Earthlings and the Earth: Adapting to Your Environment
In This Chapter
▶ Understanding how the body adapts to hot and cold
▶ Looking at the effects on the body of exercising at altitude
Being a warm-blooded mammal has its advantages. By generating your own internal body heat, you can maintain your metabolic function without having to rely on the external environment. Whether the day is cold or warm, your cells stay happy and functioning. But conditions can arise that make the body either too hot or too cold. Controlling body temperature can become quite difficult in extreme environments, a situation that can impair performance and even result in death.
In addition, as a creature that breathes air, you certainly value the oxygen that conveniently surrounds you, waiting to fill your lungs. However, as you ascend in altitude, the conditions change, and oxygen is not quite so
available, a situation that presents problems for high-flying and high-climbing humans.
In this chapter, we explore how the body maintains its temperature under both hot and cold conditions and how it’s affected by and acclimatizes to altitude changes. By the end of this chapter, you’ll see how tough the environ- ment can be on your body and the ways that your body can adapt to
conditions to continue to perform its best.
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Keeping It Just Right: The Basics of Temperature Regulation
Your body is finicky when it comes to temperature. It likes a very narrow range that hovers around 98.6 degrees Fahrenheit. Too far above or below that point, and cell function begins to decline, and systems don’t function.
Your body maintains an acceptable temperature through a series of control systems, starting with the hypothalamus in the brain. The hypothalamus signals other mechanism to either warm you up or cool you off.
The hypothalamus: Your internal thermostat
The hypothalamus is an almond-sized section of the brain located just above and slightly to the front of the brain stem. One of its key metabolic functions is body temperature regulation.
Blood continually courses through the hypothalamus, where its temperature is monitored. Each person has a hypothalamic thermostat set at a predetermined temperature, or set point. That set point is around 98.6 degrees Fahrenheit. The goal of the hypothalamus is to balance the gain of heat (due to exercise or the environment) with heat loss so that the set point is maintained.
Body temperature is not the same everywhere on the body. For example, the skin, also called the shell, is cooler than the blood, and the blood is cooler than active muscle and organs. The hypothalamus monitors the core temperature, the temperature deep within the body. The core temperature is the most important temperature to keep near its set point.
The hypothalamus does more than just regulate body temperature. It controls a range of functions like thirst, blood pressure, hunger, and hormone release. In some cases, the hypothalamus releases chemicals that impact the body directly (like releasing vasopressin to boost blood pressure), or it releases chemicals that cause other organs to release their hormones. For example, the hypothalamus may release a hormone that causes the pituitary gland to release growth hormone.
Pass the heat, please: The core-to-shell model of heat transfer
Hot things like to transfer their heat to cooler things. In order for the heat being generated in the muscle (lots of heat) to leave the body, it must come
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in contact with cooler and cooler locations. Therefore, the direction of heat movement looks something like this:
Heat in muscle → Cooler blood → Cooler skin → Cooler environment As we explain in the next section, your body has ways of moving heat away from its core by cooling the shell (skin). Problems can arise, however, when these mechanisms fail or when extreme environmental conditions overwhelm the body.
Some Like It Hot — But Not Your Body!
Hot environments can pose a serious challenge to the body. If you add the heat generated during heavy exercise, you can be hit with a double whammy!
Understanding the different ways your body gains heat from the environment can help you plan for the conditions.
Looking at the mechanisms of heat gain
The gain of heat by the body can come from within, or it can be from the external environment. Is the day sunny? Humid? Does the ground give off any heat? If the mechanisms of heat gain are combined, you could be in for a very hot day! This list shows the variety of ways you can gain heat:
✓ Heavy exercise: A large percentage — 80 percent, in fact — of the calo- ries your body burns is lost as heat. So the harder you work, the more heat you produce.
✓ Heat from the sun: A sunny day can add 20 percent more heat load than a cloudy day. The sun heats the skin, making it harder to dispel the heat.
Heat dissipates by going from higher to lower temperature environ- ments, as we explain in the earlier section “Pass the heat, please: The core-to-shell model of heat transfer.” If your skin is warmer due to heat from the sun, the internal heat can’t dissipate as efficiently.
Feeling feverish?
Although your set point is supposed to be main- tained at 98.6°F, it can be modified. Under condi- tions of illness or infection, your body activates defenses to combat the nasty invaders. One of these self-defense tools is heat! The hypothala- mus raises the body’s set point, causing a rise in
core temperature as high as 104°F. A high core temperature may fight the illness, but it can also be a danger to your cells if it goes too high — the reason why medications may be given to block the flames of a fever.
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✓ Reflected heat: Heat can reflect off many surfaces, like asphalt, metals, and water. (Ever run on a black asphalt surface? You may have noticed how hot it is!) The reflected heat is why stadiums can get so hot in the summer time, or why a city street is a lot hotter than a country lane.
✓ Hot air: A hot day can also warm the skin, making it hard for heat to get from the core to the skin. In this situation, the heat stays trapped inside the body (it’s that core-to-shell model of heat transfer principle again).
✓ Covered skin: For heat to leave the skin, the skin must be exposed. Your clothing traps the hot air near your skin and prevents heat from leaving.
Think about how hot it gets, even when you’re wearing only a light shirt.
✓ Humid days: As the next section explains, one way to cool the skin is through sweating. The sweat itself doesn’t cool you off; the sweat evaporating off the skin does the trick. Very humid air, however, already has all the water it wants, and it isn’t willing to take your stinky sweat!
On humid days, you can’t cool your skin through evaporation.
Because the environment is such a contributor to heat gain, people try to mea- sure the environmental conditions by using a variety of indexes. One is called the heat index. This index, shown in Figure 6-1, calculates a temperature by considering the air temperature and the relative humidity.
Figure 6-1:
The heat index factors together air tempera- ture and humidity.
Illustration courtesy of the National Weather Service
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Turning on your personal air conditioner:
The body’s cooling mechanisms
To cool off, your body has a variety of mechanisms to move heat from your body’s core to the shell. Some of these mechanisms work better than others:
✓ Convection: Hot air molecules can sit just above the skin, keeping the skin hot and preventing it from cooling. With convection cooling, air blows on the skin, sweeping away the hot air so that the skin can release more heat. If you’ve ever stood in front of a fan when you are really hot, you know what convective cooling feels like.
✓ Conduction: When something hot touches something cold, the heat moves to the colder object. In conductive cooling, the skin comes in
physical contact with a cooler object — a cold towel, cold water, ice cubes, or anything else that can be touched — that transfers the heat away from the skin. Some people may even wear cooling vests, which can go under- neath clothing. Ever wonder how firefighters stay cool? Cooling vests!
✓ Radiation: Infrared rays can warm a fast-food hamburger, and they also can take heat away from the skin. Remember, your blood is warmer than your skin. When the core temperature rises, the hypothalamus signals blood vessels in the skin to open wider, which lets more hot blood move to the skin. When the hot blood gets near the surface, its heat can radiate away. Another effect of this cooling mechanism is that your skin looks red; that flushed appearance is the result of the widened blood vessels. Hopefully, you’ll never get warm enough to cook a burger!
✓ Evaporation of sweat: Water vapor holds far more heat than water itself (think of steam, which is hotter even than the boiling water that produces it). When your core temperature rises above the set point, the hypothalamus triggers sweat glands in the skin to release fluid (sweat) onto the skin. This fluid has been heated by the blood coursing through the skin. When it reaches the surface of the skin, it turns to a vapor (evaporates) and takes the heat with it.
Sweat evaporation, which does 80 percent of the work in cooling the body, is the most important cooling mechanism your body has. The more skin you have exposed to the environment, the more sweat evaporation can take place (remember how humidity affects evaporation, though).
Adding insult to injury: Exercising in hot environments
Athletes can’t hide from the heat. They often must perform in conditions that would make most of us run inside for some iced tea. If their bodies’ cooling mechanisms are working, things may go just fine. However, if the athletes are unaccustomed to the heat or if they can’t cool themselves, a rise in body tem-
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perature above the set point, or hyperthermia, is inevitable, a situation that results in a decline in performance and possible serious injury.
When athletes encounter a hot environment for the first time, they notice an immediate effect on performance. Heat reduces their ability to perform well.
Seeing the effects of a hot environment on the body
Exercising in a hot environment produces certain changes in the body:
✓ A faster rate of glycogen (carbohydrate) use: As carbs are used more quickly, athletes deplete the primary energy source for much of their work. Have you ever heard of the marathon runner that “bonks,”
“crashes,” or “hits the wall” with fatigue during a race in hot weather?
He has run out of glycogen! (Chapter 4 has more information on glyco- gen and exercise metabolism.)
✓ Reduced muscle blood flow: With so much blood going to the skin to help cool the body, less blood is available for the muscles. Less blood flowing to the muscles means less oxygen for the muscles, fewer nutri- ents, and reduced performance.
✓ Increased blood lactic acid: Because the muscle is starving for oxygen, it relies on anaerobic metabolism to keep going. The resulting lactic acid contributes to bonking and a very unpleasant feeling in the muscles.
(For a detailed explanation of what lactic acid is and how it affects the body during exercise, refer to Chapter 4.)
✓ High rate of water and electrolyte loss through sweating: Sweat evapo- ration is the body’s primary cooling mechanism. In the untrained indi- vidual, the sweat contains not only water but also electrolytes (sodium and potassium), which are important for cell function and the transport of nerve signals. They must be maintained within the body.
Dehydration reduces performance. Every volume of sweat lost results in a reduction in performance. You also run the risk of heat injury. To prevent dehydration, you must replace fluids lost as quickly as possible, both during and after the activity.
✓ Increased heart rate: With blood going to the skin for cooling and more fluid being lost due to sweating, a reduction in blood volume occurs.
With less blood volume, the heart must beat even faster than usual to push the same amount of blood.
Too hot to handle: Heat injury
Athletes are known to push themselves very hard, sometimes ignoring warn- ing signs of injury. When this happens in a hot environment, the result can be very bad. Even hours after being in the heat, the illness can continue.
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Heat injury is a leading cause of death among athletes, and the injury has three primary stages of severity, listed here from least to most severe. Action should be taken at the first signs of heat injury:
✓ Stage 1 — Heat cramps and dizziness: During the early stages of heat injury, the cells become dehydrated, and fluid is lost from the blood.
The result is a feeling of nausea and dizziness, as well as muscle cramps.
Treatment: Get the person out of the hot environment! Give her cool fluids as soon as possible, massage the cramps, and have her stretch. If she is dizzy, have her lie down.
✓ Stage 2 — Heat exhaustion: If the athlete pushes past cramps and ignores the nausea and dizziness, things can become quite severe. Heat exhaustion causes extreme losses in fluid. Symptoms of heat exhaustion include the following:
• Cold clammy skin, despite a high internal temperature
• Pale complexion
• Collapsing from dizziness or even losing consciousness
• Headache and nausea
• Weak and rapid pulse
• Shallow breathing
Treatment: Call 911 immediately. Heat exhaustion is very dangerous and needs immediate care by a physician. The person with heat exhaustion may need so much fluid that it must be administered intravenously. The sufferer’s sweating mechanism is shutting down due to the loss of so much fluid, and the condition can progress to heat stroke (and death)
Determining how much fluid you need to replace
Did you know that one fluid liter weighs one kilogram? Additionally, one pound of fluid is 16 ounces. So to determine how much fluid is lost due to sweating, you do the following:
1. Weigh the person (with minimal clothing) before the activity.
2. Weigh the person after the activity.
3. Subtract the after-exercising value from the before-exercising value.
The difference in weight represents the loss of fluid!
Here’s an example: Before a long run, Bob weighs 180 pounds wearing just his running shorts. After the run, he weighs 177 pounds:
180 pounds – 177 pounds = 3 pounds fluid lost = 48 ounces that need to be replaced (3 × 16 = 48).
Get Bob drinking! Remember that he has also lost things like sodium and potassium, so sports drinks may be a nice solution to the problem.
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quickly, so you must act fast! Until emergency responders arrive, cool the sufferer with cold water and, if available, ice.
✓ Stage 3 — Heatstroke: At this point, the person has collapsed com- pletely and her ability to cool herself has shut down. Symptoms of heat stroke include
• Very high body temperature (106°F or more)
• Dry skin (sweating has shut down)
• Unresponsiveness
• Labored breathing
Treatment: Without immediate treatment, death can occur quickly (often due to things like liver or kidney failure). Call 911 immediately, and, if possible, immerse the sufferer in ice. Your goal is to cool her down fast!
Getting your body to adapt to the heat
Fortunately, your body is equipped to adapt to hot environments. It only takes about 10 to 12 days, and the recipe is pretty straightforward:
✓ You must exercise in the hot environment. Sorry, sitting in a sauna or working on your tan does not count! Adaptations only occur when you work in the actual environment.
✓ Take it easy and work up intensity. Whatever the training intensity you are used to, back it off to about 60 percent to 70 percent for three to five days. As your body starts to adapt, you can build up intensity.
✓ Show some skin. Wear minimal clothing to help your skin cool through sweat evaporation. Clothing should be light, breathable, and reflect light. If you normally wear pads (you play football, for example), start without them for a while.
✓ Take water breaks. Stay hydrated! Drinking about 8.5 ounces of water every 15 minutes helps offset sweat losses.
Within 10 to 12 days, your body will acclimatize. The changes that occur, listed here, help cool your body faster and allow better (and safer) performance:
✓ Sweat rate increases. Your sweat glands put out more sweat, which means more cooling.
✓ Sweating begins earlier. The sweat mechanism kicks in more quickly, so you don’t get too hot before things start to cool.
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✓ Sweat is more watery. The sweat glands reduce the amount of sodium and other substances in the sweat, a combo that helps conserve electro- lytes. Watery sweat also evaporates faster, which is cool — literally and metaphorically!
✓ Plasma volume increases. The kidneys retain more fluid for the blood, and because some of the sweat comes from the blood, this translates into more fluid volume.
✓ Your body uses less glycogen. Glycogen use drops after acclimatiza- tion, which means you don’t fatigue as fast. (Glycogen, which is a stored form of glucose in the muscle and liver, is discussed in greater detail in Chapter 4.)
✓ Your skin is cooler. Due to all the preceding changes, the skin stays cooler, enabling heat to move from the core to the shell more easily.
✓ Your heart rate is reduced. Because of all the fluid increases and improved cooling, the heart doesn’t have to work as hard, and your per- formance in the hot environment will be similar to your performance in a cooler environment.
When Chillin’ Ain’t Cool: Exercising in Cold Environments
Humans are capable of surviving in space, so it’s not hard to believe that cold environments can be conquered. Obviously, standing in Antarctica in your running shorts isn’t a wise move, and certainly exercising in cold environ- ments has risks that you must be aware of. But proper clothing and under- standing how much heat you generate during the activity can go a long way toward ensuring that you safely handle cold environments.
Baby, it’s cold inside: Introducing hypothermia and wind chill
When heat loss exceeds the body’s ability to generate heat, a condition of hypothermia exists. In hypothermia, the body temperature drops below 95°F, and a loss of body function results. Causes of hypothermia include
✓ Exposing skin to cold air, which rapidly draws heat away
✓ Exposing skin to cold water, which dramatically accelerates heat loss
✓ Wearing wet, cold clothing, or falling into cold water