In the late 80’s early 90’s I was coaching some 14 to 18 year olds, there was a heat wave, I took some athletes to the swimming pool for training in preference to the track. We were doing sets of ? holding your breath and going as far as possible underwater. After a couple of months everyone could do 6 x 25m on a walkback.
When I was doing the old level I was asked by an AIS coach how many died. It was good for a laugh. All the kids are now in their 30’s. One of them made the comment that if the pool was 3m deep he could 50m every time,
If you don’t to increase blood flow then an alternarive is to increase the octane in it.
Worked for me.