Mark says he was in his kitchen Friday when he collapsed. That's when he suffered a series of what he says were "massive heart attacks." EMTs had to bring him back to life 4 separate times -- it was that touch and go.

According to Mark, he had a 100% blockage in his left anterior descending artery -- his LAD. That's also called a "widowmaker," and it was almost true to that name.

He credits his wife and the team at St. Joseph's Hospital with his survival, saying without them, "I wouldn't be writing this. My chances of survival were virtually nil."

As for his condition, another shock ... "I feel great. Humbled once more. Home tomorrow!"

As for reaction, his "Supernatural" costar Misha Collins said this -- "Mark! You don't need to do the most and biggest every time! 6 heart attacks? 2 or 3 would have been impressive enough. You've impressed us, okay. Now stop with this heal up and get back on the road with us. Love you, pal."