This question will be highly related with this other How could an efficient respiratory system evolve for giant arthropods?, because both systems do an important and combined contribution to oxygenation of the animals tissues.
The circulatory system of arthropods is open, that is, there is no closed circuit of vessels through which a differentiated fluid circulates, which properly could be called blood, arthropods don't have specialized cells to transport oxygen, and the liquid which transport nutrients and oxygen is called hemolynpha. The hemolynpha is moved throughout the body by different muscles which push that till disembogue to each organ.
The two closest groups to arthropods that could evolve a closed circulatory system are anelids or worms and cephalopods, so can be possible that a common ancestor with no closed circulatory system has developed one.
So, from the current structures that the different arthropods have, how they could evolve to meet the requirements of a larger creature?
Basically this question is searching for the series of mechanisms and process that must evolve and appear and how they would work to make it possible.