Bodies Heating

﷽
Salawat

Through a forgotten path, I came to the question: How are bodies actually heated? I was delighted and blindsided upon realizing "I have no idea how"

Unrequested, I present to you my dive into correcting my own ignorance.

"[...]body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles."

Ok, how? Friction heating? The burning of energy on the cellular level? Intuitionally it makes sense that contraction make heat; until you think about it more--after which it makes less sense the more you think about it. How infuriatingly typical of thermodynamics.

Obviously, at the bottom of the stack, entropy is lossy, and heat is the primary vehicle of loss. It's the middle bit that I don't understand. (!) If it's just the staying alive bit that keeps us warm, then why aren't reptiles able to generate body heat too? (If this question intrigues you too, don't worry, there's no answer at the end.)

Below, you'll find excerpts in quotations and summarizations in parenthesis, as I translate the lay of smart people into something I can understand. The following excerpts are from the first of the two National Library of Medicine books you'll find linked at the end of my shoddily worded, intellectual head scratching. The selections are explanations of the physiology of "Temperature Increasing Mechanisms"

"Stimulation of the posterior hypothalamic sympathetic centers causes vasoconstriction of skin blood vessels."
(Heat sensors located in the back of the brain make the blood vessels in the skin constrict when they notice the body is getting cold)

"Increase in thermogenesis by promoting shivering, sympathetic excitation of heat production, and thyroxine secretion" (This is so specific, and yet so vague. Translating, we find: Increases heat production by shivering, turning on the heater, and thyroxine. How does the body's heater work? What's thyroxine and what does it do? )

Thyroxine: "Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. (...) Thyroid hormones play vital roles in regulating the body’s metabolic rate, heart, digestive function, muscle control, brain development and maintenance of bones." (Aright, presumably, this thyroxine (Who does it even think it is, huh?) aids in heating the body by way of speeding up the metabolic rate--The biological equivalent of throwing more logs on the fire. )

Now to figure out the bodies' heater -->>> "Heating via metabolic heat production"

Helpful!

Now how does the metabolic heat production work? The following emphasis is added by me to help the reader in extracting the pertinent details, and the details themselves are left for the more invested (and biologically inclined) reader.

"(Heat generation at a) Cellular Level:
The mitochondria control cellular metabolism and produce most of the heat in the body. This control is implemented with the help of the tricarboxylic acid cycle (TCA cycle) and the Electron Transport Chain. The TCA cycle, also known as the Kreb’s Cycle or Citric Acid cycle, is a central driver of cellular respiration and takes place in the matrix of the mitochondria. The TCA cycle begins with its substrates, Acetyl-CoA, and oxaloacetate derived from pyruvate, the end product of glycolysis. Besides glucose, the beta-oxidation of fats is another major source of Acetyl CoA. These two substrates are funneled into the TCA cycle, and undergo a series of redox reactions producing various intermediates and result in the formation of high energy bonds in the form of NADH and FADH2. NADH and FADH2 undergo oxidation at the Complex I and II of the electron transport chain respectively and participate in the oxidative phosphorylation process to generate ATP molecules. The process of oxidative phosphorylation occurs at the inner mitochondrial membrane, consumes oxygen, and phosphorylates ADP to form ATP molecules. Finally, this ATP is then used to power various molecular reactions throughout the body which creates heat. ATP hydrolysis is an exergonic process where the phosphate bonds are broken to release energy in the form of heat.
The inner mitochondrial membrane also contains a unique uncoupling protein (UCP-1), also known as thermogenin that is involved in regulating the inner membrane permeability and the resultant generation of heat [6][7]. "
[...]
Up to 60% of the heat generated during metabolic processes is used to maintain body temperature.”

Summary: The body heats via metabolic heat production -->> The heat comes from burning the human fuel of choice: ATP.

Now....how does the body move the heat generated to where the heat is needed? Because in the first line-->"body heat is generated mostly in the deep organs," But the stuff that gets cold first are all the outer bits. My guess is heat transfers via blood (acting as radiator fluid)?

(I told you there was no satisfaction on the reptilian front)

https://en.m.wikipedia.org/wiki/Thermoregulation_in_humans
https://www.ncbi.nlm.nih.gov/books/NBK499843/
https://www.yourhormones.info/hormones/thyroxine/
https://www.khanacademy.org/science/ap-biology/ecology-ap/energy-flow-through-ecosystems/a/animal-temperature-regulation-strategies

https://www.ncbi.nlm.nih.gov/books/NBK538294/