Abdomen is a large body cavity between the thorax (chest) and the pelvic cavity. A strong wall of muscle, called the diaphragm, separates the abdomen from the thorax. But no structure separates the abdomen from the pelvic cavity. The abdominal organs include the stomach, liver, pancreas, intestines, kidneys, adrenal glands, and spleen. A thin membrane known as the peritoneum lines the entire abdominal cavity and covers most of the abdominal organs. Two large blood vessels, the aorta and vena cava, run along the spine and pass through the diaphragm and into the thorax.