The standard answer is that a metabolic hyperemia (increased blood flow)
to the gastrointestinal tract "steals" blood flow that would otherwise
go to the brain. While there is no doubt that blood flow to the GI tract
increases during digestion (caused by metabolic signals to arteriolar
smooth muscle in the gut wall), it has always seemed to me that cardiac
output would easily increase to keep arterial pressure constant and thus
keep brain blood flow constant.
So I see two possibilities that may account for the sleepiness after
eating: 1) the vasodilation (of small arteries in the GI tract) is so
great that arterial pressure is, in fact, compromised thus reducing
brain blood flow, or 2) some of the sugars and fatty acids (or other
foodstuff molecules or the hormones that respond to them) derived from
the meal modulate CNS function in a way that makes us feel sleepy.
The textbooks preach 1), but I am one cardiovascular physiologist who
thinks 2) is more likely.
You might post this question in bionet.biology.cardiovascular.
Bob
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Robert D. Phair, Ph.D. rphair at ix.netcom.com
BioInformatics Services http://www.webcom.com/rphair
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