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Thermoregulation
Humans are subjected to vast changes in environmental temperatures; from greater
than 100F; to less than –22F. Enzymes in the body, chemicals that speed the rate of
other chemical reactions, operate over a very narrow range of temperatures. For
that reason failure to control body temperature can result in physiological changes
& damage. Body temperature < 36o C (97oF) or above 40oC (104oF) may cause
disorientation. Above 42o (108o F) convulsions & permanent cell damage. Not
only does environmental temperature affect the body but heat is also generated
internally through metabolism, biochemical reactions that produce heat as a
byproduct. Heat made in one part of the body is distributed to other parts via blood.
When cells become more metabolically active, that is any time energy use
increases, such as with physical activity or during the absorptive state-after eating,
there is more heat in the body that must be dissipated. To ensure a homeostatic
internal environment for body temperature, the body has anatomical &
physiological mechanisms that keep the body temperature within normal limits
regardless of environment & amount of metabolism. This type of homeostatic
regulation is called thermoregulation.
Normal body temperature depends on when, where and in whom it is measured. It
varies about 1o C in a 24 hour period. Core temperature is the most important body
temperature. This is the temperature of organs in the major cavities-abdominal,
cranial and thoracic. Rectal temperature gives an estimation of core temperature.
Shell temperature is the temperature closer to the surface such as skin and oral
temperatures.
Mechanisms of Heat Transfer
If body temperature is to remain constant, heat must be lost to the environment at
the same rate it is generated. When environmental conditions become too hot or
too cold or if internal metabolism increases the production of heat, the body must
control the gains or losses to maintain homeostasis. Heat exchange with the
environment involves 4 processes: 1) radiation, 2) conduction, 3) convection & 4)
evaporation. Of these four only one is of any real importance when it comes to
thermoregulation and that is evaporation.
During evaporation: water changes from a liquid to a vapor and in so doing
absorbs energy.
Mechanisms of Thermoregulation
Homeostatic regulation of body temperature occurs from two sources 1) sweat
from eccrine glands and 2) blood vessels in the skin.
Thermoregulation is achieved via negative feedback loops. It is a homeostatic
mechanism and therefore has a control center, a receptor and an effector. The
control center is found in the preoptic area of the hypothalamus. This area receives
information from thermoreceptors in the skin. Skin receptors detect a change in
temperaturesends message to preoptic hypothalamus. Heat loss and gain are
coordinated by heat loss and gain centers in the anterior hypothalamus. If the core
temperature is decreasing, receptors detect this and send a message to the control
center (preoptic hypothalamus). The preoptic area sends a message to the heat gain
center to restore body temperature.
Mechanisms for Promoting Heat Gain
When receptors notify the preoptic nucleus that core temperature has fallen below
acceptable levelsheat loss center is inhibited and the heat gain center is
activated. The sympathetic vasomotor center decreases blood flow to the dermis of
the skin (vasoconstriction) reduces heat loss by radiation, convection &
conduction. As skin coolsblood flow is restricted and the skin takes on a bluish
color-cyanosis. Blood returning from the limbs is shunted into deep veins. The
piloerector muscle is stimulatedhair stands on endtraps air near the skin.
If dermal vasoconstriction cannot restore or maintain core temperatureshivering
thermogenesis begins. Shivering is a gradual increase in muscle tone which
increases energy consumption of skeletal muscle throughout the body. If heat gain
center becomes extremely active muscle tone increases to the point at which
stretch receptor stimulation will produce brief, oscillatory contractions of
antagonistic muscles which is seen as a shiver. Shivering increases the work load
of muscles and in so doing elevates O2 & energy consumptionproduces heat
which warms deep vessels to which blood has been shunted by sympathetic
vasomotor center. Shivering can elevate body temperature very effectively. It can
increase the rate of heat generation by 400%.
Non shivering thermogenesis is a more long term mechanism for heat production
that is used in colder seasons. The sympathetic nervous system and thyroid
hormone produce an increase in metabolism. Hormones are released which
increase metabolic activity of all tissues. Heat gain center stimulates adrenal
medulla via the sympathetic ANSepinephrine is releasedincreases rate of
glycogenolysis (break down of glycogen) in liver and skeletal musclemetabolic
rate increases. The preoptic nucleus also regulates the production of TRHthyrotropin releasing hormone by the hypothalamus. TRH increases production of
thyroxin by the thyroid gland. Thyroxin is a key hormone in control of metabolism.
Mechanisms for Promoting Heat Loss
When theperipheral thermoreceptors in the skin note that the core temperature is
increasing, they send a message to the preopotic thermostat which sends signals to
either the heat-losing center in the hypothalamus. When temperature at the
preoptic nucleus becomes greater than the thermostat settingheat loss center is
stimulatedsets off a series of events to produce heat loss. These include: 1)
inhibition of vasomotor center peripheral vasodilation (increased diameter of
blood vessels in the skin)warm blood flows to skin’s surfaceskin looks
reddish. As skin temperatures rise, radiation & convection loses increase. As blood
flow to skin increases2) sweat glands are stimulatedincrease output
perspiration flows-evaporative loss increasesskin coolsbody temperature
decreasesperspiration ceasesblood flow to skin decreases. 3) respiratory
centers are stimulateddepth of respiration increases. Often one begins breathing
through the mouth increasing evaporation loss through the lungs.