This image of a patient with ascities highlights the margins of the intraperitoneal versus retroperitoneal compartments. The patient on the left has ascites, an abnormal fluid collection within the peritoneal space. This fluid appears as a density slightly darker than the surrounding abdominal wall musculature. Note how the intraperitoneal compartment is essentially bathed in ascites fluid (all around the liver, spleen, and intestines). The retroperitoneal space, where the kidneys, aorta, and vena cava reside, is not in contact with the ascites fluid. Notice these structures are surrounded by the retroperitoneal fat. The image on the right is at a comparable level in a patient without ascites.