A right is not the same as a guarantee.  A proper and true right however is inalienable or inseparable from our individual humanity and our individual existence.

If a man is very ill and his disease is such that he will die within a few hours, he still has a right to his own life.

We aren't saying that he is guaranteed life.  We are simply saying that his is morally justified in continuing to live if he can.

A right can be thus equated to an opportunity.  A right does not guarantee an outcome.

But if a man can lose his life, how can we say that his right to live is inalienable?

This is a very sophisticated and abstract level of thinking, and you may have to stand on your mental tiptoes to fully comprehend the moral concept.

Although a man might lose his life (cease to live), he cannot and does not lose or relinquish his RIGHT to life.

One man can kill another man, or through his actions cause another man to die, but he cannot alienate the other man's right to life, although he might alienate that life.

The property of that other man's life is not yours and it cannot be yours.  No man can ever own another (although he might pretend to). 

You can rob a man of his life (kill him), but you can never acquire it.  The other man's life can never be transferred to you. 

If one man kills another, the killer does not acquire the life he has ended.  The killer has a right to only one life - he can never live two lives.