ampulla
Anatomical term for a dilated portion of a tubular structure, such as the ampulla of Vater or the ampulla of the fallopian tube.
The ampulla of Vater is located at the junction of the bile duct and pancreatic duct.
From 膨大 (bōdai, 'swelling, enlargement') + 部 (bu, 'part, section'). The reading ぼうだいぶ is a straightforward compound reading.