Idealized representation of the peritoneal and mesenteric relationships at various levels of the colon and rectum

Both the transverse and sigmoid colon are located intraperitoneally; at these levels, the visceral peritoneum forms a complete covering over the exterior of the bowel (the serosa), which is continuous with the mesentery (transverse and sigmoid mesocolon). In contrast, the ascending and descending colon lie within the lateral peritoneal cavity with their posterior and lateral surfaces in the retroperitoneum. At these levels, the visceral peritoneum is only present anteriorly and medially; there is no true mesentery, since the developing mesentery has fused to the posterior parietal peritoneum. The upper portion of the rectum lies above the peritoneal reflection. The anterior surface is covered by peritoneum (which forms the rectovesical pouch in men and the rectouterine pouch in women); there is no serosa over its posterior surface. The lower rectum lies beneath the peritoneum and has no serosal layer.