Frostbite, manatees and marathons
Last Friday, I had a delightful visit from Diana Keat, a MOOC student and Facebook friend, and her husband Eddie. It was great fun to meet Diana and Eddie. I am hoping to see them again soon if I can manage to get to their home town of Philadelphia to see the Greg Dunn exhibit at the Franklin Institute. One of the great gifts that Diana gave me in our short visit was her telling me about her experience taking Understanding the Brain.
For all of you who may be wanting to get in on the UtB MOOC action, I am so happy to tell you that we will be re-launching the course in the fall of 2016, just a few months from now. When it re-launches, the MOOC will be “on demand” which means that you can join the fun anytime.
Diana found it amazing how neurobiology intersects with politics and government. She was struck in particular by the neuro-realities of thermoregulation. As this is one of my favorite examples of neurobiology in everyday life, I am going to describe it here, along with a recent example.
The basics of thermoregulatory effectors
Thermoregulation is a beautifully effective system by which we keep our body from either overheating or getting too cold. Thermal challenges can come from inside (as in the case of a fever) or from the outside (as in the case of a fall into an icy lake). To respond to either of these possible attacks (typically called challenges) on our body temperature, we need sensors. As it turns out, we sense the temperature of both our skin and our innards (for example thermoreceptors, sensory afferents that are sensitive to temperature, line the spinal cord).
But the nervous system does not simply detect bad news; it can do something about the information that it receives. The brain can change body temperature through one of several effectors. There are three basic types of thermoregulatory effectors that serve to either increase or decrease body temperature:
- Effectors that increase body temperature:
- Heat production – e.g. increase metabolic rate, shivering
- Heat conservation – e.g. skin vasoconstriction (see below), huddling
- Effectors that decrease body temperature:
- Heat loss – e.g. sweating, skin vasodilation (see below), removing clothing
In essence we have passive (heat conservation) and active (heat production) modes for cold defense but we can only passively decrease our body temperature in response to a heat challenge. To put it another way, there is no biological form of refrigeration. We don’t have a freon-like molecule. This simple fact means that high temperatures are far far more dangerous than low temperatures. We are all handicapped when the outside temperature is above body temperature.
A good analogy for thermoregulatory constraints is a tea kettle that can be set to heat water to different temperatures, depending on the type of tea leaves used.
Every morning, I drink either green or black tea, which are best steeped at temperatures of ~175°F and ~190°F, respectively. My spouse drinks herbal tea which steeps optimally at ~200°F. Most mornings I make my tea first, bringing water to either 175°F or 190°F. Then I set the kettle to 200°F and make peppermint tea for my spouse. However, every once in a while I sleep late and the kettle has been heated to 200°F when I am ready to make my tea. But I am in a bit of a pickle because as with biological thermoregulation, the Perfectemp Tea Kettle can produce heat, conserve heat or lose heat but it cannot cool water. Thus, using the capabilities of the kettle itself, my only course of action is to open up the top of the kettle and let the water cool. This is, of course, a slow process.
The situation is the same with our bodies as it is with the Perfectemp Tea Kettle. Both our bodies and the tea kettle can produce heat, conserve heat, or lose heat but neither can produce cold. But we can bring in outside help to cool overheated bodies and tea kettles. We can jump in water or turn on air conditioning to cool our bodies and we can add cool water to an overheated kettle of water.
The bottom line is that we are not biologically equipped to cool ourselves down. This limitation means that we are in mortal danger when the temperature is above body temperature (roughly 37°C or 98°F). In such circumstances, access to air conditioning is transformed from a luxury to a medical necessity and thus the responsibility of a government interested in the well-being of its citizens.
Why is ambient heat so dangerous to us and other animals?
Most of the time, when the ambient (environmental) temperature is in a comfortable range, we use vasomotion to keep our temperature within the preferred range. Vasomotion refers to the autonomic (automatic and not able to be controlled consciously) control of the diameter of the blood vessels in your skin. When you close up the blood vessels in your skin, aka vasoconstriction, less heat is lost to the environment and more heat stays within the core of your body. In contrast, when you open up the blood vessels in your skin, aka vasodilation, heat from your body is lost to the environment and body temperature goes down. You observe this all the time when people go red in the face while exercising. The red of an overheated person stems from the red, oxygenated blood present in the skin’s blood vessels which have expanded through vasodilation.
Imagine that you’re walking in Chicago in a cold winter. Thermoreceptors in your skin tell your brain, “it’s super duper cold out here” and as a result, the vessels in your exposed skin are instructed to vasoconstrict. Now imagine that the outside temperature dips really low. Depending on the wind, frostbite can set in within minutes as illustrated in the National Weather Service’s chart:
What you can see is that at temperatures below about 0°F (-18°C), frostbite can set in within 30 minutes, depending on the wind chill. In some instances, frostbite happens even faster, in less than 5 minutes. Yet most of us who experience extremely low wind chills do not get frostbite. Why not? Because of something called paradoxical vasodilation. That is, at very cold temperatures, our blood vessels dilate, they bathe our poor, vulnerable skin in nice, warm blood arriving from our warm innards.This process prevents our skin from freezing and being damaged.
Now imagine that you are in Phoenix in the middle of summer. The temperature outside is 110°F (about 43°C). Your well behaved thermoregulatory system detects the high temperature and vasodilates blood vessels in your skin. Consequently, heat from your body escapes to the environment…..except, hold on….that can’t happen because your core is cooler than the environment. So the only thing that can happen is for you to gain heat from the environment. In effect, you will send 37°C blood to your skin and your core will get back blood that is closer to 43°C. This has the potential to essentially “cook” your insides. Not so good obviously. You might be thinking that a paradoxical vasoconstriction would be great in these circumstances. Alas, there is no such thing. Paradoxical vasoconstriction does not happen.
Manatees are large aquatic mammals that live in warm waters, typically at least 70°F (21°C). Because they are surrounded by warm water, they have almost no ability to lose heat to the environment. Therefore, manatees live on a thermoregulatory precipice, always just a hair away from overheating. I have been lucky enough to watch these massive animals. They move incredibly slowly and are herbivores. They simply could not move faster and keep their body temperature low.
The lack of paradoxical vasoconstriction and the possibility of gaining heat from very hot environments reinforce the importance of governmental intervention in regions and times of extreme heat. We explore this topic in the next sections.
Heat waves, cooling centers, and prisons
As explained above, during heat waves, air conditioning is a medical necessity rather than a luxurious option. It is for this reason that cooling centers are setup by responsible governments. People need access to a cool environment when the outdoor temperature reaches core body temperature. This is particularly true of elderly people and people on certain medications that compromise thermoregulation (e.g. resperidone).
The need for air conditioning is nowhere more evident than in the case of prisoners in hot climates. A particularly egregious example of thermoregulatory negligence occurred in Texas. As reported by Manny Fernandez in the New York Times, 10 inmates died in less than a month during the summer of 2011. What makes my blood boil (or more accurately my hypothalamus spark) is the callous and ignorant response of one Keith Price, a former warden who is now the department head for Sociology and Criminal Justice at West Texas A&M. As quoted by Mr Fernandez, Professor Price said, ““Just from a statistical standpoint, that’s really not significant, particularly when you consider the population. Many inmates are poorly equipped to manage their lives and thus make poor decisions. I do not believe it is up to the taxpayers to provide air-conditioning for inmates when some simple self-discipline would avoid many of these problems.” The emphasis is mine.
Professor Price appears to believe that some lives are not significant and that the lives of some populations are less significant than those with other demographics. I simply don’t get this. And I don’t ever want to be a person that understands such reasoning. To me, regarding some lives as insignificant is extreme callousness and simply not right. Professor Price is also ignorant. No decision or measure of self-discipline can lead to body-cooling. No. Not possible. In our progressively hotter world, governments that want to incarcerate people without killing them will need to put cooling measures such as air conditioning into place in more and more locales around the world.
Exercising in a hot environment
Diana’s visit and remark about thermoregulation happened to coincide with my reading an article in the New York Times about a proposal in Phoenix to ban hiking on hot days. The municipal desire to ban hiking when the temperature exceeds 110°F (remember that this is about 43°C) is highly practical. At such high temperatures, hikers are not only generating heat by their movement but they can gain heat from the environment as discussed above. The city knows that some portion of the hikers will become overheated and will require rescues that are highly costly. According to reporter Fernanda Santos, Phoenix has already rescued 141 people this year. The cost of these rescues is not given but one can imagine that it is a sizable chunk of money running in the hundreds of thousands, money that could serve all manner of other pressing needs.
One objection to the proposed ban is predictable. Some opponents believe that such a ban is another example of unwelcome government intrusion into people’s lives, an initiative befitting a so-called nanny state. I have no comment.
An entirely reasonable objection comes from local residents who say that they go hiking all the time and are fine even in high temperatures. In fact, people who are well acclimated to a hot climate may be able to safely exercise in hot weather. They do so because they sweat at a lower temperature, have a lower resting body temperature, lower exercise-associated heart rate and so on (see Sportsci.org for more details about heat acclimation).
I am tempted to say something about sweating here but am going to refrain because this post is already really long. Suffice it to say that sweating, analogous to swamp coolers that are often used in the southwest U.S., is a very efficient way to lose heat but it requires a low humidity environment.
The problem comes when out-of-towners, Northerners, arrive to join in the fun. For a person who is not heat-acclimated, running or hiking in high temperatures is increasingly likely to result in incapacitation or even death.
Should the government step in and close hiking trails on hot days? I don’t know the answer. But it certainly would not hurt to provide clear information that explains the dangers for out-of-towners and local residents alike.
A similar calculus to the question of whether to close hiking trails in Phoenix is involved when deciding whether to hold marathons in hot climates. The Tel Aviv marathon was delayed by a week in 2013 due to a heat wave; nonetheless one young runner died of hyperthermia. Last year, the organizers stopped the marathon three hours early after treating 75 runners for hyperthermia and taking 15 of them to the hospital. The temperature outside was 21°C (70°F)!! The take-home message is that exercise heats up the body and without a facile way to lose that heat, hyperthermia can easily occur.
More posts are coming
Many of you may have noticed my many months long silence here. I was finishing up work on the second edition of my textbook. It’s truly exciting to have that completed and to be able to move on to other projects and interests. I had to be very disciplined to keep myself from writing blog posts. I simply could not justify the indulgence of spending time on writing for fun when there were items that still needed to be finished. So now that is over and I am free to indulge my right to write. Stayed tune for more neurobiological musings.