That man in the video is literally a decomposing body with a beating heart. All organs fail and the heart is one of the last to go in all the cases I've read up on. It would be better for these people if it was the first to die off.
This is the chromosomes of Hisashi Ouchi. He was exposed to around 10,000 to 20,000 millisieverts, which is highest radiation exposure a human has ever had to date. 8,000 mSv is 100% fatal. As you can see, the chromosomes are completely destroyed. Doctors couldn't even tell which one was which.
For reference, this is what chromosomes normally look like in a healthy person.
With your chromosomes destroyed, your cells cannot replicate anymore and your body starts to decay. If any managed to survive, the new cells that form are going to be cancerous tumors. Your cells frequently die and replication is vital.
You literally become a decomposing body. First to go are organs that replicate the fastest, the first being your intestines.
They tried to do skin graphs but new skin cannot grow onto dead or dying tissue. It didn't take.
Hisashi Ouchi lived for 83 days like this, but luckily the last week or two he stopped responding to stimulus.
If this interests you, I highly recommend the book
A Slow Death: 83 Days of Radiation Sickness. It's the best retelling of Hisashi's medical treatment, and no the doctors weren't sadistic like most think. They were told by the family to keep treating him, they worked together hoping for a miracle. The family basically lived at the hospital. This being the first accident in Japan's history, they had to make up medical treatments based on outdated treatments from the 1940s and 1980s, that's what they had to work with. They really tried to save him, but unfortunately this high of an exposure is 100% fatal.
The book is based on a documentary by NHK-TV, which I have searched for many times but can't find. Tried the official website, tried searching in Japanese but I can't for the life of me find it. I want to watch it so bad. He has been dehumanized so much and I would do anything to watch footage of him and see what he's like. Hisashi has been described as a good person, telling jokes with nurses and was really positive during the start of this ordeal. It's now been lost to time or censorships, sadly.
Also this photo is not him! It gets shared around constantly with his story and its misinformation. Hisashi never had an amputation and it would have made his health a lot worse if they did. This is of a burn victim after a debridement.