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.
Balloons filled with helium only float in our atmosphere because the average density of the gas and the balloon itself is less than that of the atmosphere around it. An easy way to show this is to try and put a blown up balloon underwater-the density of the water forces the balloon to rise to the surface. balloon expands to a certain level until the gas inside it is at an equal level of pressure with the gas outside, E.G: There are the same number of atoms bouncing off the inside of the balloon as there are bouncing off the outside, forming a balance of forces.