why cant the body live indefinately

[nathansphase]

Why does the body eventually have to die naturally why cant cells just keep renewing?

[parkerweb]

Some cells in your body do keep regrowing and replace old ones e.g the skin or tissues in the gut.

However some cells cannot be replaced, like nerve tissue for example.

If, in the case of a stroke, where part of the brain (an organ composed of many nerves) no longer has blood supply, and dies, the nerves in that area will never re grow, (even if blood suppy if restored) so you loose the action of that part of the brain.
If its an important area of the brain then you may die; or you may just loose a specific ability (movement, feeling e.t.c.)

Also to consider life span, one must consider organs and how well they work.

The heart (another organ whos cells (myocytes) do not regenerate) over a period of time becomes less elastic, therfore each passing beat becomes harder, and may result in heart failure, or a host of other things (like high blood pressure) before this, reducing overall life span.

The lungs over a period of time collect may different pollutants and so do their job less efficiently.
Eventually a person may start to have breathing difficulties, leading to a host of problems, which can lower a persons life span also.
Cancer causing agents, or carcinogens, like cigarette smoke, once deposited can not only affect the lungs, but can cause untold problems all over the body.
Small cell carcinoma for example (which has a high mortality rate) can spread all over the body in it's end stages. It is primarily caused by cigarette smoke inhalation.

Obviously, genetic, lifestyle, and cultural factors will also affect the way a persons body works, and how well it's cells renew, but in general, a persons ability to renew his or her cells is generally goverened by how well each of the bodys systems (respiratory, digestive, nervous e.t.c.) are working, and how well they can keep work in sync.
Overall this will affect how long a persons body will

[ryepie]

Check these out:
http://en.wikipedia.org/wiki/Telomere
http://en.wikipedia.org/wiki/Telomerase#Aging

Basically most cells in your body have a finite lifetime and new cells must be produced all the time to replace them. This obviously requires your chromosones to make copies of themselves, as each cell contains your entire DNA 'library'.

Each time a copy is made, areas called Telomeres (found at the end of chromosones) are shortened, the copy process never goes all the way to the end of a chromosone for some reason. Telomeres are areas that act like buffers containing 'junk DNA', DNA that has no active purpose in the operation of our bodies, so losing bit of it have no detrimental effect on a cell's activities. Eventually though after many replications, the telomeres run out and the replication process begin to eat into useful DNA!

The answer to this is Telomerase, an enzyme which can rebulid the genetic buffer of the Telomere regions. This would likely have an impact on the longevity of a human body.

[jake-the-peg]

Answers are like busses nothing for days then 2 at once!

I was just about to answer when ryepie did.

There is also Programmed cell death

http://en.wikipedia.org/wiki/Programmed_cell_d eath

Remember a human being is only a gene's way of making another gene and having you live forever would not necessarily be advantageous to a gene

[ryepie]

I would recommend 'Genome' by Matt Ridley, an excellent, very accessible book which introduces concepts like this longevity topic, as well as genetic therapy, eugenics, implications for free will etc. And considering he is obviously in the Genetics camp it is also very balanced.

[rojash]

The real question is "How long do we need to live in order to learn how to spell indefinitely?"

[jno]

rojash, I believe modern science has proved that is indeed the most commonly misspelt word on AB

[nathansphase]

I hope when I die its not from pedantry! but seriously thanks for the explanations.

[jno]

Genes just want to reproduce themselves, so you're basically programmed to live long enough to have and raise a child; after that they don't care if you wear out. Even so, jake, why wouldn't it be advantageous to have you live forever and keep producing children? Wouldn't that give a gene an advantage over others?

[jake-the-peg]

Because it introduces stagnation.

Now I'm not an evolutionary biologist so feel free to shoot this down.

A gene exists within an organism in competition with other organisms. If that organism fails to evolve quickly enough it will evenually be out competed by other more evolved organisms and the gene will die with it's organism.

Of course evolution itself may distroy the gene but that's probably a much better risk than the near certainty of extinction that stagnation would involve

[jno]

I sort of see what you mean, jake... and yet people's reproductive lives (meaning women's) do seem to be gradually extending, at both ends, without apparent peril to anyone or, presumably, their genes. I'm not saying this could go on indefinitely; but perhaps those genes that switch off less quickly are beginning to make some competitive headway after all?

[freddie_jay2]

jake-the-peg has it more or less covered there. The science part is answered above but the reason we have to die is thats the way we're programmed. There's no real reason why the body has to Telomerase but evolution would be a whole lot different if we all just carried on living