In my conclusion (so far):
Blood flow is important for cooling the brain (as well as the core of the body). One review says:
[Cerebral blood flow] CBF is most critical for maintaining and stabilizing the thermal environment of the brain. Under normal conditions, cerebral tissue beyond 2–3 cm from the cortical surface remains unaffected by changes in ambient temperature surface without simultaneous temperature changes in the perfusing blood (Stone et al., 1997). Core brain temperature is generally higher than body temperature; with blood temperature in the jugular vein higher than in the carotid artery (Nunneley and Nelson, 1994), CBF primarily contributes to heat removal from brain tissue. The superficial parts of the brain, however, are much more susceptible to the ambient temperature and may be cooler than arterial blood, particularly in neonates and infants (Iwata et al., 2014). Primate studies demonstrated that shifts in temperature, 5–7°C on either side of the neutral zone (28 and 32°C), did not affect deep brain structures, while superficial sites and CSF of the basal subarachnoid space were reported to change (Hayward and Baker, 1968). CBF, therefore, is critical in maintaining intracranial thermal homeostasis by reducing temperature in the deep brain but sometimes increasing temperature in the superficial brain (Iwata et al., 2014).
Frankly I'm surprised that apparently no paper has tried to estimate the amount of heat removed from the skull just by the jugular vein, since there's a difference in temperature with the carotid artery, and the blood flow is known as well. This could be a decent first-order approximation of how much the rest of the body acts as a radiator for the head. On the other hand, there's something else that has been studied intensely...
In many non-human mammals and birds there's evidence for a mechanism that allows the brain to be cooler than the rest of the body. This mechanism, called SBC (selective brain cooling) has actually several flavors, but all seems to involve just head-level "radiators" like the nose.
There was no direct evidence (as of 2004) that SBC exists/works in humans, although some speculation based exist based anatomical considerations... as well as experimental analogy with extreme body temperatures observed across species:
Actually, artiodactyls, which have the most effective mechanism of SBC, are able to tolerate extremely high body temperatures. Taylor (1970) has shown that heat stressed and water deprived Grant’s gazelles are able to maintain rectal temperature at 46.5C for as much as 6 h with no observable ill effects. Surprisingly, rectal temperatures as high as 46.5C (Khogali and Mustafa, 1984) and 47.0C (Hart et al., 1982) have been recorded also in humans who survived heat stroke. If SBC were absent, such high temperatures would be inevitably lethal.
All SBC info from above and images from
- Michał Caputa "Selective brain cooling: a multiple regulatory mechanism", https://doi.org/10.1016/j.jtherbio.2004.08.079
Until 2-3 years it seems there was no way to measure/image brain temperature in a similar way to how blood flow is. But that has changed with the introduction or MR thermometry, which is non-invasive method. One of the recent papers on this notes:
Brain thermoregulation remains an enigmatic process reflecting the complex interplay of metabolism and cerebrovascular perfusion.
Although not trying to answer systemic "where the heat goes" questions, the paper has some interesting findings, namely that after a stroke brain temperature (in monkeys) raises above systemic temperature. That finding raises more questions than it answers with respect to brain thermoregulation mechanisms, but here is the eye candy of MR thermoimaging:
The paper goes into some detail into hypotheses of why this raise happens, how much it contributes to stroke propagation etc., but there aren't any firm answers. What is clear is that the brain can lose its ability to thermoregulate, relative to the body.