Average temperature of blood versus core body temperature

Question

I have read the blood is warmer than the same body's core temperature. The core temperature is known to be 37 degrees, and I have been told the average blood temperature is 38 degrees. This was in my lecture slides. Another source I found was the book Nursing Practice: Knowledge and Care (page 570), which states the same thing, without any real explanation.

Is there a known purpose/mechanism behind this, or is it more a side-effect of another process? I haven't been able to find any reliable sources (only quora etc.)

Possible reasons I came up with were:

• Blood is responsible for heat loss (keywords: capillaries, vasodilation)
  • This doesn't make sense to me as the blood can never get hotter than the surface/system from which it is absorbing the heat (e.g. internal organs)
• Blood flows. This could generate some extra warmth due to (accumulative) frictional forces between the blood and the vessel walls.
  • This makes most sense to me, as it is a major difference between the blood and the organs, when considering their temperatures, ceteris paribus. It makes sense if looking from the perspective of simple heat transfer and thermodynamics.
• These are all just averages, and so there isn't a single answer, just "it depends..."

I am not really interested in the argument that blood feels warm on your (relatively) colder hands.

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Perhaps a separate question; if movement and work are taken into account, do the walls of the heart become warmer as the heart beats faster? Do we neglect the temperature increase of a hard-working liver as an exception to "core temperature"?

Any sources/further reading welcome!

Answer 1

1) Where are you measuring the temperature for the "blood" temperature versus "core" temperature?

Blood is used, as you have mentioned, to transmit excess heat to superficial capillary beds, but in cases of hypothermia blood is preferentially directed towards the vital organs at the expense of the periphery. If you are measuring the core temperature (typically bladder temperature in someone critically ill, but could also be rectal or esophageal), then that is the temperature of that body area. All of the fluid within that area (inside and outside of the cells of the organs and tissues, including blood) is at approximately that temperature. If you are measuring temperature peripherally, you may get vastly different readings depending upon physical activity, environmental conditions, etc.

2) Heat is generated through metabolism. In a very controlled manner, we do actually "burn" the food we eat into energy, carbon dioxide, water, and some other minor compounds during starvation when protein is broken down for energy. When at rest from a physical activity standpoint, the vast majority of this energy is generated in the "core" and typically this is the temperature our body attempts to regulate with relatively small variation. Water moving throughout the body conducts this heat from relatively warmer areas to relatively cooler areas, essentially equalizing this temperature within the "core" compartment.

3) If friction in the blood was to be a significant source of heat then, given how delicate the solid components of blood are, people could not continue to exist. Increased physical force, due to microscopic fibrin thrombus formation, causes utter destruction of the red blood cells during disseminated intravascular coagulation (DIC), which leads to the typical microscopic appearance on a blood smear (shattered red blood cells or schistocytes).