Do you know why doesn't the Oxalic acid bind with the calcium of the Rhubarb, and turns into a problem if consumed? Is there enough calcium to bind with the Oxa

Do you know why doesn't the Oxalic acid bind with the calcium of the Rhubarb, and turns into a problem if consumed? Is there enough calcium to bind with the Oxalic acid? Is the calcium in Rhubarb bioavailable? Genuine questions here, because as I understand, many of the nutricional fact sheets are calculated after the food is broken down and passed though a mass spectrometer.