Location and parts
The rectum begins from the sigmoid colon strictly 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 flexures:
- Sacral flexure (flexura sacralis) — follows the concave curvature of the sacrum;
- Perineal (anorectal) flexure (flexura anorectalis) — directed posteriorly.
The diameter of the organ is uneven. Anatomically, the rectum 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 fecal masses, after which the intestine narrows again as it transitions into the anal canal.
Wall structure: mucosa
The mucous membrane (tunica mucosa) of the rectum has several fundamental differences from the small intestine. It completely lacks villi, but contains a large number of intestinal crypts (glandulae intestinales).
The relief of the mucosa changes depending on the region:
- In the pelvic part, it gathers into transverse folds (plicae transversales recti).
- In the area of the anal canal, longitudinal elevations form — anal columns (columnae anales). Between these columns lie depressions called anal sinuses (sinus anales).
The epithelial lining also undergoes changes from top to bottom. In the upper sections, the mucosa is covered by a simple columnar epithelium. Below, it transitions to a non-keratinized stratified squamous epithelium, and in the lowest part, in the area of the anal skin, the epithelium becomes keratinized stratified squamous.
Muscular layer and sphincters
The muscular layer (tunica muscularis) of the rectum loses the typical features of the colon. The longitudinal muscle layer here is distributed evenly and becomes continuous — muscle bands (taeniae) are absent.
The circular layer of the muscular coat in the anal canal area thickens significantly, forming the closure apparatus:
- Internal anal sphincter (m. sphincter ani internus) — consists of smooth muscle and contracts involuntarily (is not subject to conscious control).
- External anal sphincter (m. sphincter ani externus) — formed by striated muscle tissue, it is a voluntary sphincter (under conscious control).
Peritoneal relations
The peritoneal coverage of the rectum varies along its length, which is crucial for understanding the pathways of pelvic infection spread and surgical approaches:
- Upper section is covered by the serous membrane intraperitoneally (anteriorly and laterally).
- Middle section is covered mesoperitoneally (anteriorly only).
- Lower section is completely devoid of peritoneal covering, meaning it is located extraperitoneally.
Blood supply and innervation
The rectum is characterized by an abundant blood supply provided by three pairs of arteries from different vascular beds:
- Superior rectal artery (a. rectalis superior) — a branch of the inferior mesenteric artery.
- Middle rectal arteries (aa. rectales mediae) — branch off from the internal iliac artery.
- Inferior rectal arteries (aa. rectales inferiores) — branches of the internal pudendal artery.
Venous outflow occurs via the rectal venous plexus. This is a critical area because portacaval anastomoses (connections between the portal and systemic caval venous systems) are formed here.
The innervation of the organ is provided by the autonomic nervous system. Sympathetic fibers approach from the hypogastric plexus (plexus hypogastricus), while parasympathetic innervation is provided by branches of the pelvic splanchnic nerves.
Briefly: answers to topical questions
Which lymph nodes drain lymph from the rectum?
Lymph drainage from the rectum occurs into the visceral lymph nodes of the pelvis and abdominal cavity. These include:
- Superior rectal nodes (nodi lymphoidei rectales superiores) — belong to the group of inferior mesenteric lymph nodes.
- Pararectal / anorectal nodes (nodi lymphoidei...) — belong to the visceral (organ) nodes of the pelvis.
Which veins form the rectal venous plexus?
The rectal venous plexus (plexus venosus rectalis) is connected with drainage pathways into the portal and inferior vena cava systems:
- Superior rectal vein (v. rectalis superior) — its roots connect with the rectal venous plexus and continue into the inferior mesenteric vein (v. mesenterica inferior).
- Middle rectal vein (v. rectalis media) — drains into the internal iliac vein (v. iliaca interna).
- Inferior rectal vein (v. rectalis inferior) — drains blood via the internal pudendal vein (v. pudenda interna) into the internal iliac vein.
To easily remember the sources of blood supply (arteries), use the level rule: Superior — from the digestive tract (inferior mesenteric), Middle — from the pelvis (internal iliac), Inferior — from the perineum (internal pudendal).
Class and exam questions
Are there muscle bands (taeniae) and haustra in the rectum?
No, unlike the colon, the longitudinal muscle layer here is continuous, so muscle bands (taeniae) and haustra are absent.
How does the internal sphincter differ from the external one?
The internal anal sphincter is smooth-muscle and involuntary. The external one is formed by striated muscle and is consciously controlled (voluntary).
What are anal sinuses?
These are depressions in the mucosa of the anal canal located between the longitudinal elevations known as anal columns.
What else to study on this topic
- Mechanism of portacaval anastomoses of the rectal plexus
- Topography of the rectal ampulla and its syntopy (adjacent organs)
- Histological epithelial transition in the anal canal
- Sympathetic and parasympathetic regulation of the defecation reflex
- Fasciae and fascial spaces surrounding the rectum
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