Natural human insulin circulating in the bloodstream only exists as a single molecule. It carries out its essential role in blood glucose level regulation by acting as a key, to permit glucose uptake into cells. Commercially available insulins are supplied in vials of concentrated solution. As a result, small insulin molecules cluster together to form a larger insulin molecule. After administration under the skin (subcutaneous delivery), this larger molecule needs to be broken down into smaller insulin molecules to enter the bloodstream more readily.