Rectum Anatomy and Physiology
The rectum is the distal portion of the large intestine and lies in the posterior part of the pelvic cavity. It is continuous with the sigmoid colon at the rectosigmoid junction in front of the third piece of the sacrum (Fig. 5.7), where there is often an acute angulation. The rectum curves downwards and forwards, lying first on the anterior surface of the sacrum and then on the upper surface of the pelvic floor. It deviates to either side of the midline and these lateral flexures become pronounced when the organ is distended. The lowest part of the rectum, the ampulla, is its most dilatable portion. Turning abruptly downwards and backwards, the rectum pierces the pelvic floor and terminates at the anorectal junction, where it is continuous with the anal canal (Fig. 5.5). The sharp angulations at the anorectal and rectosigmoid junctions must be navigated with care during endoscopy.