biology and physiology

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Irreversible?

Are you a budding Dr. Frankenstein?

Interesting question. Hope you don't mind if I audit the thread.

Mostly, it's the putrification of body cells as they die due to the lack of oxygen. As the cells break down, the internal system breakdown.

It's not an immediate effect, which is why some people that have drowned in very cold water can be brought back. The cold slows the effect of death due to oxygen starvation.

The multi-cellular structure is supported by blood chemistry, and blood chemistry is maintained by functional aspects of the multi-cellular structure.

For example, breathing is required to infuse the blood with oxygen which in turn supports the cells of the body (some of which allow breathing).

The fuel required to maintain the cells must be renewed regularly. Once the cycle is broken basic metabolic functions deteriorate and at some point the system becomes so damaged that the cycle can not be restarted again.

From your question, I assume you are trying to isolate the particular "breakage" which prevents an organism from restarting the life cycle again. The simplest answer to that is "cell damage" (to any one, or multiple cellular systems).