Temperature
EffectsTemperature
EffectsOther Topics Covered: Ways we gain heat How we conserve heat Hypothermia Susceptibility to Cold Effects of Diving in the Cold
Methods of Heat Loss
To look at the effects of temperature on the human body when we are diving, we first need to look at the different ways that we can lose or gain heat. There are four ways that we can lose heat, radiation, conduction, convection and evaporation. All of these except evaporation require that we be warmer than our surrounding in order to lose heat energy. Heat energy always moves from warmer areas to colder areas. When there is a temperature difference between two objects or substances, a temperature gradient is said to exist.
Radiation is a method of heat energy transfer that involves electromagnetic waves. All living and non-living objects radiate electromagnetic waves in the form of radiation. Think about getting into your car on a hot sunny day. Even after you have opened all the windows and begun driving, the car remains warmer than the outside air. This is because the car and all of its contents have absorbed radiant heat from the sun and are now giving off that energy. When on land, our bodies work in much the same way, radiating heat directly off of our skin to our surroundings. Under water however, this method of heat transfer has relatively little effect as compared to the other methods.
Conduction is the transfer of heat energy directly from one material to another. Substances vary in their conductance, or ability to transfer heat. When you take a metal ice cube tray out of the freezer, it feels very cold. However, when we take a cardboard box out of the same freezer it does not feel nearly as cold. This is because they have different abilities to conduct heat. The metal is a much better conductor, and it conducts heat away from our hand. The cardboard on the other hand is much less able to conduct heat away from our skin. Like metal, water is a very good conductor of heat. In water that is the same temperature as the outside air, we will feel colder and heat will quickly be carried from our bodies.
Convection is the loss of heat due to moving air or liquid. The air or water picks heat energy up in one place and carries it to another. Just think how much cooler it feels on a windy day than on a still day even when the temperatures are the same. This is the main way that we lose heat while in the water. Conduction alone will not cause us to lose large quantities of heat, because eventually the water around us would warm up to be near our body temperatures. With convection however, new water at the lower temperature is always around us giving no opportunity for the water around us to warm up. This is one reason why a wetsuit works well. Although water is a good conductor, when trapped in our wetsuit, it can be warmed up to near our body temperature, surrounding us with a non-moving layer of warm water.
Finally, evaporation causes us to lose heat energy by removing water from our skin and converting it to water vapor. When this occurs, the water is changing phases, a process that requires energy. This cools us by using heat energy for the phase change. As stated earlier, this is the one form of heat loss that does not require that we be warmer than our surroundings. It is however, important that the air be dry enough to allow for evaporation to occur at a reasonable rate. The more saturated the air is with water vapor, the less able it is to cause evaporation. Just think how much warmer eighty degrees feels in Florida where it is humid, than in Arizona where the air is drier. Evaporation however, does not play a role in heat transfer while we are immersed during a dive.
While we can gain heat through radiation, conduction and convection, our bodies are also able to generate its own heat internally. Exercise creates heat, but we are not always able to exercise enough to generate sufficient heat. When this is the case, our bodies have two other forms of heat generation, shivering and non-shivering thermogenesis. Shivering is an involuntary contraction of our muscle fibers. Because the contractions are not being used to do work (ie.move our limbs), the energy is converted into heat. Shivering can boost resting metabolism by up to three or four times. At the point that a diver begins to shiver, they are not yet hypothermic, but it is probably a good sign that it is time to get out of the water and get warm.
Non-shivering thermogenesis is the other way that our bodies produce heat internally. This is a complex chemical process that breaks down fats to produce heat. While humans are capable of producing heat in this manner, we are far less efficient at this process than are some other mammals, such as bats.
Before we talk specifically about the ways our bodies conserve heat, we need to discuss the difference between skin and core temperature. Everyone is familiar with the temperature of 98.6 F. This however, is our core temperature, not our skin temperature. Our skin temperature is usually lower than this. Part of our regulatory system is that our skin temperature can fluctuate without causing much of a change to our core temperature. Additionally, because we have ways to conserve heat, our surrounding temperature need not be 98.6 F to be thermoneutral. Our bodies continually produce heat through metabolism and exercise, so there is a balance between heat loss and heat gain/production. The temperature that is usually most comfortable when at rest in air is usually somewhere around 70 F. In water it is usually much warmer due to water's increased conductance and convection.
There are three main ways that we resist heat loss, vasoconstriction, insulation and through the use of counter-current heat exchange. The first method, vasoconstriction, reduces heat loss by reducing the blood flow to the skin. By constriction of blood vessels that run into our limbs and near our skin, we are able to get the warm core blood away from areas that have a large temperature gradient. In doing so, the skin will cool and the temperature gradient between the skin and the environment will also become smaller. Therefore, when your hands get cold, it may not be the result of "poor circulation." Rather, it may be your body constricting blood vessels in your extremities to reduce heat loss.
Insulation is another method that we use to conserve heat. Our insulation is made up of three basic parts, fat, muscle and our shell. Our shell can be thought of as the self-adjusting insulation that surrounds our central core. We can increase the thickness of this shell through vasoconstriction as described earlier. Each of these types of insulation are able to provide us with some level of warmth protection. Although muscle is able to provide some insulation while at rest, it does so less efficiently than fat. Fat provides us with a good layer of insulation. Our body insulation increases directly with the average thickness of the fat laying under our skin. It is true that the more fat a person has the more they are able to tolerate the cold temperatures and maintain their core temperature.
The last method employed in conserving heat is counter-current heat exchange. This method utilizes blood vessel arrangement to contain warm blood in the core and colder blood in the extremities. Arteries moving away from the core run in close proximity to the veins that are returning blood to the heart. This conformation allows heat from the warm arterial blood to be transferred to the veins. While humans utilize this method, we are much less adept at it than are many other animals. A moose for example, can stand in icy cold water and still maintain its core temperature. The blood in its feet will be close to the water temperature, but through counter-current exchange, the heat is maintained in the body's core. This system is important, because unlike vasoconstriction, there is no reduction in blood flow.
All of these methods work together to keep our core temperature fairly constant. This does not however mean that it keeps us "comfortable". When diving, it is important to utilize the appropriate gear for the water temperature and anticipated work load so that one does not become overheated or uncomfortably cold.
Unlike many of the topics we have discussed thus far, hypothermia is not a reflex, rather it is a condition. It is a term we use to describe a situation in which the body's core temperature has dropped below 95 degree Fahrenheit. Just because someone is shivering and feeling uncomfortably cold does not mean that that individual has hypothermia. Symptoms of hypothermia are:
Hypothermia is not a condition to be taken lightly. Treatment is needed quickly.
There are several factor that can change one's susceptibility to cold temperatures. One large determinant is related to an individual's surface-area-to-mass ratio. This is a ratio that looks at the individual's overall surface area as it compares to the overall mass. Someone who has a larger surface area relative to their mass will lose heat more quickly and become cold more readily since we lose heat through our skin's surface. This is the same way a large loaf of bread will stay warmer longer than a thin bread stick. This is also why fingers, ears and noses get cold quicker than other areas of our bodies. They are all thin and elongated so that the surface area to mass ratio is larger. While generally this factor would suggest that a heavier, round person would stay warmer longer, it is not always the case because there are several other factors that can play a role in determining someone's susceptibility to the cold.
Age and behavior can also play roles in cold susceptibility. Young children and older adults are less able to thermoregulate than middle age adults. Much of the differences are due to physical fitness and body composition. Behavior can help reduce heat loss through physical activity and changing body position. Have you ever awaken at night to find yourself curled up in a ball and shivering? That body position was not a coincidence. By changing our body position we can regulate surface area. The greater the exposed surface area, the more heat we will lose.
There are certain temperature regulation advantages to being physically fit. The increased muscle mass in a fit person allows for increased heat production and storage, as well as allowing the person to generate more heat through shivering. Additionally, the more physically fit person is more able to exercise at higher intensities to generate more heat.
Acclimatization is another factor that can increase one's susceptibility to the cold. This term should be distinguished from acclimation. Acclimatization refers to changes from seasonal or geographical exposure, while acclimation is a change produced in laboratory. People who are subject to cold temperatures over a period of time develop an increased ability to withstand the temperatures. Cold acclimatization involves three adaptations. First, individuals begin shivering at a lower body temperature because they are able to generate more heat without shivering. Secondly, these individuals tend to have an increased ability to sleep in the cold. Finally, these individuals typically have a better ability to adjust skin temperatures. All of these adaptations increase one's susceptibility to the cold; however, with a lack of cold exposure, these adaptations are lost.
In addition to these factors, cold tolerance can be affected by dehydration, lack of sleep and medications. All of the different factors come together to determine a person's individual susceptibility to the cold. No one fact alone can be used to predict how well a person will be able to tolerate low temperatures.
There are several bodily effects of diving in cold water. One effect that we have already talked about is vasoconstriction. As already stated, when skin temperatures begin to drop, blood vessels will begin to vasoconstrict in order to keep heat centralized in the body's core. Vasodilation however, can also occur. This occurs as a response to greatly reduced skin temperatures that could potentially lead to cold injury. When skin temperatures fall below about 50 degrees Fahrenheit, limb vessels will enlarge briefly a few times a minute in an attempt to raise skin temperature without sacrificing core temperature. This is why you can have rosy cheeks on a blustery day, even though the skin surface typically vasoconstricts in the cold.
Cardiovascular changes also occur when diving in cold water. The dive reflex, which is described in detail in "Immersion Effects II", causes a decrease in heart rate and a decrease in blood flow to the limbs. Endurance is also decreased as core temperature drops, with fatigue setting in sooner. Another change that occurs, irregular heart beats, is more dangerous than the first two. Irregular heart beats, or arrhythmias, occur in everyone from time to time. Typically they do no harm and most do not even know that they have occurred. With immersion in cold water; however, the frequency of arrhythmias increases. Both cold water and immersion increase the blood flow returning to the heart, distending the atria and causing a reflexive slowing of heart rate. Both distension and slowed heart rate increase the tendency of the heart to have irregular beats. In some divers, a high frequency of irregular heart beats can lead to blackouts and even death.
For divers, an important change that occurs, is a reduced ability to both absorb and eliminate nitrogen from the blood. Staying uniformly cold throughout a dive does not change the relative amount of nitrogen that is absorbed as compared to what is eliminated. However, it has been found that starting out warm and then chilling throughout the dive can place a diver at risk for decompression sickness. When warm, we absorb more nitrogen than in a cooler state, and then when we cool greatly throughout the dive our bodies' are less able to eliminate the gas. Divers should always wear clothing appropriate for the tasks and the water temperature, and they should always closely follow their dive tables.
As we have stated in earlier sections, cold water and immersion both increase diuresis. The vasoconstriction that occurs in response to the cold, and the equalization of pressure causing centralization of blood due to immersion, both increase venous blood return. This in turn causes atria distention which signals to nerve receptors that total blood volume is too high. In an effort to return blood volume to what these receptors see as normal, a signal is sent to the kidneys to release more fluid. Hence, urine output is increased.
In addition, cold water can cause some physical and mental impairment. Tactile ability, or your sense of touch, decreases in cold water, as does your physical strength. In severe cold water, intellectual functioning can also be impaired. I
With all of these effects in mind, it is very important to be aware of the water temperature where you will be diving and prepare accordingly. A case of hypothermia or decompression sickness could ruin an otherwise wonderful vacation.