Folks,
In a bit of a blast at me on a recent ASA post, Glenn cites a response
to a private e-mail conversation that I had with him.
In it he refers to research by Cabanac et al. and Caputa et al.. I
believe his representation of this literature is accurate and I thank
him for bringing it to my attention. I must confess I don't understand
it, since it does not seem to agree with the basic human physiology
that I know. Blood pumped by the heart flows generally unidirectionally
from arteries through capillary beds or bypass channels to veins. The
articles would seem to indicate that venous flow is reversed and not
that blood is simply shunted elsewhere. Moreover it is suggested that
this is a mechanism to help in the cooling process. The idea of cool
venous blood being used to cool arterial blood is, of course, standard
fare in rete mirables of creatures. Such cooling would also occur in
the brain between paired warm arteries and cooler veins but this appears
to be a different mechanism.
If someone understands the research or the brain circulation better than
me, could you clarify this for me? I got and read one of the original
articles but it did not clarify it for me that much. It would be an
interesting and novel mechanism, which so far has not gotten into my HAP
books (but then aqaporins haven't either).
I cite the references below.
Even if this research is wrong, Glenn is absolutely right that there
should be a correlation between the holes in the skull for blood vessels
and differing ability to cool the skin of the head. You would need more
capillary beds for more cooling. I just don't understand the reversal
in the cranial veins.
-- James and Florence Mahaffy 712 722-0381 (Home) 227 S. Main St. 712 722-6279 (Office) Sioux Center, IA 51250Below is Glenn's response of Sat Jul 24 22:11:01 1999 that he cited in his ASA post of Sun Nov 18 2001 - 20:50:30 EST > > "In both subjects the blood flowed rapidly from skin to brain during > hyperthermia; during hypothermia no flow was detected in one subject, > and in > the other blood flow was clearly reversed." M. Cabanac and H. Brinnel, > "Blood flow in the emissary veins of the human head during > hyperthermia," > Eur. J. Applied Physiol. (1985):54:172-176, P. 173 > > "Because of its relatively large mass the human brain needs to > be cooled > more than that of most other species. At rest, this is accomplished by > the > carotid blood; when the temperature of the artierial blood is raised, > the > brain is in jeopardy and there is need for a mechanism to cool the brain > directly. Caputa et al (1978) have shown that in humans the blood flows > in > the opthalmic vein according to a pattern compatible with a cooling of > the > brain during hyperthermia. When the subjects were hypothermic, the blood > flowed from brain to face, but when they were hyperthermic from face to > brain. Thus cool blood from the sweating face was directed towards the > sinus > cavernosus which therefore appears to be a heat exchanger between venous > blood cooled by peripheral skin and warm arterial blood. The emissary > veins > are quite numerous and do not possess valves, and the thermal dependence > of > the direction of blood flow in them coupled with the earlier results of > Caputa et al, on the flow in the ophthalmic vein, give clear indication > of a > possible mechanism for the cooling of the human brain during > hyperthermia." > M. Cabanac and H. Brinnel, "Blood flow in the emissary veins of the > human > head during hyperthermia," Eur. J. Applied Physiol. (1985):54:172-176, > P. > 175
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