Body Fluids
• PROPORTIONATELY, WHEN TOTAL
BODY WATER IS REDUCED:
• INTRACELLULAR, INTERSTITIAL, AND
PLASMA FLUID VOLUMES DECREASE.
Hypohydration and Body Fluids
• IN GENERAL, THE GREATEST
DECREASES TEND TO OCCUR IN
THE INTERSTITIAL AND
INTRACELLULAR FLUID VOLUMES.
Hypohydration and Body Fluids
• PERCENT PLASMA VOLUME
DECREASES STAY ABOUT THE
SAME REGARDLESS OF THE
DECREASE IN TOTAL BODY WATER.
Effects of Heat Adaptation on
Hypohydration
1.
ADAPTED PERSON HAS A
SMALLER PLASMA VOLUME
REDUCTION AT A GIVEN BODY
WEIGHT LOSS DUE TO
HYPOHYDRATION.
Effects of Heat Adaptation on
Hypohydration
2.
ADAPTED PERSON HAS MORE
TBW; THEREFORE, ABSOLUTE
FLUID LOSS WOULD
REPRESENT A SMALLER
PERCENTAGE OF TBW.
GENERAL EFFECTS OF
HYPOHYDRATION
1. INCREASED CORE TEMPERATURE (INCREASE
TENDS TO BE LINEAR).
2. INCREASED HEAT STORAGE DUE TO REDUCED
HEAT LOSS THROUGH BOTH SENSIBLE AND
INSENSIBLE MECHANISMS.
3. NO AFFECT ON RATE OF AEROBIC AND
ANAEROBIC METABOLISM.
GENERAL EFFECTS OF
HYPOHYDRATION
4.
DECREASED SWEAT RATE FOR A
GIVEN CORE TEMPERATURE
DUE TO HYPEROSMOLARITY
(HIGH CONCENTRATION OF
OSMOTICALLY ACTIVE
PARTICLES IN A SOLUTION)
AND/OR HYPOVOLEMIA (LOW
PLASMA VOLUME) OF
PLASMA.
GENERAL EFFECTS OF
HYPOHYDRATION
• NOTE: HYPEROSMOLARITY OF PLASMA
VOLUME HAS ALSO BEEN SHOWN TO
INCREASE CORE TEMPERATURE DUE TO
REDUCED CONVECTIVE HEAT TRANSFER
(DECREASED SKIN BLOOD FLOW) AS WELL AS
THE DECREASE IN SWEAT RATE THAT LEADS
TO AN INCREASE IN CORE TEMPERATURE.
HYPOHYDRATION AND
EXERCISE
1.
HYPOHYDRATION DURING
SUBMAXIMAL EXERCISE
WITHOUT THERMAL STRESS:
A.
INCREASED HEART RATE.
B.
DECREASED STROKE VOLUME
AS REDUCED PLASMA VOLUME
DECREASES END-DIASTOLIC
VOLUME.
HYPOHYDRATION AND
EXERCISE
C. NO CHANGE IN CARDIAC OUTPUT.
D. NO CHANGE IN VO2MAX.
E. DECREASED PHYSICAL WORK
CAPACITY DUE PRIMARILY TO
THERMOREGULATORY STRESS.
F. THERMOREGULATORY STRESS =
INCREASED CORE TEMPERATURE AND
HEAT STORAGE AS INTERNAL
CONVECTIVE HEAT TRANSFER AND
SWEAT RATE IS DECREASED (I.E.,
DECREASED HEAT LOSS).
HYPOHYDRATION AND
EXERCISE
2.
HYPOHYDRATION DURING SUBMAXIMAL
EXERCISE WITH THERMAL STRESS.
A.
INCREASED HEART RATE
B.
DECREASED STROKE VOLUME
C.
DECREASED CARDIAC OUTPUT AS
DECREASE IN STROKE VOLUME IS
GREATER THAN INCREASE IN HEART
RATE.
HYPOHYDRATION AND
EXERCISE
D.
E.
DECREASED MAXIMAL OXYGEN
OXYGEN UPTAKE RATE.
DECREASED PHYSICAL WORK
CAPACITY DUE TO
THERMOREGULATORY STRESS
AND CARDIOVASCULAR
STRAIN.
FITNESS LEVEL,
ADAPTATION LEVEL, AND
HYPOHYDRATION
1.
IN A HYDRATED STATE, AN
ADAPTED PERSON WITH A HIGH
FITNESS LEVEL WILL HAVE LESS
BODY HEAT STORAGE AND
BETTER PERFORMANCE THAN
UNADAPTED PERSON WITH LOW
FITNESS LEVEL.
FITNESS LEVEL,
ADAPTATION LEVEL, AND
HYPOHYDRATION
2.
HOWEVER, HYPOHYDRATION MAY
NEGATE THE THERMOREGULATORY
ADVANTAGE OF THE HIGHLY
TRAINED, ADAPTED PERSON,
DESPITE THE FACT THAT THE
HIGHLY FIT PERSON IS CAPABLE OF
TOLERATING HIGHER CORE
TEMPERATURES.
FITNESS LEVEL,
ADAPTATION LEVEL, AND
HYPOHYDRATION
3.
ADDITIONAL RESEARCH IS
NEEDED IN THIS AREA.
HYPERHYDRATION
1. HYPERHYDRATION CAN DELAY THE
DEVELOPMENT OF DEHYDRATION DURING
HEAT STRESS.
2. HOWEVER, EXCESS FLUIDS BY THEMSELVES
MAY NOT PROVIDE A CLEAR EXERCISE
ADVANTAGE.
HYPERHYDRATION
3.
ADDITIONAL RESEARCH IS
NEEDED IN THIS AREA.
GLYCEROL:
PERFORMANCE AID
OR FAD?
HYPERHYDRATION USING
GLYCEROL
• RAPIDLY ABSORBED WHEN INGESTED
ORALLY AND EVENLY DISTRIBUTED
TO ALL FLUID COMPARTMENTS.
• ATTRACTS AND HOLDS WATER LIKE
A SPONGE.
• INCREASES PRE-EXERCISE
HYDRATION LEVELS AS URINE
PRODUCTION IS DECREASED.
HYPERHYDRATION USING
GLYCEROL
• DURING EXERCISE:
•
INCREASED SWEAT RATE AT A LOWER
CORE TEMPERATURE.
•
GREAT SWEAT CAPACITY.
•
LOWER HEART RATE AND
CARDIOVASCULAR STRAIN.
•
IMPROVED ATHLETIC PERFORMANCE IN
HOT, HUMID ENVIRONMENTS.
QUESTIONS?
THE END!
AND THAT WINDS UP MY
PRESENTATION TONIGHT AT SJSU
WHERE THE WOMEN ARE STRONG,
THE MEN ARE GOOD LOOKING,
AND ALL OF THE STUDENTS ARE
ABOVE AVERAGE!!