AssessmentsAnatomy of the gastrointestinal organs of the pelvis and perineum
GI Organs of the Pelvis
USMLE® Step 1 style questions USMLE
When you eat, food travels over 15 feet before leaving your body! But before it leaves, it has two final stops; the rectum and the anal canal. Let’s explore these gastrointestinal organs of the pelvis and discuss the process of defecation and the structures involved.
The rectum is the terminal chamber of the large intestine that temporarily stores feces before defecation. The rectum joins with the sigmoid colon at the level of S3, forming the rectosigmoid junction.
Then, the rectum courses below in the pelvic cavity, reaching a point anterior and inferior to the tip of the coccyx. Here, the rectum pierces the levator ani muscle to join with the anal canal, forming the anorectal junction.
The relations of the rectum to the surrounding structures differs between biologically male and biologically female individuals.
In males, the rectum lies behind the fundus of the urinary bladder, the seminal vesicles, the left and right ductus deferens, and the prostate gland. In females, the rectum lies behind the lower end of the uterus and the vagina.
These flexures include the sacral, the anorectal, and the superior, inferior, and intermediate lateral flexures. The sacral flexure follows the curve of the sacrum and the coccyx, forming an anteroposterior curve with an anterior concavity.
Next is the anorectal flexure, which is a sharp postero-inferior angle at the anorectal junction. This flexure is maintained by a sling-like muscle, puborectalis, which is part of the levator ani muscle group.
Now, let’s move to the anterior view of the rectum, which shows three sharp curves called the lateral flexures. These include the superior and inferior lateral flexure, which lie on the left side, and the intermediate lateral flexure, which lies on the right side of the rectum.
These lateral flexures match three internal folds of the rectum, called the superior, middle and inferior transverse rectal folds. Just inferior to the inferior transverse rectal fold, forming the last part of the rectum, is the ampulla of the rectum.
This pouch is supported by the levator ani muscles, and by a ligament that extends from the tip of the coccyx to the external anal sphincter, called the anococcygeal ligament.
Okay, now let’s look at the relationship between the rectum and the peritoneum. To do that, let’s slice the rectum into thirds. In the upper third, the peritoneum covers the anterior and lateral surfaces of the rectum.
In females, the peritoneum reflects from the middle third of the rectum to the posterior surface of the vagina and the uterus, forming the rectouterine pouch. In both males and females, the lower third is without peritoneal cover and is considered subperitoneal.
The blood supply of the rectum comes from superior, middle, and inferior rectal arteries. The superior rectal artery is the lower continuation of the inferior mesenteric artery, which arises directly from the abdominal aorta. This artery supplies the superior part of the rectum.
Finally, the left and right inferior rectal arteries arise from the internal pudendal arteries, which are branches of the anterior division of the internal iliac arteries. These arteries supply the anorectal junction and the anal canal.
Venous blood from the rectum drains into three main tributaries: the superior, the middle, and the inferior rectal veins. The superior rectal vein drains into the inferior mesenteric vein, which is part of the portal circulation.
These three sets of rectal veins all anastomose together within the walls of the rectum and anus, forming a connection between the portal and systemic systems. This type of special connection is called a portosystemic anastomosis.
The anal canal begins at the anorectal junction, and descends posteroinferiorly to end at the anus, which is the external opening of the gastrointestinal tract. Typically, the anal canal is collapsed, and expands as feces pass through it during defecation.
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