Concerning the fear and stress issue. Jewish law forbids us from causing the animal fear and stress. There is actually requirements about how the animal is prepared for the process. Further, it has to be done quickly w/o taking a lot of time give the animal the ability to be stressed by the procedure. Also, there are very strict requirements about what type of knife can be used and how it must be EXTREMELY sharp and checked in particular ways. Further, the process does initiate a precipitous drop in blood pressure and blood perfusion of the brain, bringing about a (permanent) loss of consciousness
Concerning human anatomy, you have to remember that humans are not kosher. Thus, if you look at the anatomy it has the same issue as non-kosher animals when it comes to blood flow to the brain.
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