NUTRITION IN SPORT

rates or drips off the skin. Effective evaporation
is also prevented if the vapour pressure gradient
between the skin and the environment is low.
This latter situation will arise if the skin tempera-
ture is low or if the ambient water vapour pres-
sure is high: clothing that restricts air flow will
allow the air close to the skin to become saturated
with water vapour and will therefore restrict the
evaporation of water from the skin surface. A
large body surface area and a high rate of air
movement over the body surface are also factors

that will have a major impact on evaporative heat

loss, but these same factors may be a disadvan-

tage in that they will promote heat gain from the

environment by radiation and convection when

the ambient temperature is higher than skin tem-

perature (Leithead & Lind 1964). Smaller indi-

viduals will have a high surface area relative to

their body mass, and may be at an advantage in

hot conditions, but this will depend on the rela-

tive rates of evaporative heat loss and heat gain

by physical transfer.

thermoregulation and fluid balance 205

Air temperature

Air humidity

Sky

thermal

radiation

Solar

radiation

Reflected

solar

radiation

Ground

thermal

radiation

Running speed

Contracting

muscle

Metabolic

storage

Skin blood flow

Convection

Radiation

Work

Conduction

Convection

Respiratory

Sweat

Muscle blood flow

Convection

Fig. 15.1Main avenues of heat gain or heat loss in the exercising individual.