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Chapter 1 7 Environmental Physiology. Section I: Exercise When Exposed to Altered Pressure. Exercise at Increased Altitude.

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Chapter 17

Environmental Physiology

Section I: Exercise When Exposed to Altered Pressure

Exercise at Increased Altitude

As altitude increases, there is a decrease in pressure. This reduced pressure causes air molecules to be more dispersed. Thus, for a given air volume, even though the relative presence (gas fraction) of a gas remains the same, there is less of a given gas.

For example,

Sea level

PO2 in air = 760 mmHg x 0.2093 = 159 mmHg

At Pikes Peak, CO (4,000 m or 14,300 ft)

PO2 in air = 430 mmHg x 0.2093 = 90 mmHg

Altitude (ft)

Altitude (m)

meters feetPressure (Torr)PIO2 (Torr) PAO2 (Torr)SaO2 (%)

1 Torr = 1 mmHg ; 1 m = 3.28 ft ; PB = 760 [e-(m/7924)] ; PIO2 = (PB - 47) x 0.2093 ;

PAO2 ~ (PB - 47) x 0.146 ; SaO2 approximated from %O2-Hb dissociation curve

At rest

During steady state exercise

At VO2max



Blood lactate


Plasma volume

Blood volume


Stroke volume

Heart rate

Cardiac output


Percent change (%)

Percent change (%)

Decreases in VO2max During Acute Hypoxic Exposure

-9.2%/1,000 m > 1,500 m

1,500 m ~ 5,000 ft

sea level

3,000 m

4,000 m

Is this difference a beneficial or detrimental adaptation?



Does living at altitude improve exercise tolerance at altitude?

  • Yes,

  • VO2max decrement is not as large

  • Ventilation is higher

  • Maximal blood lactate is lower

  • AMS symptoms are less severe

Does training at altitude improve exercise tolerance at sea level? ???

This question has not adequately been answered. However, there are recent findings that may indicate a benefit of sleeping at altitude (> 7,000 ft) and training at low-moderate altitude (< 7,000 ft).

Exercise During Hyperbaria altitude?

Hyperbaria refers to exposure to increased pressure above 1 atmosphere (atm = 760 mmHg).

For example;

When submerged in sea water, pressure increases by 1 atm every 10 m. In fresh water, the pressure change is not as great and approximates 10.4 m.

  • Physiological changes associated with hyperbaria include,

    • cutaneous blood flow

    • central blood volume and venous return

    •  heart rate

    • diuresis

    • VO2

O altitude?2




Section II: Exercise And Thermal Stress altitude?

Dehydration - decrease in total body water. Occurs at a faster rate during exercise in hot and/or humid environments

for example,

sweat rates can to 2-3 L/Hr

Deleterious effects of dehydration on exercise occur with as little as fluid loss equal to 2% body weight.

For a 70 kg male; 70 x 0.02 = 1.4 kg ~ 1.4 L

This could occur with as little as 30 min of exercise!!!!

Hyperthermia - increased body temperature resulting from body heat storage

Physiological changes during dehydration altitude?

  • Core temperature

  • Plasma volume

  • Venous return

  • Stroke volume

  • Heart rate

  • Cardiac output

  • a-vO2

  • Skin blood flow

  • Catecholamines

  •  Blood lactate

  •  VO2

  • CNS dysfunction

  • Exercise tolerance

  • Sweat rate

  • Evaporative cooling

Improving Exercise Tolerance During Heat Exposure altitude?

  • Fluid intake (pre-during and post-exercise)

  • Do not rely on thirst mechanism

  • Complete heat acclimation or acclimatization

Acclimation - chronic adaptations induced by exposure to artificial environmental conditions

(eg. environmental chambers, sauna, exercise)

Acclimatization - chronic adaptations induced by exposure to foreign a foreign climat

(eg. geographical relocation)

Chronic adaptations to exercise and exercise in a hot environment that improve acclimation to exercise in the heat

Heat Illness, Heat Exhaustion and Heat Stroke environment that improve acclimation to exercise in the heat

These conditions are more severe clinical manifestations of heat exposure.

Heat Exhaustion - the decreased cardiovascular function that accompanies dehydration and mild hyperthermia

Heat Stroke - when heat stress continues, or is worsened beyond that of heat exhaustion (core temp > 39.5 C), physiological symptoms progress to CNS dysfunction - disorientation, confusion, psychoses

Heat exhaustion and heat stroke are both heat illnesses. However, heat stroke can be potentially lethal due potential organ damage and failure.

Evaluating Environmental Conditions For Risk of environment that improve acclimation to exercise in the heat

Heat Injury

An index has been developed that incorporates all contributors to thermal heat stress - Wet Bulb Globe Index (WBGI)

Dry bulb temperature - measure of air temperature

Black bulb temperature - measure of the potential for radiative heat gain

Wet bulb temperature - measure of the potential for evaporative cooling

WBGI = (0.7 x Tw) + (0.2 x Tb) + (0.1 x Td)

Cold Exposure the WBGI

Section III: Human Function and Performance During Gravitational Challenge


The early space programs of the USA and Russia revealed that prolonged exposure to microgravity was detrimental to human physiology.

The earth has a reference gravitational force = 1 g

The moon’s gravitational force = 0.17g

Outside a planetary orbit (eg. Space Shuttle), gravitatonal force = 0g

Research models that have been used to mimic physiological responses to microgravity are,

head-down bed rest lower body negative pressure tilt testing

Physiological Effects of Exposure to Microgravity Gravitational Challenge

  • Muscle atrophy

  • Bone mineral

  • Muscle enzymes

  • Blood volume

  •  Diuresis

  •  Heart rate

  • Stroke volume

  • Compromised regulation of peripheral blood vessels and vascular resistance