AssessmentsAnatomy of the gastrointestinal organs of the pelvis and perineum
USMLE® Step 1 style questions USMLE
Contributors:Ursula Florjanczyk, MScBMC, Sam Gillespie, BSc, Elizabeth Nixon-Shapiro, MSMI, CMI, Ahmed A. Abu Ajeene
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.
Let’s think of the pelvis like a bowl. The levator ani muscle acts as the bottom of the bowl to support the structures within the pelvis, especially the rectum.
Another structure that supports the rectum is the anococcygeal ligament which forms a fibrous ridge from the anal canal to the coccyx, acting as an anchor.
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.
The rectum is a Latin word that means straight, but this is a bit misleading, because the human rectum actually has many flexures.
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.
When contracted, the puborectalis muscle pulls on the anorectal flexure preventing feces from entering the anal canal, which maintains fecal continence by keeping the feces stored in the rectum.
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.
The ampulla of the rectum expands to store feces before defecation, which also helps maintain fecal continence.
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.
Then, the peritoneum continues to cover only the anterior surface of the middle third of the rectum. From here, the course of the peritoneum is a bit different in males and females.
In males, the peritoneum reflects from the middle third of the rectum to the posterior surface of the bladder, forming the rectovesical pouch.
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.
Next are the left and right middle rectal arteries, which arise from the anterior division of the internal iliac arteries to supply the middle and lower parts 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.
Finally, the inferior rectal veins drain into the internal pudendal veins, which drain into the internal iliac veins, which again are part of the systemic venous system.
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.
Next are the parasympathetic fibers, which arise from S2 - S4 levels of the spinal cord, and pass through the pelvic splanchnic nerves and the inferior hypogastric plexus to reach the rectum.
Finally, the rectum lies below the pelvic pain line, meaning that visceral afferent fibers from the rectum follow the parasympathetic fibers to the S2 - S4 sensory ganglia.
Okay, now let’s take a deep breath and have a quick quiz! Can you name this junction? All right, but can you name this part of the rectum?
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.
Similar to the urinary bladder, the anal canal has internal and external sphincters, which are circular muscles that control defecation.