An investigator is studying the embryology of the gastrointestinal tract and discovers an anatomical line called the pectinate line within the anal canal. Which of the following best characterizes the region of the anal canal above the pectinate line?

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. Regina 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 ai 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 analcoccygeal 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. Rectum is a Latin word that means straight, but this is a bit misleading because a human rectum actually has many flexures.

Flexures of the rectum1:45–3:42

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 anterior posterior curve with an anterior concavity.

Next is the anorectal flexure. Which is a sharp posterior inferior angle at the anorectal junction.

This flexure is maintained by a sling-like muscle, pubil rectalis, 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 ampula 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 analcoccygeal ligament.

The ampula of the rectum expands to store feces before defecation. Which also helps maintain fecal continence.

Peritoneal relations of the rectum3:42–4:52

OK, 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. The lateral extension of the peritoneum forms a fossa on each side of the rectum called the parareal fossa, which allows the rectum to expand when it fills with feces.

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 rective vesicle 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 recto uterine 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.

Arterial supply of the rectum4:52–5:43

The superior rectal artery is a 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 anal rectal junction and the anal canal. Venous blood from the rectum drains into the three main tributaries the superior, the middle, and the inferior rectal veins.

Venous drainage of the rectum5:43–6:42

The superior rectal vein drains into the inferior mesenteric vein, which is part of the portal circulation. The middle rectal vein drains blood from the ampula of the rectum into the internal iliac veins.

Which are part of the systemic Venus system. 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 anastomos 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. Finally, the motor innervation of the rectum is supplied by the sympathetic and parasympathetic divisions of the autonomic nervous system.

Innervation of the rectum6:42–7:33

Let's start with the sympathetic fibers, which arise from the lumbar spinal cord and pass through the lumbar splankic nerves and the superior and inferior hypogastric plexuses to reach the rectum.

Next are the parasympathetic fibers, which arise from S2 to S4 levels of the spinal cord and pass through the pelvic splankate 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 through S4 sensory ganglia.

Quiz7:33–7:54

OK, now let's take a deep breath and have a quick quiz. Can you name this junction?

Alright, but can you name this part of the rectum? Let's move a little farther down and talk about the anal canal.

Anal canal7:54–8:22

Which is a small tube that forms the last part of the gastrointestinal tract. The anal canal begins at the anorectal junction and descends posterior inferiorly 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.

Sphincters of the anal canal8:22–9:05

Let's start with the internal anal sphincter, which is an involuntary muscle surrounding the anal canal. Typically, this sphincter contracts to prevent the leakage of feces or gas and relaxes as the rectal ampula fills with feces.

The external anal sphincter is a voluntary muscle that surrounds the anal canal and has deep, superficial and subcutaneous parts.

This muscle blends superiorly with the puborectalis muscle and attaches posteriorly to the coccyx by the analcoccygeal ligament.

Now, let's look inside the anal canal. The upper half of the anal canal has longitudinal ridges called the anal columns, which contain the terminal branches of the superior rectal artery and vein.

Internal features of the anal canal9:05–10:19

At the upper ends of these columns, the wide rectal ampula narrows to join the anal canal, forming the anal rectal junction.

Also, the lower ends of adjacent columns are joined by folds called anal valves. These valves have a small space above them called the anal sinuses.

These sinuses make defecation a little bit easier by adding mucus to the feces. The lower borders of the anal sinuses and valves form an irregular line called the pectinate line.

This line divides the anal canal into an upper and lower part, which differ in embryonic origin. The upper part is derived from the embryonic hindgut.

While the lower part is derived from the embryonic proctoidium. For this reason, the pectinate line is an important feature to help distinguish the blood supply, nerve supply, and lymphatic drainage patterns in the upper and lower parts of the anal canal.

Blood supply and innervation above the pectinate line10:19–11:45

First, let's discuss some general patterns, and then we'll talk about some exceptions. Above the pectinate line, arterial supply comes from the superior rectal artery, which is a continuation of the inferior mesenteric artery.

The internal rectal venous plexus drains above the pectinate line into the superior rectal vein. Which continues to become the inferior mesenteric vein which is part of the portal system.

The lymphatic vessels drain into the internal iliac lymph nodes. Lastly, the nerve supply above the pectinate line is visceral innervation from the inferior hypogastric plexus, providing sympathetic, parasympathetic and visceral afferent fibers.

First are the sympathetic fibers which stimulate and maintain the contraction of the internal anal sphincter. Next are the parasympathetic fibers which relax the internal anal sphincter and initiate peristaltic movement of the bowel during defecation.

Finally, the afferent fibers from the upper part of the anal canal are visceral, making it sensitive to stretch only. The upper part of the anal canal also lies below the pelvic pain line, which means the visceral afferent fibers travel with the parasympathetic fibers to the spinal sensory ganglia at the level of S2 to S4.

Blood supply and innervation below the pectinate line11:45–13:04

Below the pectinate line, the arterial supply comes from the two inferior rectal arteries, which are branches from the internal pudendal arteries, which come from the internal iliac arteries.

The internal rectal venous plexus also drains the lower part of the anal canal. However, instead of traveling to the portal Venus system, it drains into the systemic venous system.

The internal venous plexus below the pectinate line drains into the inferior rectal veins, and then to the internal pudendal vein, and then into the internal iliac vein.

The lymphatic vessels below the pectinant line drain to the superficial inguinal lymph nodes. The lower part of the anal canal is innervated by branches from the pudundal nerves, the inferior rectal nerves.

These nerves control the contraction and relaxation of the external anal sphincter. Finally, the afferent fibers from the lower part of the anal canal are somatic, making the lower part sensitive to pain, touch, and temperature.

To help you remember the differences above and below the pectinate line, everything above gets VIP access. Unfortunately, we don't live in a perfect world, so there are a few cases where things aren't so straightforward.

Exceptions in blood supply and innervation of the anal canal13:04–13:58

The first comes from the arterial supply. The middle rectal arteries anastomo the superior and inferior rectal arteries and supply blood both above and below the pectinate line.

The middle rectal veins also anastomose the superior and inferior rectal veins from the internal rectal plexus to drain the anal canal both above and below the pectinate line.

The internal rectal venous plexus also receives arterial blood from the superior and middle rectal arteries through short vessels, forming arteriovenous anastomosis.

This forms pouches around the anal rectal junction called anal cushions. All right, as a quick recap.

Review13:58–16:37

The gastrointestinal organs of the pelvis are the rectum and the anal canal, which form the last parts of the gastrointestinal tract.

In males, the rectum lies behind the fundus of the urinary bladder, the seminal vesicles, the right and left ductus deferens, and the prostate gland.

In females, the rectum lies behind the lower end of the uterus and the vagina. The rectum has 5 external flexures the sacral, the anorectal, and the superior, inferior, and intermediate lateral flexures.

Its 3 internal folds, the superior, middle, and inferior transverse rectal folds, correspond to the lateral flexures. The blood supply of the rectum comes from the superior, middle and inferior rectal arteries.

As for venous drainage, the superior rectal veins drain into the portal venous system. Whereas the middle and inferior rectal veins drain into the systemic venous system.

The anal canal begins at the anorectal junction and descends posterior inferiorly, ending at the anus. The anal canal has internal and external sphincters, which are circular muscles that control defecation.

The internal sphincter is an involuntary muscle that contracts to prevent the leakage of feces or gas and relaxes as the rectal ampula fills with feces.

The external anal sphincter is a voluntary muscle that controls defecation. Inside the anal canal there are the anal columns, and at their lower ends there are the anal valves which have a small space above them called the anal sinuses.

Finally, the lower borders of the anal sinuses and valves form an irregular line called the pectinate line. Which is a landmark that marks important anatomical changes.

Above the pectinate line, arterial supply is ensured by the superior rectal artery, while venous blood drains into the portal venous system.

Below the pectinate line, arterial supply comes from the inferior rectal arteries, while venous blood drains into the systemic venous system.

However, the middle rectal arteries and veins also ensure vascularization both above and below the pectinate line.