What anatomical structure divides the ventral cavity into an upper and lower cavity?

The diaphragm is the anatomical structure that divides the ventral cavity into an upper and lower cavity. This muscle plays a crucial role in respiration, as it separates the thoracic cavity, which houses the heart and lungs, from the abdominal cavity, which contains various organs such as the stomach, liver, and intestines. During inhalation, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and allowing air to flow into the lungs. Conversely, during exhalation, the diaphragm relaxes and moves upward, pushing air out of the lungs.

The other structures listed do not serve as dividers of the ventral cavity. The stomach is an organ within the abdominal cavity, while the liver and spleen are organs also located within that same cavity, functioning in digestion and filtration respectively. None of these organs provide the clear separation between the thoracic and abdominal regions that the diaphragm does.