What, physiologically, makes us hot and sweaty when we exercise? Obviously the sweat is the body’s way of cooling down, but what creates the heat in the first place?
AFAIK, it’s a byproduct of the energy used to produce movement.
The body’s thermoregulatory processes like vasodilation and sweating, as you noted, are used to dissipate the heat.
You are breaking chemical bonds to release energy. As everything works, heat is released as a by-product. Your auto engine heats up because of the chemical reaction when fuel is converted into energy. The body is truly an engine in that energy is released to power the machines (muscles).
Simplified answer: your body produces energy by oxidizing (burning) fuel (food, & food stored as fat). Heat is a by-product of this.
During exercise, that is – sometimes your body oxidizes fuel just for the purpose of keeping you warm. That’s why we are called 'warm-blooded animals.)
Contrary to the previous answers, it’s not primarily due to breaking of chemical bonds, nor is it really comparable to the heat produced in a car engine.
The human body is not a heat engine. While the process of oxidation produces some waste heat, it is a relatively minor component to the excess heat produced by exercise. A car engine works by heating gases, it is the heating of the gases that does the work. In contrast muscles are electrochemical motors. While heat is released by the oxidation of ATP, it’s an entirely accidental byproduct. Most of the heat produced by working muscles is simply friction heat. The muscles consist of multiple fibres, all sliding past each other, and multiple attachment points, all ratcheting across each other, the amount of friction is quite impressive.
Also worth noting, adult humans never “oxidize fuel just for the purpose of keeping you warm”. Bats can do that. Babies can do that. Adult humans can’t do that. We lose that ability at the age of about 6 months. After that age, if we want to generate heat to keep ourselves warm we have to do it by inducing friction in the muscles. We call that shivering. As adults we simply have no tissues that are capable of oxidize fuel just for the purpose of keeping us warm
That’s not true, Blake. Babies have much more brown fat than adults as they do not shiver and need it to survive, but it’s more recently been found that thin adults (especially women, who have less muscle mass) have brown fat too. And we can increase the amount and activity of our brown fat by exposing ourselves to cold.
I’m pretty sure I have a lot more brown adipose tissue than the average adult because I am thin as a pin with low bodyfat yet I seem to ‘burn off’ everything extra I take in as body heat (the skin of my torso gets hot to the touch after large meals). Also I seldom feel cold and am very sensitive to heat. I do not have hyperthyroidism, but many people with it have a similar metabolism and have been shown to have very large brown fat deposits.
When you convert energy from one form to another you nearly always get heat too. If you convert chemical to kinetic or electrical to light there is always some that is lost as heat. This is essentially the efficiency of something. For example the light bulbs we used to use had poor efficiencies. You turn them on and in no time at all there were too hot to touch. The energy was being converted from electrical to light and and some was being lost as heat. Now we have energy saver light bulbs where a lot less is converted to heat. Most of it is converted to light like it should be. So they are advertised as: 20W (equivalent to 100W). They convert less of the energy to heat (energy lost) and more of it too its intended purpose(light).
Every mechanical or electrical motor has its efficiency rating. Technically I’m sure some of the energy is lost as sound as well in a mechanical motor but this makes up such a small amount that the energy lost is nearly all heat.
I cant think of any example of macro energy conversion where heat is NOT given off. It comes out of every energy conversion I can think of.
The body’s efficiency is described here
Every loss will have heat given off. While these seem like low energy conversion rates I’m sure the reason it prevails was that giving off heat is a big advantage. Keeps me warm so I can sit at computer for hours without much strenuous movement.