Which structure separates the thoracic cavity from the abdominal cavity?

The diaphragm is a muscular structure that plays a crucial role in respiration and separates the thoracic cavity from the abdominal cavity. It is dome-shaped and located at the base of the rib cage, serving as a barrier between the lungs and the organs in the abdominal cavity. Its contraction during inhalation increases the volume of the thoracic cavity, allowing air to flow into the lungs, while its relaxation during exhalation decreases the thoracic volume, forcing air out. This functional partition also helps maintain the pressure differentials necessary for effective breathing.

In contrast, the rib cage provides structural support and protection for the thoracic organs but does not actively separate the two cavities. The sternum, which is a flat bone at the front of the rib cage, also supports the thoracic structure but does not serve as a separator. The lungs, while located within the thoracic cavity, do not have any role in separating it from the abdominal cavity. Thus, the diaphragm is the key structure that effectively divides these two major body cavities.