Page 237 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Thermoregulation When the core temperature falls below set
point, the body checks heat loss by constrict-
Thermoregulation maintains the core ing the blood vessels in the shell (! A, left) and
temperature (! A) at a constant set point increases heat production by generating vol-
(! 37!C) despite fluctuations in heat absorp- untary and involuntary (shivering) muscle ac-
tion, production, and loss (! p. 222). The core tivity (! D). Although infants can quickly be-
Thermal Balance and Thermoregulation set-point fluctuations happen during the men- ambient temperatures, these three mecha-
temperature exhibits circadian variation. It
come hypothermic because of their high sur-
face/volume ratio, their brown fat allows them
fluctuates by about 0.6 !C and is lowest around
to produce additional heat (non-shivering ther-
3 a.m., and highest around 6 p.m. (! p. 381 C).
mogenesis; ! p. 222). Upon exposure to low
The set point changes are controlled by an in-
trinsic biological clock (! p. 334). Extended
nisms are activated by the cold receptors of the
skin (! p. 314) before the core temperature
strual cycle (! p. 299/A3) and fever.
The control center for body temperature and
falls.
The range of ambient temperatures be-
central thermosensors are located in the hy-
pothalamus (! p. 330). Additional thermosen-
tween the sweating and shivering thresholds
sors are located in the spinal cord and skin
is known as the thermoneutral zone. It lies be-
tween ca. 27 !C and 32 !C in the nearly un-
(! p. 314). The control center compares the ac-
tual core temperature with the set-point value
tory measure necessary within this range is
viations (! D and p. 4f.).
variation of blood flow to the skin. The narrow
range of this zone shows the thermoregulatory
When the core temperature rises above the
9 and initiates measures to counteract any de- clothed test subject. The only thermoregula-
set point (e.g., during exercise), the body in-
importance of behavior. It involves choosing
creases the internal heat flow (! p. 222) by di- the appropriate clothing, seeking shade, heat-
lating the blood vessels of the skin. Moreover, ing or cooling our dwellings, etc. Behavioral
arteriovenous anastomoses open in the pe- adaptation is the chief factor in survival at ex-
riphery, especially in the fingers. The blood treme ambient temperatures (! C).
volume transported per unit time then not The thermoneutral zone is subjectively per-
only conveys more heat, but also reduces the ceived as the comfort zone. 95% of all subjects
countercurrent exchange of heat between the wearing normal office attire and performing
arteries and their accompanying veins (! B). normal office activities perceive an indoor cli-
In addition, venous return in the extremities is mate with the following conditions to be com-
re-routed from the deep, accompanying veins fortable: ambient and radiant (wall) tempera-
to the superficial veins. Sweat secretion also ture ! 23 !C, wind velocity " 0.1 m/s, and rela-
increases. The evaporation of sweat cools the tive humidity ! 50%. A resting, unclothed sub-
skin, thereby creating the core/skin tempera- ject feels comfortable at about 28 !C and ca.
ture gradient needed for the internal heat flow. 31 !C to 36 !C in water depending on the thick-
Central warm sensors emit the signals that acti- ness of subcutaneous fat (heat isolator).
vate the sweat glands. (In this case, the ther-
mosensors of the skin do not detect warmth Fever. Exogenous (e.g., bacteria) and endogenous py-
rogens (various interleukins and other cytokines from
because their environment is cooler than the macrophages) can cause the set-point temperature
core temperature). The efferent nerve fibers to to rise above normal. This is triggered by prostaglan-
the sweat glands are cholinergic fibers of the din PGE 2 in the hypothalamus. In the initial phase of
sympathetic nervous system (! D). fever, the core temperature (although at its normal
level) is too low compared to the elevated set-point.
Acclimatization to high environmental tempera- This results in shivering to raise the core tempera-
tures (e.g., in the tropics) is a slow process that often ture. As the fever decreases, i.e. the set-point returns
takes years. Characteristically, the sweat secretion toward the normal temperature, the core tempera-
rate rises, the salt content of the sweat decreases, ture is now too warm compared to the normalized
and thirst and thus H 2O intake increase. set-point, resulting in vasodilatation and sweating to
lower the core temperature again.
224
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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