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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 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. 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