The retroperitoneum is a space situated behind the parietal peritoneum and in front of the transversalis fascia. It contains further spaces that are separated by the fasciae, between which communication is possible with both the peritoneal cavity and the pelvis, according to the theory of interfascial spread. The perirenal space has the shape of an inverted cone and contains the kidneys, adrenal glands, and related vasculature. It is delineated by the anterior and posterior renal fasciae, which surround the ureter and allow communication towards the pelvis. At the upper right pole, the perirenal space connects to the retrohepatic space at the bare area of the liver. There is communication between these two spaces through the Kneeland channel. The anterior pararenal space contains the duodenum, pancreas, and the ascending and descending colon. There is free communication within this space, and towards the mesenteries along the vessels. The posterior pararenal space, which contains fat, communicates with the preperitoneal space at the anterior surface of the abdomen between the peritoneum and the transversalis fascia, and allows communication with the contralateral posterior pararenal space. This space follows the length of the ureter to the pelvis, which explains the communication between these areas and the length of the pelvic fasciae.