29-30) *** Environmental Influences *** Flashcards
Body temperature Regulation:
Physiological body temperature is regulated to keep it nearly constant even when environmental temperature changes
* Only 25% of the of the energy the body produces (ATP) is used for physiological functions such as muscle contraction; the rest is ?
* All active tissues produce ? that must be precisely offset by heat loss to the environment to maintain the internal temperature of the body
* Must balance ? (metabolic heat, heat from environment) with ? (radiation, conduction, evaporation, convection)
Physiological body temperature is regulated to keep it nearly constant even when environmental temperature changes
* Only 25% of the of the energy the body produces (ATP) is used for physiological functions such as muscle contraction; the rest is lost as heat
* All active tissues produce metabolic heat (M) that must be precisely offset by heat loss to the environment to maintain the internal temperature of the body
* Must balance heat gain (metabolic heat, heat from environment) with heat loss (radiation, conduction, evaporation, convection)
Thermoregulatory Control
What brain region is responsible for maintaining body temperature?
Pre-optic-anterior hypothalamus (POAH)
Thermoreceptors detect changes in temperature and relay information to the POAH
* Hypothalamus activates mechanisms to heat/cool body
Four Effectors activated by POAH
Preoptic anterior hypothalamus
(1) Skin Arterioles
- Sense change in skin/core temp → POAH activates SNS → Smooth mm in arterioles to skin dilates/constricts to alter BF
(2) Exocrine sweat glands
- Increase in skin/core temp → POAH activates SNA → Eccrine Sweat Glands activated → Sweat released onto surface of skin → evaporates and cools
(3) Skeletal MM
- When need to generate more body heat
- Thermoreceptors in skin/core sense cold → POAH activates brain centers controlling mm tone → Shivering activated
(4) Endocrine Glands
- Hormones that increase metabolic rate: Thyroxine, NE/E (enhance SNS respones)
What two Cardiovascular changes occur in response to exercise in Heat and Dual demand (Skin and sk mm both need high bf)
2 changes to meet increased demand:
(1) CO increases above the increase that would be seen with a similar exercise intensity in cool conditions by increasing heart rate and contractility
Elevated core temperature →
(a) POAH sends signals to arterioles (SNS) to cause blood vessels to vasodilate to deliver heat to body surface;
(b) POAH sends signals to heart (SNS) to increase HR and contractility;
BUT ability to increase SV limited as blood is pooling in periphery (skin) and less is returning to heart: Cardiovascular drift
(2) Blood flow shunted away from nonessential areas to the skin
- SNS signals to kidneys, liver, intestines -> vasoconstriction -> allows available CO to reach skin without compromising Skeletal mm bloodflow
Physiological responses to exercise in Heat:
What is Heat Stress?
Heat stress → any environmental condition that increases body temperature and jeopardizes homeostasis
CV System:
Dual demand: In hot environment cardiovascular system must transport blood to working muscles and to skin
2 changes to meet increased demand:
(1) CO increases above the increase that would be seen with a similar exercise intensity in cool conditions by increasing heart rate and contractility
Elevated core temperature →
(a) POAH sends signals to arterioles (SNS) to cause blood vessels to vasodilate to deliver heat to body surface;
(b) POAH sends signals to heart (SNS) to increase HR and contractility;
BUT ability to increase SV limited as blood is pooling in periphery (skin) and less is returning to heart: Cardiovascular drift
(2) Blood flow shunted away from nonessential areas to the skin
- SNS signals to kidneys, liver, intestines -> vasoconstriction -> allows available CO to reach skin without compromising Skeletal mm bloodflow
- In response to prolonged submaximal activity (aerobic/aerobic exercise in hot environment)
- Describes the time-dependent downward shift in several cardiovascular responses
- SV ↓ – rise in core temperature redistributes blood to the periphery for cooling; progressive decrease in plasma volume decreases central venous cardiac filling pressure, reducing SV
- Concomitant heart rate ↑
Other factors:
* CO remains constant – maintain O2
* Arterial blood pressure ↓
What is Cardiovascular Drift in response to prolonged submax activity in HOT env’t
Cardiovascular Drift
* Describes the time-dependent downward shift in several cardiovascular responses
* SV ↓ – rise in core temperature redistributes blood to the periphery for cooling; progressive decrease in plasma volume decreases central venous cardiac filling pressure, reducing SV
* Concomitant heart rate ↑ (Exercise tachycardia)
Other factors:
* CO remains constant – maintain O2
* Arterial blood pressure ↓
- ↓SV: (more blood going to skin decreases preload which decreases SV)
- HR↑ to compensate for ↓SV
Decreased filling pressure → ↓VR → ↓EDV → ↓SV
What limits exercise in heat?
Exercise in heat becomes limited when HR approaches maximum
With increasing temperature:
* SV decreases
* HR increases (will reach max)
* CO increases and then plateaus
* At lower temperatures CO increases
Any factor that overloads the cardiovascular system or interferes with heat dissipation can impair performance, increase risk of overheating or both
Critical Temperature Theory:
Critical temperature theory → the brain will send signals to stop exercise when some critical brain temperature is reached (40 – 41 °C); especially in well-trained, acclimated athletes
Sweating involves which glands? Stimulated by?
Eccrine sweat glands (Thermoregulatory):
* Elevated body temperature → POAH stimulates SNS of eccrine sweat glands found on body’s surface
How is sweat changed by training?
Affected by training: (Fluid balance)
* With training and repeated heat exposure more sodium is reabsorbed and sweat is more dilute due to increased sensitivity to aldosterone
* No mechanism to conserve other electrolytes
Health risks during exercise in heat:
(1) Heat Cramps
- severe cramping in lrg mm
- due to sodium loss and dehydration (high sweating)
- Prevent/minimize with proper hydration and increasing salt intake
(2) Heat Exhaustion
- Effects beyond mm
- CV system unable to meet body demands as it is severely dehydrated
- Can’t get enough blood to both sk mm and skin
- Due to excess fluid loss from sweating that decreases BV
- Heat cannot be dissipated quickly enough (insufficient BV)
(3) Heatstroke
- Life threatening due to failure of body’s thermoregulatory mechanisms
- Increase in internal body temperature exceeding 40 °C
* Confusion, disorientation or unconsciousness
* Coma and death if core temperature continues to rise
- Sweating may cease
Acclimation to Heat exercise
Heat acclimation → changes in plasma volume, cardiovascular function, sweating and skin blood flow that allow for subsequent exercise bouts in the heat to be performed with a lower core temperature and heart rate response
* Changes occur over short periods of time (days/weeks)
Studies have shown that repeated exercise in the heat causes a series of relatively rapid adaptations that enable us to perform better and more safely in hot conditions
Effects of Heat Acclimation
With acclimation:
* Heat loss ability of the body is enhanced at a given rate of work
In response to a submaximal exercise bout:
* Core temperature increases less than before
acclimation
* HR increases less
* More work can be done before a maximal tolerable temperature or HR is reached
- Athletes must exercise in heat to attain acclimation (sauna/steam room do not achieve same effects)
- Trained athlete acclimates faster than untrained subject (training even in cooler environment already increases heat tolerance)
COLD Stress: (4) MECHANISMS to avoid heat loss
(1) Peripheral Vasoconstriction
- SNS → contraction of smooth muscle of arterioles to skin → reduce blood flow to minimize heat loss
- Tonic Skin Vasoconstriction even at thermoneutral temp (Tone continuosly modified)
(2) Non-shivering Thermogenesis:
* A cold-induced increase in heat production not associated with the muscle activity of shivering; increase in metabolic rate primarily in brown adipose tissue
* Controlled SNS
(3) Shivering
(4) Behavioural responses
3 Patterns of adaptation to repeated cold exposure:
(1) COLD HABITUATION
- People repeatedly exposed to cold environment where significant body heat loss does not occur
* Cutaneous cooling (hands/feet), but no decline in core temperature, provoked either by brief or mild whole-body or localized cold exposures
* Skin vasoconstrictor and shivering responses are blunted
(2) METABOLIC ACCLIMATION
Occurs when increases in metabolic heat production can minimize heat loss
* Non-shivering and shivering thermogenesis will generate heat
(3) INSULATIVE ACCLIMATION
* Occurs when increased metabolism cannot maintain core temperature
* Enhanced skin vasoconstriction occurs to increase peripheral insulation and
minimize heat loss
