Final answer:
In a PA recumbent position upper GI exam of a sthenic patient, barium sulfate fills structures such as the fundus of the stomach, aiding in the diagnosis of GI diseases without being absorbed into tissues.
Step-by-step explanation:
When a sthenic patient undergoes an upper GI examination in the PA recumbent position, a suspension of barium sulfate is used to coat the intestinal tract, allowing for enhanced visual detail over traditional X-rays. In this position and body habitus, structures such as the fundus of the stomach are more likely to be filled with barium. This helps in diagnosing conditions like acid reflux disease, Crohn's disease, and peptic ulcers. Due to its high atomic mass, barium is opaque to X-rays, thus providing a clear outline of these digestive organs. It is worth noting that while soluble barium salts can be toxic, barium sulfate's insolubility enables its safe use for medical imaging without absorption into tissues.