如何在MVC中更新模型

I'm wondering how you keep a constant value of the Model in an ASP.NET MVC framework. Like when adding something to the Model through the view. You go back to an action in the controller but where do you keep the Model? Is it a private in the controller? or is it passed back and forth from the view to the controller because if it gets big then you are passing alot of data back and forth to add/delete a single item from the Model.I'm wondering how you keep a constant value of