What describes the diaphragm?

The diaphragm is an essential muscle within the respiratory system, located beneath the lungs and shaped like an inverted dome. It plays a critical role in breathing; when it contracts, it moves downward, increasing the volume of the thoracic cavity and allowing air to flow into the lungs. Conversely, when it relaxes, it moves upward, helping to expel air from the lungs. This muscle not only supports the lungs but also helps regulate the pressure in the thoracic cavity, which is integral for both breathing and maintaining proper cardiovascular function.

The other options present structures that do not accurately describe the diaphragm. The flexible membrane around the lungs refers to the pleura, which is different from the function and structure of the diaphragm. A chamber inside the heart describes the atria or ventricles, which are unrelated to the diaphragm. Finally, an organ that filters blood typically pertains to the kidneys or spleen, not the diaphragm. Thus, understanding the diaphragm's role as a muscle that supports respiration highlights its importance in both breathing mechanics and overall health.