General structure and peritoneal relations
The large intestine is divided into several sections: the cecum, the vermiform appendix, the colon (which is divided into ascending, transverse, descending, and sigmoid parts), the rectum, and the anal canal.
The various parts of the intestine are covered by the peritoneum differently:
- The cecum, ascending colon, and descending colon are covered by visceral peritoneum only anteriorly. The posterior wall of these segments (about 50% of the surface) is retroperitoneal and separated from the surrounding fat by an adventitial layer.
- The transverse and sigmoid colons are intraperitoneal. They have their own mesentery (mesentery), a peritoneal fold containing blood vessels, nerves, and lymphatic pathways.
Features of the muscular wall
The wall of the colon has characteristic features:
- The muscular coat (Muscularis propria) consists of inner (circular) and outer (longitudinal) layers. The myenteric plexus (Auerbach's plexus) lies between them.
- The taeniae (Taeniae coli) are three narrow longitudinal bands formed by the aggregation of the outer muscle layer. They are rich in collagen and elastic fibers.
- Haustra are sac-like dilations between the taeniae. In the distal part of the intestine, they are formed only upon contraction of the taeniae.
- Epiploic appendages (Epiploic appendages) are small accumulations of adipose tissue within peritoneal folds, located mainly on the cecum and sigmoid colon.
Blood supply to the colon
The arterial bed of the colon is supplied by two large vessels:
- The superior mesenteric artery (SMA) supplies the proximal part up to the splenic flexure.
- The inferior mesenteric artery (IMA) supplies the distal sections.
The terminal branches of these arteries anastomose to form the marginal artery along the mesenteric border of the intestine. Straight vessels (vasa recta) arise from it and penetrate the organ wall: short ones enter near the mesenteric taenia, while long ones enter between the omental and free taeniae. Venous drainage goes into the portal vein.
Rectum and anal canal
The rectum (Rectum) begins at the level of the 3rd sacral vertebra. Unlike the colon, it lacks haustra, but has intraluminal folds known as transverse rectal folds (valves of Houston).
An important structure here is the mesorectum. This is a layer of adipose tissue surrounding the posterior and lateral walls of the upper part, as well as the entire lower third of the rectum. It is enclosed within the mesorectal fascia and contains perirectal lymph nodes.
The sphincter apparatus of the anal canal includes:
- Internal sphincter (smooth muscle).
- External sphincter (striated muscle).
- Longitudinal muscle layer between them.
The canal is supported by the pelvic floor muscles, the basis of which is the levator ani muscle (levator ani).
Briefly: answers to topical questions
Where does venous drainage from different parts of the rectum go?
Venous drainage from the rectum flows into the portal and inferior vena cava systems, depending on the section.
- Upper third — drainage goes via the superior rectal vein (v. rectalis superior) into the portal system.
- Middle and lower thirds — drainage occurs via the middle and inferior rectal veins (v. rectalis media and inferior veins) into the internal iliac veins, and then into the inferior vena cava system (systemic circulation, bypassing the liver).
To which regional lymph nodes does lymph drainage from the colon occur?
Lymph drainage from the colon occurs into regional lymph nodes located along the feeding arteries, as well as into celiac lymph nodes. First-tier nodes are located on the intestinal wall and along the mesenteric border, while subsequent-tier nodes are located along the branches of the mesenteric arteries.
| Colon segment | Localization of regional lymph nodes |
|---|---|
| Ascending colon | Along a. ileocolica, a. colica dextra, a. colica media |
| Hepatic flexure | Along a. colica dextra, a. colica media |
| Transverse colon | Along a. colica dextra, a. colica media, a. colica sinistra, a. mesenterica inferior |
| Splenic flexure | Along a. colica media, a. colica sinistra, a. mesenterica inferior |
| Descending colon | Along a. colica sinistra, a. mesenterica inferior |
| Sigmoid colon | Along aa. sigmoideae, a. colica sinistra, a. rectalis superior, a. mesenterica inferior |
Which nerves provide parasympathetic and sympathetic innervation to the colon?
Innervation of the colon is provided by sympathetic and parasympathetic fibers.
- Parasympathetic innervation: up to the level of the left colic flexure is provided by the vagus nerve (n. vagus); more distally — via the pelvic splanchnic nerves (nn. splanchnici pelvini) from the sacral segments S2–S4.
- Sympathetic innervation: is provided via the splanchnic nerves (nn. splanchnici).
To memorize the arteries: SMA (Superior) supplies the 'right/upper' half (up to the splenic flexure), while IMA (Inferior) supplies the 'left/lower' half of the colon.
Class and exam questions
At what level does the rectum begin?
At the level of the 3rd sacral vertebra.
What are the longitudinal muscular bands of the colon called?
They are called taeniae (Taeniae coli).
Which parts of the colon normally have a mesentery?
The transverse and sigmoid colons.
Where does venous drainage from the middle part of the rectum go?
Middle rectal veins drain into the internal iliac veins, i.e., into the inferior vena cava system (systemic circulation).
What else to study on this topic
- Histological structure of the mucosa and submucosa
- Features of lymph drainage from various segments
- Structure and functions of the pelvic floor
- Clinical significance of epiploic appendages (hidden diverticula)
Master the entire topic in the "Diagnostic Radiology" course
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Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
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