Final answer:
The diaphragm is the muscular structure that separates the thoracic cavity from the abdominopelvic cavity.
Step-by-step explanation:
The physical structure that separates the thoracic cavity from the abdominopelvic cavity is the diaphragm. This muscular partition plays a critical role not only in separating these two major body cavities but also in the process of breathing. The diaphragm is dome-shaped when at rest; its superior surface is convex, forming the floor of the thoracic cavity where the lungs and heart reside. Meanwhile, its inferior surface is concave, serving as the curved roof over the abdominal cavity that houses the digestive organs and the pelvic cavity that contains the reproductive organs.