Download effect of different environment

Thermoregulation
Thermoregulation is the balance between heat production
mechanisms and heat loss mechanisms that occur to maintain
a constant body temperature.
heat production mechanisms and heat loss mechanisms that
occur to maintain a constant body temperature.
Heat flows from higher temperature to lower temperature.
Conduction is the transfer of heat between objects that are in
direct contact with each other. For instance, if a person sits on
the cold ground, heat moves from the body to the cold ground.
Convection is the transfer of heat by the movement of air or
liquid moving past the body. This explains why a breeze
across the skin may cool one down, whereas trapping air
inside clothing keeps the body warm.
When the body is too cold, it increases heat production and
decreases heat loss. Vasoconstriction, the constriction of the
vessels of the skin, helps prevent heat loss. Shivering, which is
a rhythmic contraction of skeletal muscles, produces heat.
Heat can also be produced by nonshivering thermogenesis, an
increase in metabolic heat production.
When the body is too hot, it decreases heat production and
increases heat loss. One way of increasing heat loss is through
peripheral vasodilation, the dilation of blood vessels in the
skin. When these vessels dilate, large quantities of warmed
blood from the core of the body are carried to the skin, where
heat loss may occur via radiation, convection, and
conduction. Evaporation of fluids from the body also causes
heat loss. Humans constantly lose fluids from the skin and in
exhaled air. The unconscious loss of fluid is called insensible
perspiration.
How the body loses heat
Physiologically, heat is generated in the muscles by metabolic
chemical reactions, mainly in the liver. Some heat is lost
through the lungs, although 90-95% is lost through the skin.
Heat is transferred from the core to the skin by blood passing
through peripheral blood vessels.
The rate of heat loss is determined by the extent to which the
peripheral blood vessels dilate; fully dilated they will allow
blood to travel 100 times faster than when constricted, thus
losing body heat faster. Heat loss rates are also greatly
increased by sweating, especially in dry environments.
How the body controls heat loss
The body controls heat loss by tightening the blood vessels
under the skin, restricting the flow of blood - to the peripheral
blood vessels ('Vasoconstriction'). The development of
peripheral vasoconstriction allows a cooler, outer 'shell' to
form an insulating barrier that slows heat loss from the body's
core. Hands and feet have fewer large blood vessels, and when
the flow of blood is restricted it is harder for the blood to keep
flowing to these areas which quickly become cold.
How heat transfers from the skin to the surrounding
environment
Heat loss is due to one or more of the following - convection,
conduction, evaporation or radiation. In comfortable
environments, about 65% is lost through radiation, with most
of the rest through evaporation. In cold environment, most
heat lost is via convection and conduction.
Convection happens when air or water with a lower
temperature than the body comes into contact with the skin
and then moves away. An example of convection is blowing
on hot food to cool it down. The amount of heat loss depends on
the temperature difference between the body and
environment plus the speed with which air or water is
moving.
Conduction is the transfer of heat to objects or substances the
body comes into direct contact with. Metal and stones are
good heat conductors, which is why they feel cold to the touch,
even at room temperature.
Evaporation is responsible for 20 - 30% of heat loss in
temperate conditions. The remainder happens in the lungs and
airways. In cold conditions, airway evaporative heat loss
increases as the incoming air is humidified and warmed.
Conduction
Conduction is the flow of heat energy from regions of warmer
temperature to regions of cooler temperature
Conduction is the movement of heat from a warmer object to a
cooler one when they are in direct contact with one another.
For example, when you lay directly upon a cold rock the heat
from your warm body will transfer into the rock. You become
cooler as the rock becomes warmer.
The rate of heat transfer between two objects of different
temperatures depends upon several factors. These include

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The temperature difference between the two objects
The total surface area where the two objects are in
contact
The greater the temperature difference between two objects in
contact, the more heat is transferred between them in a given
time. For example, when you place your hand on a very hot
stove top you will quickly receive a great heat input from the
stove to your hand. If the stove top is only warm, it will take
much longer to receive the same amount of heat into your
hand.
The more surface area in contact between two objects, the
more quickly heat is transferred between them. Stick your
finger on an icicle for a minute and it feels cold but you will
probably not feel too uncomfortable. Strip naked and lay on a
block of ice for a minute and you will most likely be very
uncomfortable indeed as the ice absorbs heat from your body
at very fast rate.
Convection
Convection is somewhat like conduction as mentioned above,
but the two objects in contact are also moving relative to one
another. Once again, the amount of heat transferred between
the two objects is dependent upon their differences in
temperature and the amount of surface area in contact.
However there is a third important component and that is the
speed with which the cold object is moving.
For example, when your warm face is exposed to a blast of
cold air the speed of that air matters. If the cold air is moving
slowly it may not cool your face very much at all.
The blood in your body also transfers heat by convection. As
our body cools, its response is to move blood away from the
extremities in order to keep the body's core at optimal
temperature. The result is that our hands and feet become
cooler and may eventually lead to frostbite. This gives impetus
to the fact that in order to keep you feet cool you should put
on your warm hat.
Radiation
Radiation is the transfer of electromagnetic energy between
two objects of different temperatures. Since our bodies tend to
be 98.6 degrees F, we are often warmer than our surroundings
and so we radiate heat to them. In turn we can receive
radiative heat from the rays of the sun, fires, and light
reflected off from snow, rocks and sand, or water.
To minimize the amount of radiative heat you lose to your
environment make sure all exposed areas of your skin are
covered. This includes the head, face, neck, and hands.
Evaporation
When water evaporates its change in state from liquid to a
gas takes up a great deal of energy and lowers the
temperature of the surface on which it occurs. This is the
process of evaporation.
In hot environments evaporation is a welcome process and we
may even encourage the process by wetting ourselves down
when the need and opportunity arises. In cold environments
however, evaporation can be a killer as it consumes a large
amount of energy and warmth from your body and transfers
it to the outside world. In addition, when the clothing you
need to stay warm becomes wet it loses much of its insulative
value and exposes you to the risk of hypothermia
Additional evaporative heat loss occurs through breathing.
When a dog is hot he will pant. The air he brings into body is
filled with moisture that is heated by the body. When the dog
exhales he sends this hot moisture laden air out of his body
and into the outside world. The dog becomes cooler.
Conduction:
Conduction is the transfer of heat from a warm object to a
cold object when the two objects are in contact with each
other. This can include rods, reels, dip nets (hands), sunglasses
or goggles (face).
Conduction is also a major source of heat loss in wet clothing.

Conductive heat loss can be minimized by:
Using an insulated cushion or seat when sitting. Heat
cushions that maximize dead air entrapment by
minimizing passive convection within the pad are very
effective.
For the Hands the best prevention against conductive
heat loss is the use of minimally compressible insulation
in the palms of your gloves or mitts with easily
compressible insulation for the back of the hand (to
minimize weight and maximize warmth). Mitts are the
better choice.
Heat loss through the feet (and specifically, through the
soles of your shoes or boots) simply requires a barrier
between your bare feet and the surface.
This barrier should include socks, insoles, and the sole of
your shoe or boot.
Thin poly-pro or silk socks under a minimal compressible
sock made with a high-density merino wool blend,
combined with insoles made of closed cell foam or loden
(felted) wool provide good in-shoe protection. Shoes with
thick mid-soles and those with lugged soles (which
minimize direct contact with the cold surface) provide
the basis for good winter footwear.
Convection:
Convective heat loss occurs in response to movement of a fluid
or gas.
The two types of Convection Loss are Active and Passive.
In outdoor clothing, convective heat loss occurs when warm
air next to the body and in the clothing is displaced by cool air
from the outside environment.
Windproof clothing, worn over insulating clothing capable of
trapping dead air air in its thickness, provides reasonable
insurance against convective heat loss.

Active Convection:
The biggest factor contributing to convective heat loss is
wind, especially when the boat is moving.
Consequently, a wind proof jacket should be standard
ware.

Passive Convection:
This occurs by the “chimney effect” that draws cool, dense
air into our clothing system from pants cuffs and waist
hems, displacing warm, light air that exits out of our
neck and cuffs.
Passive convection can be better controlled with
garments that includes plenty of ventilation options.
Clothing with a hood and adjustable cuffs and hems that
can be fully opened or fully closed provides the most
versatility – tighten the cuffs and hems to preserve heat
and loosen them up to vent heat and cool down and
minimize overheating.
Top
Evaporation
Evaporation occurs when a liquid (such as sweat) changes
phase to a vapor (sweat vapor). This phase change requires
heat. Unfortunately, your body heat drives this phase change.
Evaporative heat loss may be most noticeable in context of the
“flash-off” effect, which occurs after a period of intense
physical activity and sweating in cold conditions, followed by
rapid evaporation and chill after stopping to rest.
Evaporative heat loss from perspiration can occur in one of
two ways.

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Sensible (or “active”) perspiration is caused by the
formation of liquid sweat droplets at the skin surface in
response to excess heat. This excess heat is usually a
result of being dressed too warmly for a given activity
level.
Insensible (or “passive”) perspiration is the direct
emission of sweat vapor from the skin in response to a
humidity gradient (i.e., your skin is “drying out”).
Insensible perspiration is most significant while at rest,
or while sleeping, while sensible perspiration is most
significant during periods of activity.
Top
Respiration
Technically, respiration combines the processes of evaporation
(of moisture in the lungs) and convection (displacement of
warm air in the lungs by cold air from the outside
environment). Because humidity in the lungs is 100%,
respiration is an important heat sink in cold, dry conditions.
Significant moisture (and thus, body heat) can be lost when
that moist air is exchanged with much drier outside air. In
addition, some body heat is lost to the process of warming the
cold air entering your lungs.

Minimizing Respiratory Heat Loss
Respiratory heat loss can be significant in cold, dry
conditions. Respiratory heat losses can be minimized by
breathing air that has been pre-warmed and/or prehumidified prior to taking it into the lungs. Breathing
through a fleece balaclava or face mask can improve
respiratory comfort by increasing the humidity and
warmth of air being breathed prior to its entry to the
lungs.
Innovative products are appearing in the market
specifically designed to magnify this effect, and are
informally known as “heat exchange face masks.”
Radiation
Radiation heat loss occurs primarily on cold, clear nights, and
is readily noticeable after sunset. Radiation heat loss from the
body occurs primarily due to infrared emission. Cloud cover
dampens the effects of Radiation heat loss somewhat, by
reflecting a significant portion of radiant heat back to the
earth’s surface.

Radiation heat loss is most significant between sunset
and sunrise, when the atmosphere loses tremendous
amounts of heat that was absorbed by sunlight
throughout the day. The best defense against Radiation
heat loss is thick insulation.
o
Minimizing Radiation Heat Loss:
Unless you fish well into a starlight cold night don't
worry too much about Radiation heat loss.
But if you do or if you are spending the night
beneath the cold stars read on.
Radiation heat loss is assumed by many to be
negligible relative to other heat loss (wind-induced
convective, respiration and evaporative). However
in windless conditions when the body is not active it
can be significant, especially at night.
Radiation heat loss can be minimized by one of two
methods.
The first is by wearing a reflective barrier (such as
aluminized nylon or mylar) near the skin capable of
reflecting infrared radiation back to the body.
The second is by wearing thick clothing (down or
high-loft synthetic fill garments). The latter strategy
is effective because infrared radiation cannot travel
through thick insulation, and thus, most of the
infrared radiation lost by the body can remain
entrapped in the clothing system rather than exiting
out to the environment.
Stay Warm On the Water, and always wear your
life vest.
Mechanisms of Heat Loss
In order to design appropriate clothing and sleep systems, we
must first understand the primary mechanisms of heat loss.
Conduction
Conduction is defined as the transfer of heat from a warmer
object to a cooler object when the two objects are in direct
contact with each other. Backpackers experience conductive
heat loss anytime the body is in direct contact with cold
ground. While hiking, the primary source of conductive heat
loss is out of the feet via soles of your footwear. While at rest,
conductive heat loss occurs while sitting or lying on the cold
ground surface. Conduction is also a major source of heat loss
in wet clothing, due water’s excellent conductive properties.
Convection
Convective heat loss occurs in response to movement of a fluid
or gas. In outdoor clothing systems, convective heat loss occurs
when warm air next to the body and in the clothing is
displaced by cool air from the outside environment. The
biggest factor contributing to convective heat loss, of course, is
wind.
In addition to wind-induced or “forced” convection, “passive”
convection occurs via the “chimney effect” that draws cool,
dense air into our clothing system from pants cuffs and waist
hems, displacing warm, light air that exits out of our neck
hems and other vents.
Evaporation
Evaporation occurs when a liquid (such as sweat) changes
phase to a vapor (sweat vapor). This phase change requires
heat.
Conduction, convection, and radiation can cause both heat loss
and heat gain to the body, evaporation is a mechanism of heat
loss only, in which a liquid is converted to a gas. Perspiration
evaporating off the skin is an example of this heat loss
mechanism.
Body temperature is regulated by a system of sensors and
controllers across the body. The brain receives signals
regarding body temperature from the nerves in the skin and
the blood. These signals go to the hypothalamus, which
coordinates thermoregulation in the body. Signals from the
hypothalamus control the sympathetic nervous system, which
affects vasoconstriction, metabolism , shivering, sweating, and
hormonal controls over temperature. In general, the posterior
hypothalamus controls responses to cold, and the anterior
hypothalamus controls responses to heat.
Hypothermia, or low body temperature, is a result of
prolonged exposure to cold. With a decrease in body
temperature, all metabolic processes begin to slow.
Hypothermia can be life-threatening.
Hyperthermia describes a body temperature that is higher
than normal. One example of hyperthermia is fever. A fever is
generally considered to be a body temperature over 38 degrees
Celsius (100.4 degrees Fahrenheit). A fever is the body's
natural defense to an infection by a bacterium or virus.
Fevers are one of the body's mechanisms for eliminating an
invading organism. Fevers may even make the immune
system work more effectively. Heat exhaustion and heatstroke
are other examples of hyperthermia. These occur when heat
production exceeds the evaporative capabilities of the
environment. Heatstroke may be fatal if untreated.
Temperature (C)
28
30
33
37
42
44
Symptoms
muscle failure
loss of body temp control
loss of consciousness
normal
central nervous system
breakdown
death
Reducing Heat Loss
To reduce the amount of heat you lose through conduction,
place quality insulation between you and the object you are
touching. The insulation must have plenty of dead air space
within its structure and resist compression if weight is to be
applied to it.
To reduce heat lost through convection, have a windproof
outer shell that you can wear over your insulation layers. This
will help prevent wind from penetrating your clothing and
removing the body heat you have stored there.
To reduce the amount of heat you lose to your environment
through evaporation you need to stay dry. Rain, snow, fog,
water, and sweat can make you wet and increase the amount
of evaporation the occurs on your body. In addition, when
your clothing becomes wet its efficiency as insulation drops off
dramatically. Avoid deep heavy breathing as this too will
serve to move large amounts of heat from your body and out
into your environment via the evaporative process.
Reducing the heat you lose through radiation means covering
all exposed areas of your skin so that none of it shows to your
environment. This includes the head, face, neck and hands