Final answer:
The diaphragm is the muscular structure that divides the upper and lower halves of the torso, separating the thoracic cavity from the abdominopelvic cavity, which is further supported by the skeletal structure of the pelvic girdle.
Step-by-step explanation:
The structure that separates the upper half of the torso from the lower half is the diaphragm. This muscular partition forms the floor of the thoracic cavity and separates it from the abdominopelvic cavity. In addition to its role in respiration, the diaphragm also coordinates with the abdominal muscles for various processes such as defecation, urination, and childbirth, a combined effort known as the Valsalva maneuver.
The thoracic cavity houses vital organs such as the lungs and heart and is protected by the thoracic or rib cage. Below the diaphragm lies the abdominopelvic cavity which, while not physically divided by a membrane, can functionally be separated into the abdominal cavity (housing the digestive organs) and the pelvic cavity (containing organs related to reproduction).
When considering the skeletal structure, the pelvic girdle plays a significant role in demarcating the lower portion of the torso. It is where the lower limbs are attached and supports the organs of the abdomen. The pelvic girdle is made of the ilium, pubis, and ischium bones, and connects to the spine through ligaments.