Complex sentence: Santa Ana is the county seat and second most populous city in Orange County, California, and with a population of 324,528 at the 2010 census, Santa Ana is the 57th-most populous city in the United States.
Split into simpler sentences: Santa Ana is the county seat and second most populous city in Orange County, California. With a population of 324,528, as of the 2010 census, Santa Ana is the 57th most-populous city in the United States.