Final answer:
The diaphragm is formed by the fusion of the inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes onto the central tendon, complemented by skeletal muscle that originates from various structures including the xiphoid process, ribs, and lumbar vertebrae.
Step-by-step explanation:
The sections that fuse to form the diaphragm are primarily composed of the central tendon and surrounding skeletal muscle portions. The inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes, also known as the parietal pleura, fuse onto the central tendon of the diaphragm. To the sides of this tendon, the skeletal muscle portions insert, with various origins including the xiphoid process of the sternum anteriorly, the inferior six ribs and their cartilages laterally, as well as the lumbar vertebrae and 12th ribs posteriorly.
The diaphragm is not only essential for breathing, but it also includes three significant openings: the caval opening for the inferior vena cava, the esophageal hiatus for the esophagus and related nerves, and the aortic hiatus through which the aorta, thoracic duct, and azygous vein pass. It effectively separates the thoracic and abdominal cavities and assists in bodily functions such as defecation, urination, and childbirth through the Valsalva maneuver.