So every human being is in heat balance. Meaning that normally our body temperature is fixed at around 37 degrees centigrade and we have exquisite regulatory mechanisms to accomplish that, and by heat balance, we mean that heat in equals heat out. We're at equilibrium. So we always have heat coming into our body. We always have heat going out of our body and our body has regulatory mechanisms for making sure that that stays in equilibrium. However, there are times when heat in is greater than heat out, and if that happens over a sustained period of actually not too long a time, our core body temperature begins to increase, and also there are times when heat in is less than heat out, and in those cases, our core body temperature starts to decrease. So a lot of what I'm going to talk about in thermal regulation is intuitive and obvious. Just from living, we know that these things happen. But I think it's good to kind of formalize and give a basic overview of how thermoregulation works. So first, what about heat in? Where does heat in come from? There are external sources. So radiation from the sun or other bodies that emit radiation such as a lamp. Also convection, which is transfer of heat from ambient air to the skin. Another mechanism or external source is conduction, but it's relatively minor, so I'm not going to deal with it. There are also internal sources of heat in, and this is heat generated by metabolism. So it turns out that 80 percent of energy production for muscle contraction is actually released as heat. So muscle contraction is a relatively inefficient process. We have a basal or resting metabolism that's a roughly 85 watts. So even if we're let's say lying in bed or sitting in a chair, not really moving around very much, our heart is still beating, we're still breathing, our cells are still metabolizing. There are other functions going on her body. So there's a basal or resting metabolism. We're always generating heat. However, there's muscle metabolism over and above basal metabolism once we're using our muscles. So for example, if we're jogging, we're generating 600 watts and just to give you an idea what that means. That means that would raise our core body temperature by one degree centigrade every 10 minutes in the absence of heat out. So where does heat out come from? So one source is transfer of heat from skin to ambient air. So that's just naturally occurring all the time. However, also as we'll get into in more detail in a few minutes, if the body finds it necessary to increase the amount of heat out, it has a mechanism for skin vasodilation, dilation of the skin of arteries. So that increases the skin blood flow and it transfers heat from the core to the skin, and that heat is then transferred from the skin to the ambient air. The second mechanism for heat out, we all know about is sweating, and the third is behavioral adaptation, and I'll go into all three of these mechanisms in more depth. So let's look at this chart. It shows the physiological responses to heat loading. By heat loading, I mean is that the body is subjected to a stress of increased heat in. So let's take a look at this. So we have the environmental or external heat that I mentioned coming from radiation or convection that contributes to the total body heat load. Then we also have the metabolic heat that I mentioned, coming from both basal metabolism and muscular work also contributes total body heat load. So let's say that we now place a heat stress on the body and it could be by going jogging for example. So that increases the heat load and it starts to increase body temperature, and that sends signals to the hypothalamic thermoregulatory center which is located right in the middle of the brain. So there are various sensors that we have on our skin and other parts of our body that are able to send those signals, and in fact, the sensors exists in the hypothalamus itself as well, and so then there's a response that is organized by the hypothalamic thermoregulation center. Let's not worry about inhibition of chemical thermogenesis or inhibition of shivering. But the two main responses I want to talk about are skin vasodilation and sweating that I mentioned, and the skin vasodilation results in radiation and convection loss of heat from the body. So decrease in body temperature and then the sweating through evaporation also decreases body temperature. Evaporation results in cooling. So I also want to point out that there's a cost to this mechanism which usually isn't a big cost. But it could become one and that's what leads to some heat related disease or illness, and when they're sweating, there's an increase in fluid and sodium potassium losses with sweating, and also with skin vasodilation, we see an increase in heart rate and cardiac output that in certain instances could be problematic. So here's a man who's both sweating and also his skin is red due to the vasodilation, and so with skin vasodilation, the determinants of the rate of heat transfer from the skin to the ambient air are the temperature of the skin minus the temperature of the ambient air. So the greater that differential, the greater the rate of heat transfer, and you know that from experience of course, and also air velocity such as wind speeds if we're outside, air velocity over the skin. So if we're outside, as you know from experience on a windy day, the wind will tend to blow away the heat that's coming off of your skin and that's helpful. So in essence, the heat transfer rate decreases with increasing ambient temperature and decreasing air velocity. So on a very still day as you know that's very hot, it's hard to transfer that heat out of our bodies. So here you see a man sweating and let's look at sweating. So with sweating, our skin becomes moist and we get evaporation and the reason there's cooling is that the skin heat is actually used to convert the liquid sweat to water vapor then evaporates. Determinants of the evaporation rate are the ambient humidity and again, air velocity over the skin. So the evaporation rate will decrease with increasing ambient humidity. So as you already know from experience, the higher the humidity, the less good the sweating does in terms of increasing heat out, and then evaporation rate also decreases with decreasing air velocity over the skin for obvious reasons. So this is a hot day and these people jumped into the swimming pool and that's a behavioral adaptation. So let's look at behavioral adaptations. So these are based on conscious perception of the thermal environment. So if it's a hot day, people perceived the heat. They might jump into a swimming pool. If they're standing in the sun, they might decide to move into the shade instead if they have that option. If a person is jogging and it's really too hot and they're starting to feel the heat, they might stop jogging, reduced their physical activity. If we're inside, we might decide to turn on the air conditioner or a fan, or we might take off our sweater. So and I'm sure you can think of many other behavioral adaptations. So now I wanted to turn to acclimatization or adaptation to local climates. Those could be behavioral or cultural such as the custom of taking a siesta in certain in Mexico and Central America, resting in the middle of the day in order to avoid the heat. The acclamation should be accomplished through the built environment such as maximizing cross ventilation in housing or it could be physiological. So let's talk briefly about the built environment. So this is a traditional house in Indonesia that utilizes passive cooling. So you can see it's well-ventilated. It's raised off the ground so that allows ventilation underneath the house. You see these shady verandas. It looks like you can't tell for sure, but there are probably windows on all four sides in order to get the cross ventilation, and so this kind of housing evolved in order to meet the needs of people living in a hot climate. So then there's also physiological acclimatization to chronic heat exposure, and so for people who are permanent residents in a hot climate, let's stick with Central America. So they're acclimatized just from having grown up there. Now, if living in New Haven, if I take a trip to Nicaragua let's say, I would need to get acclimatized and it generally takes several weeks. It's facilitated by exercise and also it's reversible. So let's say I take a month trip to Nicaragua. By the end of the trip, I would be acclimatized. But when I come back, that would start to decay over a several week period. It's actually quite remarkable what the body is able to do and people spend their lives trying to understand and study these mechanisms of acclimatization. So what happens is that you develop a lower core temperature among other things. I'm just listing some of the main ones, a higher sweat rate, reduced loss of sodium from sweat, expansion of your blood volume, and that allows more blood to be sent to the skin for the vasodilation, higher rate of skin blood flow, improved cardiovascular function, and improved aerobic exercise capacity.
