Physofex final better hydration

Radiation (heat transfer)

Transfer of heat via electromagnetic waves without direct contact; occurs between objects of different temperatures

Requirement for radiation

A temperature gradient between the body and surrounding surfaces must exist

Conduction (heat transfer)

Direct transfer of heat between objects in physical contact

Requirement for conduction

Direct physical contact between objects of different temperatures

Convection (heat transfer)

Transfer of heat via movement of air or fluid across the skin

Difference between air vs water convection

Water removes heat much faster than air because it has a higher heat capacity and density

Evaporation (heat loss)

Heat loss through the conversion of sweat from liquid to vapor

Requirement for evaporation

A water vapor pressure gradient between the skin and environment must exist

Normal core temperature

~37°C (98.6°F)

Dangerously high core temperature

40–41°C (104–106°F)

Core temperature

Temperature of vital organs (brain, heart, lungs)

Shell temperature

Temperature of skin and peripheral tissues

Core-to-shell gradient

The temperature difference between core and skin that allows heat dissipation

Why the core-to-shell gradient is important

Without a gradient, heat cannot move from the core to the skin for dissipation

Process of evaporative cooling

Sweat absorbs heat from the skin as it changes from liquid to vapor, lowering body temperature

Role of water vapor pressure in evaporation

Evaporation depends on the gradient between water vapor pressure at the skin and in ambient air

Environmental factor most limiting evaporation

High relative humidity

Why high humidity impairs cooling

Reduces the vapor pressure gradient, limiting sweat evaporation

Strategies to reduce heat illness risk

| Gradual heat exposure (acclimation), proper hydration, lighter clothing, adjusting intensity, exercising in cooler parts of the day

Role of heat acclimation in prevention

Improves sweating efficiency and cardiovascular stability

Heat cramps

Painful muscle spasms due to fluid and electrolyte imbalance

Heat exhaustion

Inability to continue exercise due to cardiovascular strain and dehydration

Heat stroke

ife-threatening condition characterized by core temperature >40°C and CNS dysfunction

Most dangerous heat illness

Heat stroke

Dry bulb temperature (Tdb)

Ambient air temperature measured in the shade

Black globe temperature (Tg)

Measures radiant heat from direct sunlight

Wet bulb temperature (Twb)

Reflects evaporative cooling potential and humidity

Wet Bulb Globe Temperature (WBGT)

Composite index of environmental heat stress combining Tdb, Twb, and Tg

WBGT formula

WBGT = 0.1Tdb + 0.7Twb + 0.2Tg

Primary source of heat during exercise

Metabolic heat production from voluntary muscle contraction

sources of heat gain

Shivering and hormone-induced increases in metabolic rate (non-shivering thermogenesis)

Role of the hypothalamus

Acts as the body’s thermostat, regulating heat gain and heat loss

Hypothalamus response to heat gain

Anterior hypothalamus increases sweating and skin blood flow

Hypothalamus response to heat loss

Posterior hypothalamus increases heat production and reduces skin blood flow

Effect of heat stress on performance

Increases cardiovascular strain, reduces VO₂max, and accelerates fatigue

Why performance declines in the heat

Competition between muscles and skin for blood flow and impaired thermoregulation

Heat acclimation

Physiological adaptations to heat in controlled environments

Heat acclimatization

Physiological adaptations to heat in natural environments

Key heat acclimation adaptations

Increased plasma volume, earlier onset of sweating, increased sweat rate, reduced heart rate

How acclimation improves performance

Enhances heat dissipation and cardiovascular stability

Total body water percentage

50–70% of body mass

Euhydration

Normal body water content within homeostatic range

Hypohydration

State of body water deficit

Dehydration

The process of losing body water

Hyperhydration

State of excessive body water

Trigger for renin release

Low blood volume or low blood pressure

Source of renin

Kidneys

Overall function of RAAS

Conserves sodium and water to restore blood volume and pressure

Hyponatremia definition

Dilution of plasma sodium concentration <135 mmol/L

When hyponatremia most likely occurs

Prolonged exercise with excessive intake of low-sodium fluids

Risk factors for hyponatremia

Long duration exercise (>4 h), excessive drinking, small body size, high sweat sodium losses, females

Symptoms of hyponatremia

Headache, nausea, confusion, swelling, seizures in severe cases

Urine color method

Urine color 1–3 indicates hydration; ≥5 suggests hypohydration

Body weight method

1 kg body weight loss ≈ 1 L sweat loss

Hydration cutoffs via body weight

>2% body mass loss = hypohydration

Urine specific gravity (USG)

Values ≥1.020 indicate dehydration

Planned drinking

Scheduled fluid intake based on sweat rate and conditions; best for long, intense exercise in heat

Drinking to thirst

Adequate for shorter, lower-intensity exercise in cooler conditions

Pre-exercise hydration goal

Begin exercise euhydrated

Dehydration threshold to avoid

>2% body mass loss

Post-exercise rehydration guideline

Consume 1.25–1.5 L fluid per 1 kg body weight lost

Sodium recommendation

Include sodium when exercise lasts >2 h in heat or sweat sodium losses are high