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Sorry if this question seems strange, but in the recent time I have been interested in the question of what life is and how you can define life.

My question: How long can individual cells live on a human body from the point the human dies? When a human dies, legally he is dead, but how is the medical explaination? Can a human be partly dead, that the heart and brain has stopped, but the cells on the body are still alive?

When you talk about a human life, is it one life or could you consider each cell on the body as a living piece and the whole human body itself including the consciousness is just a container for the cells?

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    $\begingroup$ Or: "Is Henrietta Lacks still alive?" $\endgroup$ – nico Feb 3 '13 at 13:33
  • $\begingroup$ This is a question of emergence $\endgroup$ – shigeta Feb 5 '13 at 17:20
  • $\begingroup$ I'd love to see answers to the specific questions 'how long can individual cells live in a human after death' (in normal circumstances - HeLa excluded :P) I'm thinking alveolar squamous epithelium might do fairly well in terms of maintaining oxygen supply. $\endgroup$ – Rory M Feb 5 '13 at 18:43
  • $\begingroup$ Humans are social animals. If you entirely isolate a human from all human contact (e.g., solitary confinement in prison, or lone shipwreck survivor on deserted island) for years at a time, usually the person will go insane, to some degree. So, can you say that an individual human is alive? Certainly, no individual human can reproduce, thus failing one of the usual criteria for life. $\endgroup$ – dmm Oct 18 '13 at 16:32
  • $\begingroup$ After heart dies, unless oxygen and nutrients can be supplied, cells will die within a short time. $\endgroup$ – One Face Mar 5 '15 at 12:35
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The definition of life is the main issue here. That is one of the great philosophical questions in Biology (and not only Biology) and is way beyond the simple Q&A format of this site. However, most definitions of life can be boiled down to1:

A self replicating entity that interacts2 with its environment.

Or, even more simply [1]:

Life is self-reproduction with variations

In both cases, the answer to your question depends on which level you are focusing on, your resolution if you like. If you take a human cell and apply either of the definitions above, it is alive. A cell is capable of autonomous reproduction, and metabolizes substances harvested from its environment. The same holds true of you look at the human body. However, the definition will not hold for a part of the human body, for example an arm or kidney.

Consider also that a human cell is capable of making more copies of itself, more individual cells. A human body is also capable of making more copies of itself, more human bodies. They can, therefore, both be taken as alive.

There are even more complex examples like Dictyostelium discoideum an amoeba that can exist (and be alive) either as a single cell or as a colony sporting different cell types and rudimentary organization. However, the organism is alive in both of these stages. In one case, you would consider the cell to be the fundamental unit of D. discoideum life, and in the other you could consider the colony.

So, to answer your question both a human cell (as long as it is alive, actively metabolising and capable of reproducing) and the entire human body can be taken as alive. That my cells are alive does not lessen the fact that I am too. It is just a matter of perspective.

References

  1. Trifonov EN., Vocabulary of definitions of life suggests a definition. J Biomol Struct Dyn. 2011 Oct;29(2):259-66.

1 Yes, I know this is a simplification, I just don't want to be drawn in to a long winded discussion on what the definition of life is. My answer is applicable to any of them really.
2 Metabolism can be seen as a form of interaction.

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