Biomedical Engineering Reference
In-Depth Information
if the dog were to get overheated, the solution is to hose down the dog, which would
reduce the trapped air insulation via the fur. The same effect occurs when skiing in a cold
environment. A ski parka has trapped air that serves as an insulator. However, if the parka
were to become wet, the trapped air would be mixed with water, thus reducing its effective-
ness as an insulator.
14.3.8 Heat Loss inside the Body
Many of the mechanisms for heat transfer that have been discussed refer to heat loss
from the body to the environment. However, heat must be transferred from the body core
to the skin or lungs. The two mechanisms available for such transfer are via conduction
and via convection. Conduction is a slow process. Convection is accomplished via blood
flow. As blood reaches the body core, it absorbs heat. Blood then is channeled toward the
periphery, where heat is released. Ruch and Patton (1965) discuss the mechanisms by which
the blood circulation affects internal heat distribution in three ways:
1.
It minimizes temperature differences within the body. Tissues having high metabolic
rates (e.g., the liver) are more highly perfused and are thus kept at nearly the same
temperature as less active tissues. Cooler tissues are warmed by blood coming from the
active organs.
2.
It controls the effective body insulation in the skin region. Warm blood is increased to the
skin via vasodilation, when the body wishes to reject heat. Blood is bypassed from
arteries to veins via deeper channels through vasoconstriction when conservation of
body heat is vital.
3.
Countercurrent heat exchange between major arteries and veins often occurs to a
significant extent. If heat conservation is necessary, arterial blood flowing along the
body's extremities is precooled by loss of heat to adjacent venous streams.
A model of heat transfer from the core toward the skin can be split into two sections:
from the core to a muscle region and from the muscle region to the skin (Cooney, 1976).
The heat transfer includes both a conductive term using Fourier's law and a convective
term using a forced convective form as follows:
k
cm
A
ð
T
c
T
m
Þ
Q
¼
Z
cm
þð
dm
b
=
dt
Þ
C
pB
ð
T
c
T
m
Þ
D
conduction
convection
k
ms
A
ð
T
m
T
s
Þ
Q
¼
Z
ms
þð
dm
b
=
dt
Þ
C
pB
ð
T
m
T
s
Þ
D
Since the heat transfer rate is assumed to be the same for both zones and the convection
term is likely far greater than the conduction term, then an overall heat transfer rate from
core to skin is of the form
T
s
Þ
This form does not account for any shunting of blood to allow for heat gain or loss via
vasoconstriction or vasodilation.
Q
¼ð
dm
b
=
dt
Þ
C
pB
ð
T
c
