Board Thread:Questions and Answers/@comment-75.70.197.147-20130224222919/@comment-1504072-20130225133719

Raiden's white blood is based on the same idea as Oxycyte, and not necessarily identical. If it has a similar function to Oxyxyte, then it may not be a complete red blood substitute. i.e. there will be other components that perform the role of platelets in clotting.

As for why he would switch, faster oxygen transfer, means a faster respiration rate, and hence faster energy production for his body. It probably wasn't his own choice initially, as the change to white blood was probably a part of the experimentation performed on him by the Patriots.