1. CONVECTION
Heat transfer - dependent on movement - fluid or gas
Determinants:
1. air or fluid velocity
2. size
3. shape
4. medium properties
a. density
b. viscosity
c. thermal conductivity

2. NATURAL CONVECTION - motion due to change - density
– air contact with skin (e.g. warms - becomes less
dense - rises - replaced by cooler, denser air
Natural convection - human - with skin temperature - 10C
above air temperature
may be equal to 30 - 40 W/m2 - which is a significant
portion of Resting Metabolism
Definite advantage to having fur or plumage that traps layer
- air close to skin - resisting convection

3. FORCED CONVECTION - transfer due to motion from external
energy source
Layer of air around skin - thickness depends on wind speed
Little movement of this layer –
Even at high wind velocities - thin layer stuck to skin in still air -
and up to 0.2 m/sec wind velocity = natural convection
Above this point = forced convection predominates
Simple movement - arms, legs, head - do not affect forced
convection - and natural convection predominates
BUT - walking or running - forced convection = major role

4. Non-linear response or heat loss to increased wind velocity.
At low velocities - relatively rapid increase - heat loss with
increase velocity
Increase becomes less at higher velocities (60 - 80 km/hr)
(40-50 miles/hr)
At extreme velocities - *** 620 miles/hr *** - heat may be
generated and reverse heat flow.

5. CIRCULATION EFFECTS - Convective Heat Loss
Heat loss & gain with environment - primarily at level of
Capillaries
Blood flow changes with Ta involve mainly changes in resistance
vessels - but larger capacitance vessels may also contribute.

6. Vasodilation - skin >> decreases kidney blood flow (as much as
50% during exercise in heat)
Splanchnic blood flow also reduced in heat
Little change - muscle blood flow – heat
Decreased total peripheral resistance may >> decreased BP -
even with compensatory change.