Location and Parts
The rectum begins from the sigmoid colon precisely at the level of the 3rd sacral vertebra and ends at the anus. Despite its name, it is not completely straight. Along its course, the intestine forms two distinct curvatures:
- Sacral flexure (flexura sacralis) — follows the concave curvature of the sacrum;
- Perineal (anorectal) flexure (flexura anorectalis) — points posteriorly.
Its diameter is uneven. Anatomically, the organ is divided into two main parts: the pelvic part and the perineal part (canalis analis — anal canal). In the pelvic part, the intestine expands significantly, forming the rectal ampulla (ampulla recti). This ampulla serves as the physiological reservoir for the accumulation of feces, after which the tube narrows again as it transitions into the anal canal.
Wall Structure: Mucous Membrane
The mucous membrane (tunica mucosa) of the rectum differs fundamentally from that of the small intestine. It completely lacks villi, but contains a large number of intestinal crypts (glandulae intestinales).
The relief of the mucosa varies by region:
- In the pelvic part, it forms transverse rectal folds (plicae transversales recti).
- In the anal canal region, longitudinal elevations form — anal columns (columnae anales). Depressions between these columns are called anal sinuses (sinus anales).
The epithelial lining also changes from top to bottom. In the upper sections, the mucosa is lined by simple columnar epithelium. Lower down, this transitions to non-keratinized stratified squamous epithelium, and in the lowest part near the perianal skin, the epithelium becomes keratinized stratified squamous.
Muscular Coat and Sphincters
The muscular coat (tunica muscularis) of the rectum loses the typical features of the colon. The longitudinal muscle layer here is distributed evenly and becomes continuous — longitudinal bands (taeniae) are absent.
The circular layer of the muscular coat thickens significantly in the region of the anal canal, forming the closing apparatus:
- Internal anal sphincter (m. sphincter ani internus) — composed of smooth muscle and contracts involuntarily (not under conscious control).
- External anal sphincter (m. sphincter ani externus) — formed by skeletal (striated) muscle tissue and is a voluntary sphincter (under conscious control).
Peritoneal Relations
The peritoneal coverage of the rectum changes along its length, which is crucial for understanding the pathways of pelvic infection spread and surgical approaches:
- Upper section is covered by the serosa intraperitoneally (anteriorly and laterally).
- Middle section is covered mesoperitoneally (anteriorly only).
- Lower section completely lacks peritoneal coverage, meaning it is located retroperitoneally (extraperitoneally).
Blood Supply and Innervation
The rectum features an abundant blood supply provided by three pairs of arteries from different vascular sources:
- Superior rectal artery (a. rectalis superior) — a branch of the inferior mesenteric artery.
- Middle rectal arteries (aa. rectales mediae) — branch from the internal iliac artery.
- Inferior rectal arteries (aa. rectales inferiores) — branches of the internal pudendal artery.
Venous drainage occurs via the rectal venous plexus. This is a critical area because it forms portocaval anastomoses (connections between the portal and systemic venous systems).
The organ is innervated by the autonomic nervous system. Sympathetic fibers arrive from the hypogastric plexus (plexus hypogastricus), while parasympathetic innervation is provided by branches of the pelvic splanchnic nerves.