It is a hollow muscular tube approximately 12 to 20 cm in length and 6.25 cm in diameter at its widest point.
It extends from the lower end of the sigmoid colon along the anterior surface of the sacrum and the coccyx into the posterior part of the pelvic cavity. The rectum begins at the level of S2-S3 and ends at the perineum.
The rectum is the last part of the large intestine at its lower end, the rectum tapers slightly before ending in the anal canal.
The rectum can be subdivided into three parts:
- The upper third is found intraperitoneally.
- The middle third retroperitoneally.
- The lower third below the pelvic diaphragm and therefore extraperitoneally.
The rectum is supplied by the superior rectal artery (branch of the inferior mesenteric artery), the middle rectal artery (branch of the internal iliac artery), and the inferior rectal artery (branch of the internal pudendal artery from the internal iliac artery).
Lymphatic vessels extend along the veins, so proximal rectal carcinoma preferentially metastasizes to the liver, whereas distal rectal carcinoma metastasizes to the lungs.
The inferior mesenteric plexus carries the sympathetic innervation, the pelvic splanchnic nerves, and the inferior hypogastric plexus carries the parasympathetic innervation.
The upper, middle, and lower rectal veins are carried out by venous drainage from the rectum.
The superior rectal veins drain the upper part of the rectum into the portal venous system (via the inferior mesenteric vein).
On the other hand, the middle and lower rectal veins drain the lower rectum into the internal iliac vein (systematic circulation) through the internal pudendal vein.
Between these two different types of veins (those that drain into the hepatic portal system and those that drain the systematic circulation) are anastomoses that are clinically important in cases of portal hypertension.
The rectum has two flexions: the sacral flexion (dorsal curve) resulting from the concave shape of the sacrum, the perineal flexion (ventral curve) from the rim of the rectum by the levator ani muscle (puborectalis sling). Here is the transition point to the anal canal (anorectal junction).
Morphologically, the rectum is similar to the rest of the large intestine. However, it does not have tapeworms, haustra, epiploic appendages, and semilunar folds.
The three constant transverse folds (Houston valves) are characteristic. The middle rectal fold (Kohlrausch valve) is the strongest and is located about 7 cm from the anus.
Tumors below this fold may be palpable during digital rectal examination.
The rectal ampulla (section between the Kohlrausch valve and the anorectal junction) is quite elastic and serves as a reservoir during defecation.
The mucosa has the typical intestinal epithelium with simple columnar enterocytes and numerous goblet cells. In the anal transition zone, the columnar epithelium flattens more and more and eventually becomes non-keratinized stratified squamous epithelium.
- The epithelial layer is followed by the connective tissue layer (lamina propria) with blood and lymphatic vessels and a muscular layer (mucosal muscle layer).
- The submucosa contains loose connective tissue with blood vessels, lymphatic follicles, and Meissner’s plexus. It has a dense network of veins (rectal venous plexus) and is thickened in the transverse folds.
- The muscularis has the typical internal circular and external longitudinal muscles, between which is the Auerbach plexus.
- The annulus musculature continues as the sphincter and externus muscle extend within the sphincter system, while the external longitudinal musculature continues.
Stool enters the rectum from the sigmoid colon, where it is stored until it can be eliminated through defecation.
During this storage period in the rectum, a small amount of water is absorbed by the walls of the rectum, returning it to the blood supply.
The fermentation of organic fecal matter that begins in the colon and carried out by bacteria continues, and the remaining nutrients are released and absorbed by the rectal walls.
When gases or feces accumulate in the rectum, they put pressure on the rectal walls gradually increasing.
This distention stimulates the stretch receptors in the walls of the rectum and sends nerve impulses to the brain.
Nerve impulses accumulate in the brain causing discomfort and a need to empty the rectum. Causing relaxation of the smooth muscle of the internal anal sphincter to allow defecation.
The brain then decides whether the rectal contents can be released or not.
Functions of the rectum
The rectum receives stool in the colon and accumulates the stool is the last stop before eliminating the stool through the anal canal.
As in the colon, electrolytes (sodium, potassium, chloride) are absorbed in the rectum and indigestible food ingredients are broken down by anaerobic bacteria. Thus the stool thickens by absorption of water and mixes with mucus.
In addition, the rectum is part of the continence organ and plays an important role in the defecation mechanism. If stool enters the rectal blister, which is usually empty, it is recorded by stretch receptors.
This information transferred to the central nervous system causes the individual to need to defecate and he decides whether to initiate or delay defecation by relaxing or tensing the levator ani muscle and the sphincter and the external muscle.
However, increased pressure in the bleb leads to increased relaxation of the involuntary smooth cutaneous corrugator muscle and the internal and sphincter muscle (rectoanal inhibitory reflex), so that holding the stool for a long time involves increasing “strain.”
The rectum supports defecation through contraction. In addition, intra-abdominal pressure increases through voluntary tension of the diaphragm and abdominal muscles (abdominal pressure).
Functional anorectal disorders, the etiology of which is currently unknown or related to abnormal functioning of normally innervated and structurally intact muscles.
These disorders include functional fecal incontinence, functional anorectal pain, including levator ani syndrome and proctalgia fugax, and pelvic floor dyssynergia.
A rectal exam is used to diagnose certain diseases, such as certain types of cancers that are diagnosed through endoscopy.
This procedure uses a flexible scope attached to a camera and light to examine areas within the intestine.