Boundaries, length, and curvatures
The anatomical origin of the rectum is located where the sigmoid colon transitions into the rectum and its mesentery disappears—at the level of the third sacral vertebra. The total length of the organ is no more than 15 cm.
In the sagittal plane, the organ forms two curvatures:
- Flexura sacralis (sacral flexure), which has a posterior convexity following the curvature of the sacrum.
- Flexura perinealis (perineal flexure), curved anteriorly and wrapping around the coccyx, corresponding to the location of the third sphincter.
In the frontal plane, a curvature to the right is formed, which is of great practical importance during rectoscopy.
Peritoneal relations and division into segments
The organ is divided into two main parts relative to the pelvic diaphragm:
- Pelvic part, which lies above the pelvic diaphragm. It includes the supra-ampullary part (the rectosigmoid segment, sometimes having a mesentery, mesorectum) and the wide ampulla. The upper part of the intestine is covered by peritoneum on all sides (intraperitoneally), while the middle part (at the level of the IV sacral vertebra) is covered anteriorly and laterally (mesoperitoneally).
- Perineal part (canalis analis or anal canal), located below the pelvic diaphragm. The lower part of the organ is extraperitoneal: it is not covered by peritoneum and is surrounded by a prominent visceral fascia (Amussat's capsule).
Syntopy in men and women
The mutual arrangement of pelvic organs varies depending on gender.
In men, anterior to the rectum are:
- The urinary bladder and prostate gland;
- The ampullae of the ductus deferens and seminal vesicles;
- The terminal parts of the ureters.
Practical nuance: through the anterior wall of the intestine per anum, a physician can palpate the prostate gland and seminal vesicles.
In women, anterior to it are:
- The uterus and vagina, which are adjacent to the extraperitoneal floor of the rectum.
- Between the uterus and the rectum lies the rectouterine pouch (excavatio rectouterina), which is the deepest part of the pelvic cavity where loops of the small intestine accumulate. These structures are separated by a thin layer known as the rectovaginal septum (septum rectovaginale).
Muscular apparatus and sphincters
The longitudinal muscular layer of the rectum is developed evenly and does not form taeniae, unlike the more proximal parts of the large intestine. At the level of the pelvic diaphragm, bundles of the levator ani muscle (m. levator ani) weave into the wall.
The sphincter apparatus is represented by three formations:
- M. sphincter ani externus — the external sphincter composed of striated fibers (voluntary).
- M. sphincter ani internus — the internal sphincter composed of smooth muscle (involuntary), forming rings at a distance of 3–4 cm from the anal opening.
- M. sphincter ani tertius (Gerota's muscle) — a thickening of longitudinal fibers at a distance of 10 cm from the opening.
Vascular system and venous plexuses
The blood supply to the organ is provided by five arteries:
- The unpaired a. rectalis superior — a continuation of the inferior mesenteric artery;
- The paired a. rectalis media (from the internal iliac) and a. rectalis inferior (from the internal pudendal artery system).
The venous outflow forms plexuses (plexus venosi rectales): subcutaneous, submucosal with hemorrhoidal plexuses (zona hemorrhoidalis), and subfascial. Important portacaval anastomoses are formed here:
- The superior rectal vein carries blood into the portal vein system;
- The middle and lower veins drain blood into the inferior vena cava system.
Briefly: answers to topical questions
What fascial spaces surround the rectum?
The rectum is surrounded by well-documented fat-containing spaces and structures: ischioanal fossae are located on both sides of the perineal part; visceral perirectal adipose tissue is described around the rectum; the lateral and posterior surfaces of the upper part of the rectum, as well as the entire lower third, are surrounded by the mesorectum — loose adipose connective tissue; posteriorly, the mesorectal fascia is separated from the presacral fascia by a thin retrorectal space.
From which plexuses is the innervation of the rectum derived?
The innervation of the rectum is derived from several nerve plexuses and trunks.
- Inferior mesenteric plexus (plexus mesentericus inferior).
- Superior and inferior hypogastric plexuses (plexus hypogastricus superior et inferior).
- Pelvic splanchnic nerves (nn. splanchnici pelvini).
- Pudendal nerve (n. pudendus).
Five arteries supply the rectum: one superior (unpaired from the mesenteric system) and two middle and two lower ones (paired from the iliac and pudendal systems).
Class and exam questions
Where does the rectum begin and what is its length?
The organ begins at the level of the III sacral vertebra at the site of transition of the sigmoid colon, where its mesentery disappears. The total length of the rectum does not exceed 15 cm.
What anatomical structures are located anterior to the rectum in men?
Anteriorly, it is adjacent to the urinary bladder, prostate gland, seminal vesicles, ampullae of the vas deferens, and terminal parts of the ureters. Due to this, the prostate gland can be palpated through the rectal wall.
What are the features of venous outflow and the formation of hemorrhoidal nodes?
In the rectal wall, subcutaneous, submucosal (containing the zone of hemorrhoidal plexuses), and subfascial plexuses are formed. Blood outflow goes both to the portal vein system (via the superior artery) and to the inferior vena cava system, creating natural portacaval anastomoses.
How is lymphatic drainage distributed from various parts of the organ?
From the supra-ampullary part, lymph drains to the lymph nodes of the inferior mesenteric artery; from the middle segment, to the internal iliac and sacral lymph nodes; and from the lower segment, to the inguinal lymph nodes.
What else to study on this topic
- Topography of the ischioanal fossae and their connection with purulent processes (paraproctitis)
- Similarities and differences in the course of the middle and inferior rectal arteries
- Structure and clinical significance of Amussat's capsule
- Detailed analysis of the neural apparatus and pelvic splanchnic nerves
- Anatomical structure of the rectouterine pouch in women
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