Page 217 - Industrial Ventilation Design Guidebook

I 78 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

FIGURE 5.1 Simple representation of physiological temperature regulation in man.

compartments are characterized as having relatively uniform temperatures
throughout. The bigger compartment (85 to 95% of body weight) represents
the body's core and contains all of the muscles and other significant heat- and
energy-generating tissue. Blood profusion of the muscles and internal organs
distributes the heat fairly well so the core can be represented as having an ap-
proximately uniform temperature (T c). The smaller compartment represents
the skin with uniform temperature T sk. The temperature uniformity of this
simple lumped parameter model is reasonable for people at sedentary to me-
dium activities (0.7-5 met) in conditions where healthy people feel slightly
cool to very hot.
Essentially all the energy produced in the body by the various metabolic
activities is generated in the core. The skin functions as a protective and heat
transfer surface for the core. As such, the skin, which is about 1.6 mm thick
on average, has tissue with very small oxygen needs and heat-producing capa-
bilities. The energy (M) produced by the core includes the extra heat generated
by muscles in tensioning and shivering (Table 5.3) under active control for
therrnoregulation. In humans, shivering thermogenesis potentials are small
with a maximum incremental heat increase capability of about 1 met.
The metabolic energy generated by the core (M) is lost by (1) doing work,
(2) respiration, (3) passive heat conduction to the skin, and (4) active blood
flow to the skin. Any heat not transferred from the core is stored, with a re-
sulting increase in core temperature. Work is energy that leaves the body as in

CABLE 5.3 Active Physiological Controls: Shivering, Sweating
md Skin Blood Flow