Ep. 27 - Thermoregulation: The Physics and Physiology of Body Temperature Regulation

In this episode of Wilderness Medicine Updates, host Patrick Fink delves into the topic of thermoregulation, explaining how our bodies regulate temperature in response to environmental changes. Listeners will learn about methods of heat exchange (conduction, convection, radiation, and evaporation), how to measure core temperature, and the body's physiological responses—such as shivering, sweating, and blood flow control. 

The episode also touches on the effects of age, metabolic stressors, and pregnancy on thermoregulation, as well as the importance of understanding these principles in the context of treating conditions like hypothermia and heat-related illnesses.

00:00 Introduction and Episode Overview
01:29 Importance of Thermoregulation
03:12 Human Thermoregulation Mechanisms
04:20 Measuring Core Temperature
07:55 Heat Exchange Methods
14:52 Behavioral and Physiological Responses
24:48 Sex and Age Differences in Thermoregulation
27:14 Conclusion and Next Steps

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Transcript

Patrick Fink: 0:13

Hi everyone. Welcome back to another episode of Wilderness Medicine Updates. I'm your host, Patrick Fink. Today we're gonna talk about a nerdy topic. It is the topic of thermo regulation or how the body regulates its temperature in response to the environment. You can expect to learn about how we exchange heat or cold with the environment, what responses the body has to try to regulate its temperature, how we should be measuring. Core temperature and how this can inform our next steps, our next few episodes on frostbite, hypothermia, and then moving into next season into heat related illness. Before we start, I wanna say a big thank you to Ana Kino Medical Director up in Seattle of Seattle Mountain Rescue. Thanks for reaching out with kind words and give a shout out to all the members of Seattle Mountain Rescue who are perhaps listening to this podcast. I'm very glad that you got pointed this way. Happy to have you here. if there's anything that you want me to address on the show, topics that are unclear to you. No issue. Too small, no question, too menial. Send those questions to Wilderness Medicine updates@gmail.com or just reach out and say, Hey, I love to hear from you guys. You might be thinking, could I skip this episode? I'm really a practical person. I don't know if I need this information. In short, you probably can skip the episode. But I, I recommend that you don't, because I'm gonna be spending a fair amount of time on environmental illness. That's gonna be the next series of educational topics that I wanna focus on, and I think it's important to have a strong grounding in the background because if you just learn, you know, the stages of hypothermia, you learn your protocols for treating hypothermia that works 90% of the time. But then when you encounter a unique situation where you're starting to move outside of your protocols, having to make judgment instead of just following an algorithm, it's helpful to know a little bit more than you otherwise might have to know, knowing more background, knowing the reason for those protocols, knowing the reason why they say this and not that gives you the basis for making decisions in unique situations. Still, if you'd rather not spend the time, I will give some background and some rehashing of the basic concepts when we get to those environmental sections, but I do hope you'll stick with me here. This has been a particularly troublesome episode two record. This is my fourth time recording my way through this episode because making it useful to you and concise is challenging. So today I am ditching the notes. I'm ditching all the details, and I'm just gonna give you the fireside chat version. Let's dive in. humans are what are called homeo THMs, and that means that we maintain our core body temperature across a range of environments. This is best contrasted with something like a lizard or an iguana whose core body temperature is subject to the environment. They cannot get up early in the morning and record a podcast because they have to wait for the sun to come up to warm them and start driving their metabolism. Humans in contrast are more like a house with a thermostat. We have a central core temperature that has a certain set point, and we can regulate that core temperature across a range of environments. What determines our set point is a default temperature, and then things can shift it. Metabolic stresses or fever can shift that set point. It can also reduce the range of temperatures across which we can effectively regulate, and that happens more as we get older or when we're ill or if you're pregnant. The range of temperatures across which you can safely regulate your own core temperature starts to decrease. So how do you measure core temperature? Why do we care about core temperature? The reason that we care about core temperature is that the skin temperature can vary widely. In cold temperatures, for example, we pull blood away from our skin and our core temperature. The temperature of us on the inside can vary quite significantly in comparison to the outside. if we're thinking about defining heat or cold related illness, we need to define those on the basis of core temperature and not skin temperature. There isn't actually a consensus place to measure a core temperature. If you were to ask an intensive care doctor, it would probably be in the pulmonary artery, in the central circulation, right in the middle of the bloodstream. But that's pretty impractical for anyone outside of an intensive care unit because you need a big IV line in the neck, and a probe that sits within that circulation. So that's not an effective means of measuring core temperature in the field. There are other invasive means that leave an indwelling temperature sensor. So I'm thinking here of a Foley catheter that's a urinary catheter that has a temperature probe or an esophageal temperature probe, something you would slide down into the esophagus. The Foley catheter has obvious downsides. You can't really walk with a Foley catheter and the esophageal probe. Likewise, not very comfortable in an awake patient, but both of those are fine in a patient who is not with it because you can then have a constant measurement of core temperature that is less affected by the environment. A third orifice that is more commonly used would be a rectal temperature. You can leave an indwelling rectal temperature probe in someone who is unconscious. That obviously doesn't work in someone who's up and walking around, but the most common ways that we might measure core temperature are the most convenient ways would be the tympanic membranes. So in the ear, The tympanic membrane is a super fast way to measure core temperature, and it closely approximates that central circulation core temperature. It is however variable between devices. There are crummy devices out there and there are good devices, and technique is important in that you have to actually completely block the external ear canal when you take this measurement, or it can be affected by wind or other environmental conditions. The temperature under the tongue is a good place to take a temperature, provided that the mouth has been closed for a period of time. You can't just suck on ice cubes and mouth breathe for the 10 minutes before you take that temperature and expect it to be accurate. The axilla or the armpit does also closely approximate the core temperature, however. It is very subject to environmental conditions, patient positioning, sweating, et cetera. So this is a less accurate way, which I wouldn't recommend. in general, for minimally invasive means of measuring core temperature, using a typa membrane thermometer, or under the tongue is an entirely reasonable way to do it. in patients who are unconscious, an esophageal temperature probe is convenient as something that you can slide into the esophagus and leave in place to continuously monitor core body temperature. In general, all of these methods are gonna be within about 0.4 degrees Celsius of that pulmonary artery. Temperature in the central circulation, and you can just pick one and go with it as the means of measuring temperature in a given patient. You don't have to worry about how they differ one from the other. Now, let's think about how we actually exchange heat with our environment. There's four ways that we exchange heat with the environment. conduction, convection, radiation. And evaporation. Conduction is pretty basic and easy to understand. If two objects are in contact, like my butt and this chair, they will exchange heat through that direct contact. Heat moves from whatever is warmer to whatever's cooler. energy's always trying to move to lower energy states. That's why in the final accounting, the entire universe ends up this thin spread of extremely cold nothingness. But for right now, we are concentrations of energy, dumping energy into our environment because humans constantly produce energy. The most common context for conduction to happen is between someone who is sitting or lying down in whatever surface they're on. How much heat can be exchanged this way depends on the conductivity of the material, so some things are very poorly conductive. Error specifically is poorly conductive, while other things are very highly conductive, think metal. Water and actually granite Water is 25 times more conductive than air, and granite is about halfway in between. So if we're looking for something to be an insulator, we want it to have low conductivity. We wanna have, you know, a puffy jacket. What's the puff? It's air. And if. You're wondering what is gonna, cause most conductive heat loss water is, is a good touchstone. We don't encounter a lot of lying flat on cold metal. Convection is basically conduction. That happens faster because of a moving liquid or gas. So think here about a convection oven. So you have a hot oven and you the air moves around and it, and that causes faster heat exchange. Think about, sitting in a cold plunge. If you get into a very cold tub of water, and you stay very still, it feels a lot less cold than if that cold water is circulating around you. blowing wind, blowing air will cool a body much faster than still air, and moving water is gonna cool a body faster than still water. In basically all situations where there is air or water involved, there's some degree of convection happening because our body will warm the air or water at the interface with the skin, and that causes it to move away from us. the usefulness of this concept is that if you want to limit convective heat loss, you have to limit the movement of the water or air at the skin barrier if I'm wearing a puffy jacket, it limits wind movement next to my skin. Or if I'm in the water, I might wear a wetsuit, which is gonna trap a layer of water next to my skin. I only have to warm that water, and then the heat loss is much less. It's conductive. Heat loss instead of convective radiation is the third way that we can gain or lose heat in the environment. All objects in the universe that are not at the temperature, absolute zero are radiating energy into the environment. It's called black body radiation. The hotter they are, the higher the energy state, the more energy is lost through radiation. The two biggest radiative objects we need to think about in terms of heat loss or heat gain are our own bodies and the sun. When we think about heat loss from radiation, the enemy here is outer space, which is essentially a cold void of energy. So the biggest source of radiative heat gain is solar energy. the best way we can lose radiative heat is we radiate it out into the universe on a cold, clear night. Reducing radiative heat gain is about increasing reflectivity, so we want to reflect radiative energy away from ourselves. Clothing is minimally effective in this respect. dark clothing is more absorptive of, the heat of the sun than light clothing, but both are still pretty absorptive. In contrast, a metal surface, think Mylar space blanket is very effective at reflecting away radiative heat. So while your clothing might reflect 10% of radiative heat away from you, a Mylar space blanket can reflect 98% of that. that's a very. Effective tool. Likewise, it can help you retain your radiative heat. If your snowmobile broke down in the middle of winter, in the middle of nowhere, and you have to survive the night. A Mylar space blanket can help keep you from losing your radiative heat out into space. The fourth way that we exchange heat with our environment is evaporation, and that's taking water in the liquid phase, putting energy into it and turning it into a gas, into steam or water vapor. This is essentially a one way street. no practical situation out there is taking steam and condensing it into water to warm the body. The body uses this tactic of evaporation to cool the body, so you're producing sweat. That sweat evaporates, and when it evaporates, that is a reaction that pulls energy from the body to make that phase change. This is a highly effective means of cooling the body in hot situations because it takes a remarkable amount of energy to create this phase change in water. If you evaporate just a hundred milliliters of water, it can reduce the core temperature of someone who's 70 kilos or about 160 pounds by about 0.6 degrees Celsius, which is a substantial change when you think about the mass of the body. So those are the four ways that we exchange heat with the environment. conduction, objects in contact, convection, swirling water or gas, radiation. We're all glowing and losing our heat to the universe and the sun is trying to fry us. And evaporation, which you can conceptualize as sweating, but also we can do that for someone. We'll talk about that when we get to heat illness, but misting people with water and then blowing fans helps that water evaporate and wick heat out of that person. Now let's talk about how we regulate our temperature in response to the environment. The first thing to know is that the brain is in charge. There are some reflexive. Reactions to our environment that don't involve the brain. For example, if your hand is placed in a very cold environment, just the contact of cold with the hand will cause blanching of the capillaries there. or a sweat gland actually produces less sweat if the skin on top of it is wet. But for all intents and purposes, the central nervous system, the brain and the spinal cord are regulating the core body temperature. The majority of the temperature sensors of the body are out in the periphery on the skin. There are a few core temperature sensors closely related to the central circulation and your gut, but the majority of the sensing that we're doing in response to our environment is at the interface with the environment, and we have separate cold and heat sensors in the skin. Those can sense a wide range of temperatures. If you get outside of the temperatures that you can sense, you just perceive that as a burning pain. If you've ever stuck your hand in a cold bucket of water when your skin becomes too cold, you can't sense the temperature anymore, and it just feels like burning those sensors, the hot and cold sensors are concentrated on. The hands on the face, on the areas that are commonly exposed, they're less common in the legs. In the feet. They give information about the environment back to the brain, and the brain uses what are called effector responses to regulate the core temperature in response to those signals. The most effective effector response is behavioral change, and this is actually driven by the central nervous system. It's not all free will here. Modifying the environment, choosing where the body is, or trying to change that environment is the strongest means of. Protecting yourself from environmental stresses. So either finding shelter or assuming a different posture is the single biggest mover in terms of managing core body temperature. And this is intuitive, right? If, if you're cold because you're outside, the easiest way to get warm is to go inside. the body also regulates what you find. Pleasurable. So on a hot day, cool things are pleasurable. On a cold day, warm things are pleasurable, and that drives behavioral adjustment. It also drives changes in posture if you're feeling super hot, you're more likely to assume a sprawled posture spreading out arms and. Versus if you're cold, you tend to curl up to reduce your body surface area, protect your groin, your armpits, areas where the circulation is exposed, and try to retain your body heat. The second most effective and the most energetically cheap way to regulate our temperature is to control blood flow to the surface of the skin. You can think of this as a radiator. We have heat. We're producing heat all of the time. It's a metabolic byproduct We're actually trying to get rid of some of that heat to our environment. Most of us don't live in an environment that is warmer than 36 degrees Celsius or 98 degrees Fahrenheit, so we are constantly losing that heat to the environment. We can control how much of that happens by where we drive our blood flow. the body will direct blood flow to the surface of the skin. If we need to lose heat to the environment, and if we need to protect our core body temperature from cold, it will direct that blood flow away from the surface of the skin. Below the skin, we have a layer of subcutaneous fat that's an insulating layer. if you keep the blood below that layer, you're gonna help preserve your core body temperature. Whereas if you direct it above the layer, it will cause you to lose heat to the environment, assuming the environment is colder than you are. This can be a very effective response in very cold environments, bringing that blood below the subcutaneous fat layer is highly insulating Blood flow up towards the surface can actually approach zero in very cold conditions. So the difference between that skin temperature and the core body temperature can become pretty stark in a hot environment. You'll notice people become very flush. They get red in the face. Children, particularly because they're, they're sweating, sweating, responses are less than in adults. And that's blood coming up to the surface to try to increase heat loss to the environment. That leads us into the idea of sweating or evaporative cooling. the cardiovascular response, the driving of blood circulation is limited in very hot situations. You can only lose so much heat that way'cause we're talking about just kind of convective heat loss. So anytime the temperature becomes much higher where you have a higher activity level, such as exercise, we have to start engaging in evaporative heat loss, which is using sweating to put water on the surface of the skin that can then evaporate and wick heat away from the body. This is going to be more effective in drier climates, less effective in humid climates because. The tendency of water to evaporate is gonna be less in those settings. You can develop an adaptation. If you live in a hot environment for a long time. People who are adapted to heat will actually sweat more, produce more liquid, and that can be something like one to two liters of sweat per hour in a peak setting. This, is a very effective way to reduce core body temperature because water takes a lot of energy to undergo that phase transition. There are impairments of this effect in people who are quite young or quite old, kids who have not yet gone through puberty. Don't sweat as much as adults do. folks over the age of about 60 have less sweating less shivering and other such responses. they are less able to regulate their temperature in significant heat. That leads us to the idea of shivering, which I think is the conceptual opposite of sweating shivering, is using our metabolic energy to try to warm ourselves in response to a cold environment. What actually is shivering. Shivering is muscle contraction that isn't doing any work. the central nervous system triggers your muscles to fire in opposition to one another. So it would fire my bicep and my tricep at the same time and fire over, over, over, over, over again. The purpose of those repeated muscle contractions is that using our metabolic energy produces heat. So if you burn. Glucose to produce a TP, which is the energy currency that the cell uses 75% of that energy is actually wasted as heat into the environment. As soon as you do work, the remaining 25% is also released as heat into the environment, with the exception of any energy that turns into kinetic energy, movement of the limbs. shivering is a way that you can use calories to burn energy, start a fire to warm you up. It's worth knowing that if you get up and move around, activity can help warm you. but right away, as soon as you start moving, that actually suppresses shivering because obviously if your muscles are contracting involuntarily, you can't move effectively. you need a rigorous amount of activity to offset the gains that are lost when you suppress your shivering. you can also produce heat without shivering. That's called non shivering thermogenesis. And this is using what's called brown fat. Brown fat is a ki. Most of our fat is white, meaning that if you look at it under a microscope, the cells look clear and white, but brown fat looks. Huh Brown. And that's because it's full of mitochondria, typically referred to as the powerhouse of the cell. These are the metabolic factories of the cell. brown fat can take the energy that it stores as fat and burn it in these mitochondria to produce heat. It uncouples the burning of calories from the capture of energy. It says, yeah, we're not gonna capture any of that energy and just starts a bonfire to produce heat. This is most common in infants. Infants are not able to shiver, so they have a higher amount of brown fat, but it's preserved in adults across the back of the neck and across the shoulders, it's more common in individuals who spend a lot of time in the cold, particularly in cold water, you will increase the amount of brown fat that you have. Finally, the last main response that we have to regulate our temperature in the environment is using a hormonal response. So cold stimulus in particular, but any stress in general is going to increase levels of epinephrine. That's adrenaline for my European colleagues. Epinephrine increases predominantly during cold exposure, and it stimulates both increased baseline metabolism and also increases, nons shivering thermogenesis from our brown adipose tissue from our brown fat. There are other hormones which do contribute to the regulation of temperature. The main one there is going to be thyroid hormone, which. You know, it is not something that is going to change significantly in response to our environment. However, if someone has low thyroid hormone, they have hypothyroidism and they're not taking their thyroid medication, it will cause them to have a lower resting body temperature and impaired heat generation. The last thing that I want to touch on before we wrap up with this little fireside chat on thermo regulation is the idea of sex differences. I've mentioned that there are some differences across the lifespan in that infants and children don't sweat, infants in particular, cannot shiver, and that those responses get better as you get into childhood and into the teenage years when people start. Reaching peak ability to regulate their temperature. As you get older, you have less ability to shiver, less ability to regulate your vasodilation blood flow to the surface, reduced nons shivering thermogenesis, so less ability to regulate the temperatures at extremes of age. One might wonder whether there are sex differences. We have an intuitive thought that there might be sex differences. For example, my wife always complains that she's cold. When I feel warm, does that mean that we have different abilities to regulate our core body temperature? the answer is essentially no. women have a smaller blood volume thinner extremities, higher body fat, and lower lean body mass. However, in well-controlled trials, there is no difference in their ability to regulate core body temperature except at very extreme levels of exercise in heat. So at that level, we see a slight impairment in comparison to males. In contrast, pregnant women maintain a different core body temperature. over the course of pregnancy, the set point, the thermostat temperature of the pregnant woman slowly declines so that they have a lower core body temperature by the end of pregnancy. the purpose of that is to protect the fetus from heat stress. heat does bad things to a fetus in early pregnancy, fever or hyperthermia can induce birth defects because in early pregnancy, that's when you're forming your neurological system. Heat stress in the later phases of pregnancy can lead to low birth weights, but not birth defects. So when we come back to heat illness, we'll touch on this again, in women who are of reproductive age. Heat stress is something that we have to address effectively and early because women who are in the very early stages of pregnancy when their fetuses most vulnerable, might actually not know that they're pregnant. So something just to bookmark there. So that wraps up a quick run through on the principles of Thermo regulation to just touch back and review what we've gone over. We are homeo THMs. We're regulating our body temperature against the environment. We have a set point which we're targeting and our ability to regulate our temperature can be affected by metabolic stressors. Medications, age, many things. Our best ways to measure our core body temperature are using a tympanic thermometer or under the tongue in someone who's awake a rectal temperature's not bad. And in people who are. Completely obtunded. We can use an esophageal temperature or a Foley catheter temperature probe to measure the core temperature on an ongoing basis. When we think about exchanging heat with the environment, we're talking about conduction, touching things, convection, swirling liquids or gases, radiation, the glowing sun, and the glowing body and evaporation, using water to cool our body temperature. when we are creating a response to our environment, the brain is the one that's in charge that regulates the set point and uses different effector responses to protect itself from the environment. The best protection being change your environment, move, change your body position, go somewhere warm if you're cold. If you can't address that, then cardiovascular responses. Blood flow to the surface is very energetically cheap and is our main way of regulating core body temperature. But that doesn't work when it gets really hot. so then we're talking about sweating or if we're getting too cold, we're talking about shivering or non shivering thermogenesis. That's our brown fat bonfire. there are some minor changes that we get from hormonal responses as well, namely epinephrine, and we can have impaired temperature responses if we have hypothyroidism. there aren't big sex differences between men and women, except in the pregnant person who is going to have a progressively lower core temperature set point over the course of pregnancy and heat stress is particularly problematic in that setting. So I hope this was useful to you. Maybe you learned something. This will provide a good foundation moving forward as we talk about environmental illness and heat and cold stress. If anything was unclear to you, let me know. Reach out by email and I will clarify. This might be a good episode to bookmark and revisit when we get back to those. Episodes, but I will also rehash the pertinent physiology. When we get to hypothermia, for example, we'll talk about shivering, and we'll also talk about how as you reach the extremes of temperature as you become more hypothermic or my more hyperthermic, unfortunately, those conditions also cause impairment of the responses that are supposed to protect us from them. So if you get too hypothermic, you can't shiver, and that's a problem. That's it for this episode of Wilderness Medicine Updates. If you found this valuable, the best way that you can support the show is to share the episode with someone who you think might enjoy it. It doesn't have to be this episode. We don't have to start with nerdy. You can share the episode on the Safe back device on Avalanche resuscitation and physiology. On space blankets for tourniquet use. Whatever episode you think your friend, your fellow ski patroller, your fellow nurse, medical student doctor, backcountry ski partner, climbing partner, might appreciate sharing the show, puts it in front of more faces, more ears, and increases the reach of this little project. Also, if you've never given the show a rating on Apple Podcasts or on Spotify, please do so. If you have listened all the way through episode 26 or so, and you've never done this, just right now, open up the app, open up Spotify or Apple Podcast, tap on the show, go the show page, and quickly give it a five star rating. That helps us game the algorithm and move up the search rankings and get in front of more people. You don't have to type anything. It takes about two seconds and I really appreciate it. Thank you all for listening. Until next time, this is Dr. Patrick Fink. Stay fit, stay focused, and have fun.