I've never fainted but the couple of occasions when I felt faint interested me.
As a student at Melbourne Uni I was a frequent user of the Uni Bookshop. I spent the time from late 1968 to early 1970 away in Antarctica and, when I first went past the bookshop after I'd returned, I noticed they'd done a refurbishment. So I went inside and browsed. While browsing I noticed the St John Ambulance mob had put out a new edition of their First Aid handbook, an octavo-sized edition about 3/4" thick. Now, I had had my father's edition of the same handbook for a few years and, having been issued to him in the AIF during WWII, it was small enough to fit into a shirt pocket; I wondered what had been changed to warrant the increased size.
It turned out that it was almost exactly the same except for (1) new versions of the illustrations that were now full-page and full-colour and (2) the addition of three new chapters, one of which was setting up an emergency field hospital, one was on Triage, and one was on Emergency Childbirth. I swiftly read these (in the shop, between the stacks and standing up) and, when I got to the bit about cutting the umbilical cord, was almost overwhelmed by the sensation of "about to faint"; I had to support myself against the stacks to stop falling over.
This amazed me because I'd grown up exposed to the delivering of cows, ewes, bitches and moggies and never had any such reaction. About a year later I was again in the Uni Bookshop looking for something else when I again noticed the First Aid manual. Wondering whether it would induce the same response I read the Emergency Childbirth chapter again. Sure enough, when I got to cutting the umbilical cord, I had the same response, diminished this time.
Interestingly, when it came to assisting at a childbirth (a "home delivery" of a friend of my partner) I was far too involved to even feel such a response, let alone indulge it. Ditto when it came to assisting my own partner at the births of our daughters there was no such response detected.