The rectum is the last part of the large intestine and connects the sigmoid colon to the anal canal. It begins at the height of S2-S3 and ends at the perineum. The rectum is about 12 to 16 cm long und may be subdivided into three parts: The upper third lies intraperitoneally, the middle third retroperitoneally and the lower third under the pelvic diaphragm and therefore extraperitoneally. The rectum has two flexures: The sacral flexure (dorsal bend) results from the concave form of the sacrum, the perineal flexure (ventral bend) from the encirclement of the rectum by the levator ani muscle (puborectal sling). Here is the point of transition to the anal canal (anorectal junction).
Morphologically the rectum is similar to the rest of the large intestine. However it does not have taeniae, haustra, appendices epiploicae and semilunar folds. Characteristic are the three constant transverse folds (Houston’s valves). The middle rectal fold (Kohlrausch’s valve) is the strongest and located about 7 cm from the anus. Tumors below this fold may be palpable during digital rectal examination. The rectal ampulla (section between Kohlrausch’s valve and anorectal junction) is quite stretchable and serves as a reservoir during defecation.
The rectum is supplied by the superior rectal artery (branch of the inferior mesenteric artery), medial rectal artery (branch of the internal iliac artery) and inferior rectal artery (branch of the internal pudendal artery from the internal iliac artery). The venous blood drains through the correspondent veins into either the inferior mesenteric vein (→hepatic portal system) or external iliac vein (→body circulation). The lymph vessels course along the veins which is why proximal rectal carcinoma metastasizes preferentially to the liver whereas distal rectal carcinoma rather metastasizes to the lungs. The sympathetic innervation is carried by the inferior mesenteric plexus, the parasympathetic innervation by the pelvic splanchnic nerves and the inferior hypogastric plexus.
Histologically the rectum is similar to the rest of the large intestine with its usual structure: mucosa, submucosa, muscularis and serosa/adventitia. The mucosa has the typical intestinal epithelium with simple columnar enterocytes and numerous goblet cells. At the anal transitional zone the columnar epithelium flattens more and more and eventually becomes stratified squamous non-keratinized epithelium. The epithelial layer is followed by the connective tissue layer (lamina propria) with blood and lymph vessels and a muscle layer (lamina muscularis mucosae). The submucosa contains loose connective tissue with blood vessels, lymph follicles and the Meissner's plexus. It has a dense network of veins (rectal venous plexus) and is thickened at the transverse folds. The muscularis has the typical inner circular and outer longitudinal musculature between which the Auerbach’s plexus lies. The ring musculature continues as the sphincter ani externus muscle within the sphincter system whereas the outer longitudinal musculature continues as the corrugator cutis ani muscle inserting at the skin around the anus.
The rectum is the last stop before the feces is eliminated through the anal canal. Similar to the colon electrolytes are absorbed (sodium, potassium, chloride) and indigestible food ingredients are decomposed by anaerobic bacteria. The stool is thickened through water absorption and mixed with mucus.
Furthermore the rectum is part of the continence organ and plays an important role in the mechanism of defecation. If stool enters the rectal ampulla which is usually empty it is registered by stretch receptors. The information is transferred to the central nervous system giving the person the urge to defecate. He can now decide to either initiate or delay the defecation by relaxing or tensing the levator ani muscle and sphincter ani externus muscle. However the rising pressure in the ampulla leads to an increasing relaxation of the involuntary smooth corrugator cutis ani muscle and sphincter ani internus muscle (rectoanal inhibitory reflex) which is why holding in the stool for a long time involves an increasing “effort”. The rectum supports the defecation through contraction. Additionally the intra-abdominal pressure is increased through the voluntary tension of the diaphragm and abdominal muscles (abdominal press).