Regulation of Body Temperature
Rodbard[41]
suggested that the
neural mechanisms governing homeostatic temperature control developed in the hypothalamus
from circulatory control neurons in the course of evolution from reptile to mammal
(see Chapter 40
). The heat-sensitive
center consists of the heat-loss center in the anterior hypothalamus and the heat-producing
center in the posterior hypothalamus.[42]
[43]
[44]
Body temperature is regulated when changes
in blood temperature stimulate heat-sensitive receptors in the hypothalamus. Nerve
impulses from cold receptors in the skin can also activate heat-producing neurons.
The most important heat producers are the skeletal muscles, brain, liver, and heart.
The greatest heat radiator is the skin, especially that on the hands. Local electrical
stimulation of the heat-producing center induces shivering and constriction of blood
vessels in the skin, activating vasomotor nerves and decreasing blood flow. Warming
of the heat-loss center suppresses vasomotor nerve activity and in this way increases
blood flow through the skin.
In brain death, the neural connection between the temperature-regulating
center and peripheral body tissues is lost, and the patient becomes poikilothermic.
When criteria from the National Institute of Neurological Diseases
and Stroke (NINDS) were applied to establish brain death, such patients showed only
a "tendency for the temperatures to be subnormal."[17]
In contrast, when brain death was established using the criterion of cessation of
all brainstem functions, "poikilothermia was found in all patients later than 24
hours after brain death."[45]
Even if infection
occurs, fever should not develop in cases of brain death, because the temperature-regulating
centers no longer function. After brain death, body temperature tends to be hypothermic,
even with vigorous application of external heat.
