Page 57 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

34 BIOMECHANICS OF THE HUMAN BODY

1. Mass transfer in support of body metabolisms. Blood transports oxygen to the rest of the body
and transports carbon dioxide and other waste from the cells.
2. Regulation of systemic blood pressure. The vascular system is a primary effector in the regula-
tion of systemic blood pressure through its ability to alter the distribution of blood flow and reg-
ulate the cardiac output and thereby buffer systemic pressure fluctuations.
3. Heat transfer for systemic thermoregulation.

As for the third primary function, blood is known to have a dual influence on the thermal
energy balance. First it can be a heat source or sink, depending on the local tissue temperature.
During wintertime, blood is transported from the heart to warm the rest of the body. On the other
hand, during hyperthermia treatment for certain diseases where the tissue temperature is elevated
to as high as 45°C by external devices, the relatively cold blood forms cold tracks that can
decrease the treatment efficacy. The second influence of the blood flow is that it can enhance heat
dissipation from the inside of the body to the environment to maintain a normal body tempera-
ture. Theoretical study has shown that if the heat produced in the central areas of the body at rest
condition could escape only by tissue conduction, the body temperature would not reach a steady
state until it was about 80°C. A lethal temperature would be reached in only 3 hours. During exer-
cise, our body temperature would have typically risen 12°C in 1 hour if no heat were lost by
blood flow. Maintaining a core temperature of 37°C during thermal stress or exercise in the body
is achieved by increasing the cardiac output by central and local thermoregulation, redistributing
heat via the blood flow from the muscle tissue to the skin, and speeding the heat loss to the envi-
ronment by evaporation of sweat.
Thermal interaction between blood and tissue can be studied either experimentally or theoreti-
cally. However, for the following reasons it is difficult to evaluate heat transfer in a biological
system:
• The complexity of the vasculature. It is not practical to develop a comprehensive model that
includes the effect of all thermally significant vessels in a tissue. Therefore, the most unusual and
difficult basic problem of estimating heat transfer in living biologic systems is modeling the
effect of blood circulation.
• Temperature response of the vasculature to external and internal effects is also a complex task. In
a living system, the blood flow rate and the vessel size may change as a response to local tem-
perature, local pH value, and the concentration of local O and CO levels.
2 2
• The small thermal length scale involved in the microvasculature. Thermally significant blood ves-
sels are generally in a thermal scale of less than 300 μm. It has been difficult to build temperature-
measuring devices with sufficient resolution to measure temperature fluctuation.
For the above reasons, even if the heat transfer function of the vascular system has been appreciated
since the mid-nineteenth century, only in the past two decades, has there been a revolution in our
understanding of how temperature is controlled at the local level, both in how local microvascular
blood flow controls the local temperature field and how the local tissue temperature regulates local
blood flow.
Until 1980, it was believed that, like gaseous transport, heat transfer took place in the capillaries
because of their large exchange surface area. Several theoretical and experimental studies (Chato,
1980; Chen and Holmes, 1980; Weinbaum et al., 1984; Lemons et al., 1987) have been performed
to illustrate how individual vessels participate in local heat transfer, and thus to understand where
the actual heat transfer between blood and tissue occurs. In these analyses, the concept of thermal
equilibration length was introduced. Thermal equilibration length of an individual blood vessel was
defined as a distance over which the temperature difference between blood and tissue drops a cer-
tain percentage. For example, if the axial variation of the tissue and blood temperature difference can
be expressed as ΔT = ΔT e −x/L , where ΔT is the temperature difference at the vessel entrance, and L
0
0
and 4.6L are the thermal equilibration lengths over which ΔT decreases to 37 percent and 1 percent,
respectively, of its value at the entrance. Blood vessels whose thermal equilibration length is com-
parable to their physical length are considered thermally significant.