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I'm working on a character who can emit fire from her arms and forearms. I read in a previous question that an increased amount of capillaries can increase the body's ability to carry heat away. If this is true, what is the downside to having as many as 5x as many capillaries in your hands and forearms?

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There are a lot of possible downsides that come to mind, some dependent (but not much) on how the capillaries are distributed. There are 60,000 miles of capillaries in the body, with muscles, lungs, and metabolic organs having the most, but synovial membranes (what lines the joints) are well supplied with capillaries, and the hand has a large number of joints, Hamds and feet also have more capillaries for the purpose of heat maintenance and dissipation.

  • Her hands would be significantly larger and somewhat swollen from normal capillary leakage, making fine motor skills slightly more difficult and making her stand out from others noticeably.
  • Her hands would be significantly redder, making identification of her uniqueness easier. A thicker epidermal layer would hide this, but might interfere with heat transmission (fire from hands) and movement.
  • Your character would lose a lot of heat from her hands. That means in order to keep one's body temperature in the normal range, you would need to generate a more core heat. Different organs respond differently to above-normal temperatures, and this might be a problem at times, especially for her brain. Wearing gloves might help, but would hamper her ability to use her hands a bit further. Conversely, dissipating heat in warmer temperatures would also be hampered.
  • Her heart would need to work slightly harder to keep blood moving through those capillaries. Her arms would need larger arteries and veins. Extremity injuries are very common, and the larger the blood vessel, the greater the blood loss (or in unbroken skin, the larger the hematoma).
  • The risk of significant bleeding from a hand injury is normally fairly low (unless it's a larger artery that is cut neatly). This would increase in your character. Cutting one's hands/fingers is as common as dirt making a hand injury about as bloody as a scalp injury. People can bleed out from a significant scalp injury, which would be somewhat more mitigated in the hand because one can apply direct pressure to the injury or, if necessary, apply a tourniquet, whereas it's harder to do those things with large scalp injuries (a tourniquet would strangle the injured party.) Almost any hand injury would be made worse (think of hitting your thumb with a hammer and having it swell a lot more than it normally does, which would also make it more painful.)

Those are the problems that come immediately to mind. I'm sure with some digging, more can be found.

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It's not likely to kill you

Your heart rate will rise as you try to maintain blood pressure. The capillaries are where arteries become veins. Which means the blood is turning around and heading back to your heart. Having more capillaries means you're demanding more work from that heart.

But as with many things it's really about how much. 5x seems like a lot but you're limiting it to hands and forearms so it's not 5x the whole body.

The human body comes in many varieties so it is designed to be fairly tolerant. For some perspective on how tolerant consider these words:

"Every pound of weight we put on is 5 miles of blood vessels. If your heart beats 100,000 times a day, that's 500,000 miles a day for one pound of fat," says Dr. Kopecky. "So you do the math. If you're 10 pounds overweight, it's a lot and your heart gets tired. The blood pressure goes up. The heart attack rates go up, etc."

Mayo Clinic - fat is not inert

As far as your heart is concerned, you now have 10 hands and forearms. According to de Leva, 1996 that adds about 15.52% to your effective body weight: 8 x (0.56 + 1.38).

While it isn't exactly heart healthy, it's not instant death.

But it will give you a noticeable birth mark

You've got a Capillary Malformation. This will discolor the skin.

CMs are commonly known as port wine stains. They look like a pink, red or purple patch of skin and occur in 1 in 300 newborns. CMs are present at birth and can occur anywhere on the skin.

It is very rare to develop CMs as an adult. These patches may be light in color at first, but can get darker over time. In adults, CMs can thicken and develop bumps or ‘blebs’ that can sometimes bleed.

Hopkins Medicine - Capillary Malformation

While you can dissipate more heat, you are likely to favor wearing gloves and long sleeves. Not only keeps you warm but prevents the need to BBQ anyone who feels like teasing the girl with red arms.

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  • $\begingroup$ A bit nit picky (just FYI, since it's a main point) , but port wine birthmarks (PWB) don't occur with 5x the normal number of capillaries. Normally capillaries are under 10 microns in diameter (liver capillaries are larger (~30 microns). In PWB, the capillaries are malformed, with diameters ranging from 10 microns nearest the skin surface to ~500 microns deeper into the skin. That's 50 times the size of normal skin capillaries, which causes blood to pool in them, causing the coloration. $\endgroup$ Commented Dec 10, 2022 at 22:01

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