Final answer:
The pancreas lies in a retroperitoneal position behind the stomach and has both exocrine and endocrine functions. It is situated across the abdomen and ends near the spleen. Similarly, the kidneys are also retroperitoneal and play a crucial role in filtering blood.
Step-by-step explanation:
The organ that occupies a retroperitoneal position partially behind the stomach is the pancreas. The pancreas extends horizontally across the abdomen, from the curvature of the duodenum to the spleen. Functionally, it is unique, combining both exocrine (secretion of digestive enzymes) and endocrine (release of hormones into the bloodstream) functions.
The pancreas is not the only organ in a retroperitoneal position; the kidneys also share this characteristic, being situated between the parietal peritoneum and the posterior abdominal wall. They are well protected and are essential for filtering blood and producing urine.