The pathogenesis of ectopic gastric mucosa is a matter of debate between a congenital-embryological-and an acquired origin-consecutive to a gastroesophageal reflux (GER) like Barrett's esophagus -, the most commonly accepted theory being that these areas are embryological remnants.
Standard size latex filled helium balloons stay afloat for approx 8 - 12 hours, metallic helium filled balloons float for 2-5 days. I have found that a 36 latex helium filled balloon can stay afloat for a number of days due to the thickness of the balloon and the massive quantity of helium.
The less dense the gas inside the balloon, the more air it displaces in proportion to its own weight. Consequently, balloons that are more dense than air will tend to sink, while balloons that are less dense than air will tend to rise, and that's why helium balloons fly while oxygen-filled balloons fall.