Riva Health, founded by scientist Tuhin Sinha and Siri co-founder Dag Kittlaus, wants to help people measure their blood pressure in a clinically approved way. Blood pressure can help indicate at-risk patients before they are actually at risk, showing early signs of heart disease. While other hardware solutions on the market promise the same end goal, Riva wants to be a purely software solution that integrates with hardware that it thinks its end user has anyway: their smartphone. The company, launching out of stealth today, has raised $15.5 million in seed funding in a round led by Menlo Ventures, with participation from True Ventures. Greg Yap of Menlo, who talked to Sinha for three years before investing, will be joining the board.