Biomedical Engineering Reference
In-Depth Information
environment is also affected by outside temperature and wind speed. The latter affects
windchill, which can produce a substantial heat loss from the body. Heat transport also
occurs in blood heating and cooling devices, which are often used during surgical proce-
dures and as part of heart-lung machines. Finally, heat transport is vital for extreme condi-
tions such as a firefighter's suit or a spacesuit, both used to protect the body from extreme
temperatures in the environment.
The heat generated by the body at rest is called the
, equal to 72 kcal/hr.
Obviously, as metabolic activity increases, such as during exercise, the level of heat
production rises.
The equations describing heat transport are similar in form to the equations relating
transport of mass (Fick's Law) and momentum. Such transport is always in the direction
of a decreasing gradient. For mass, the gradient is the concentration difference between
two points or across membranes. For momentum transport of fluids, it is the pressure dif-
ferential causing fluid flow. For heat transfer, the driving gradient is the temperature differ-
ence between two points.
basal metabolic rate
14.3.1 Conduction, Convection, and Radiation
Heat transport can occur by three basic mechanisms. Thermal
is the process
by which heat transfer occurs by molecular interaction. It can occur in gases, liquids, and
solids. An example is heat transfer across a closed window or across a wall. In the human
body, the example of conductive heat transfer is across tissue from the core toward the
periphery (skin). Thermal
conduction
is the process by which heat transfer occurs via bulk
motion of a fluid. An example might be forced air flow from an air conditioner vent or
windchill on a windy day. In the body, examples of convective heat transfer include blood
flow from the core toward the periphery or air flow in the lungs from the alveoli through
the trachea and out the mouth and nose. Both conduction and convection require that there
be a material involved, although convection does not occur in solids, whereas conduction
can. The third method of heat transfer is thermal
convection
, which occurs as a result of elec-
tromagnetic transport processes. An example is heat gain from the sun to the earth (and
your own skin). Radiation is a surface-to-surface phenomenon and does not require a mate-
rial interaction. The greenhouse effect is an example of thermal radiation. As an example,
Figure 14.42 depicts the three types of heat transfer from a campfire.
radiation
14.3.2 Thermal Conduction
Conduction (or heat conduction) is the transfer of thermal energy between neighboring
molecules in a substance due to a temperature gradient. It always takes place from a region
of higher temperature to a region of lower temperature and acts to equalize temperature
differences. Conduction takes place in all forms of matter, including solids, liquids, and
gases, but does not require any bulk motion of matter. In solids, it is due to the combination
of vibrations of the molecules in a lattice and the energy transport by free electrons. In gases
and liquids, conduction is due to the collisions and diffusion of the molecules during their
random motion. Conduction is the movement of heat from a warmer object to a cooler one
when they are in direct contact with one another. This serves to even the temperature
