How does the Bhavanga State relate to the Jhanas? When we sit down and practice samadhi we would naturally want to enter absorbtion.But what if by practicing samatha we manage to calm and refine all the five senses,to the point where all five senses dissapear.

What's left would only be the mind sense with no outer stimuli from the five senses.

If this happens then won't the mind grasp the Bhavanga State? So my question is How does the Bhavanga State relate to the Jhanas?Are you suppose to go through one before entering the Jhanas.Or are you suppose to avoid lapsing into one in the first place.
Jhana is more than just the giving up of the five senses. Each of the jhanas is the result of very strong samadhi, and has specific factors that go along with it. For example, the first jhana has five factors: rapture, pleasure, initial application of mind, sustained application of mind, and oneness of mind. As one goes deeper into the second jhana, one leaves behind initial application and sustained application of mind, and so on.

Bhavanga is very different than this however. Bhavanga is like a blank slate. All it has are the mental factors that are necessary for mind to be there and nothing else. It only occurs in the space between other kinds of mindstate. None of the jhana factors are present at all, and there isn't even any meaningful degree of awareness in it.