Heat and Thermoregulation

00:01 In this lecture, we’re gonna talk about heat related emergencies.

00:04 Specifically, the life-threatening heat related emergency of heatstroke.

00:08 So under normal circumstances, heat production in the body is equal to heat dissipation.

00:16 That’s what allows us to maintain a constant core body temperature.

00:20 However, when we produce more heat than we’re able to dissipate, that’s gonna cause the core temperature to rise and that’s the physiologic basis of heatstroke, heat exhaustion, and other heat related illnesses.

00:34 Under normal circumstances, our basal metabolism, even without any skeletal muscle activity is about 100 kilocalories per hour and if we didn’t have mechanisms to dissipate heat from the body, our core temperature would rise one degree per hour.

00:50 When we add in skeletal muscle activity like exercise, that actually can increase our heat production up to 20 fold.

00:58 So bottom-line as humans is that we make a lot of heat and we need to have mechanisms to get rid of it.

01:04 Heat production is further influenced by metabolic factors.

01:09 Things like hyperthyroidism will increase your heat production, medications like sympathomimetics and anticholinergics also increase heat production, convulsions, fever, and of course, environmental heat stress.

01:22 These are all things that will add heat into the system.

01:25 So we need to have a way to get rid of it and there’s four major physiologic mechanisms for dissipating heat.

01:32 The first one is conduction.

01:33 Conduction is transfer of heat from the body by direct contact with some other surface.

01:39 This is pretty minimal in air.

01:41 Air’s a pretty good insulator but when you’re in water, there’s a lot of conductive heat loss from the body because water has a very high heat capacity and the body can dissipate a lot of heat, placing it into the water as a reservoir.

01:57 Convection is the next major source of heat dissipation and that’s transfer of heat via air circulations.

02:05 So basically moving air, carrying heat away from the body.

02:08 Now, this is gonna vary directly with wind velocity and this is why you feel cooler when you have a fan blowing on you than you do on a still day, even if the temperature is the same.

02:19 Radiation is another mechanism of heat dissipation.

02:24 This is the transfer of heat via electromagnetic waves out of the body.

02:29 And this is bidirectional, so we can absorb radiant heat from the sun and we can also radiate heat out away from our bodies.

02:38 This can actually result in that heat gain in hot environments.

02:42 So if you’re in the sun, in the heat, you will absorb more radiation than you will get rid of.

02:48 So this is a bidirectional process that can actually make things worse.

02:52 The last mechanism for heat dissipation is evaporation.

02:56 And this is the transfer of heat via conversion of liquid to gas.

03:00 So basically, when liquid on the skin evaporates, it decreases the body temperature by consuming heat.

03:09 So every cc of sweat that evaporates from the surface of the skin consumes just a little over half a kilocalorie.

03:17 This is the dominant mechanism of heat loss in hot environments and this is why when you’re in a hot environment, your body’s natural physiologic response is to sweat because it significantly increases your heat dissipation and helps maintain a constant core body temperature.

03:34 So thermoregulation is taken care of by the central nervous system.

03:39 So we have central heat sensors that monitor the core body temperature and temperature inputs are monitored by the hypothalamus.

03:47 So basically, the hypothalamus gets input from the temperature sensors about whether the core temperature’s rising or falling and it precedes accordingly.

03:57 When the temperature is increasing, we have thermoregulatory mechanisms that are activated to try to help prevent the body’s core temperature from rising.

04:10 So in particular, the two things we do are one, we vasodilate, and two, we sweat.

04:16 Now, when we live in a hot environment or we’re exposed to repeated heat stress, we actually acclimatize which is why on the first day of your vacation in a tropical place, you feel really hot and run down, but by the end, you feel pretty good because you acclimatize to the warm environment.

04:37 We do this by increasing sweat volume and also, decreasing sodium concentration so you don’t waste as much salt when you sweat but you actually get better evaporative benefit, and we also increase our plasma volume to compensate for the vasodilation associated with heat.