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Let’s continue our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.
Play USMLE Audio MP3 04 09 Anatomy Rectum Below
Begin 04 09 Anatomy Rectum Transcription
Let’s take a detailed look at the rectum and anus.
About how long is the rectum?
- Fifteen to twenty centimeters long.
And what three superficial structures of the entire colon are not present on the rectum?
- The three teniae colie.
And what does the rectum have instead of the three teniae?
- A continuous layer of longitudinal muscle.
Because the teniae coalesce to form one continuous layer of longitudinal muscle which is not significantly shorter than the length of the rectum, what other two features are absent from the rectum that are present in the colon? Haustra, or sacculations, and epiploic appendices.
As the sigmoid colon merges with the rectum, in what direction is the sigmoid colon traveling?
- Transversely toward the right. The most distal portion of the sigmoid colon then makes a bend.
In what direction does this bend occur?
During this posterior bend, what transition is occurring?
- The transition between the sigmoid colon and the rectum.
What happens next to the most proximal portion of the rectum?
- It bends down or inferiorly.
And what curve is introduced once it is traveling in an inferior direction?
- It begins to curve forward slightly or anteriorly.
As it does this, first going inferiorly and then beginning to curve slightly forward what concavity is it following?
- The concave or pelvic surface of the sacrum down to the top of the coccyx. Now let’s go back to the rectosigmoid junction.
When the area of the rectosigmoid junction bends posteriorly and goes back to the concavity of the sacrum, what is this flexure called?
- The sacral flexure of the rectum.
And as the sigmoid colon transitions to the rectum at the rectosigmoid junction and the sacral flexure, what six external features are lost which are present on the sigmoid colon, but not on the rectum? Hint: We’ve covered five of these already.
- The three teniae colie, the haustra, the epiploic appendices, and the sigmoid mesocolon. Let’s review the internal mucosa for just a second.
What two features are present in the internal mucosa of the colon?
- Semilunar folds and crypts.
What feature of the mucosa of the small intestines is absent from the mucosa of the colon?
- Villi. The colon has crypts but no villi.
Now, going back to the rectosigmoid junction, what bony prominence is the sigmoid colon ducking under to begin traveling posteriorly back to the concavity of the sacrum?
- The sacral prominence at S1.
So, at what sacral levels is the transition from sigmoid colon to rectum occurring?
- S1 and S2.
Now at the level of the coccyx, the rectum bends posteriorly and passes through what structure?
- The pelvic floor.
And what is the name of this lower posterior bend the rectum makes as it passes through the pelvic floor?
- The peroneal flexure. Let’s look at the lumen of the rectum.
What three structures separated the rectum above the pelvic floor into quarters?
- The three anal valves.
And what are they called?
- The superior, middle, and inferior anal valves.
What do the anal valves look like?
- Large half circular folds in the rectal wall. They correspond to the sinuous shape of the rectum.
How many are on the right and how many on the left?
- There is one on the right and two on the left.
And the names of the two on the left?
- The superior and inferior rectal valves.
And the name of the one, then, on the right?
- The middle rectal valve.
Now where the middle rectal valve is on the right in the lumen, what is seen externally above and below?
- Two bulges above and below on the right and a notch between the two bulges corresponding to the location of the middle rectal valve in the lumen.
Where the superior rectal valve is then, on the left, inside the lumen what is seen externally on the left side of the rectum and underneath what structure?
- There is a notch on the left just under the rectosigmoid junction.
And again, what is seen on the left outside of the rectum, corresponding to the inferior rectal valve inside?
- Another notch. This is the lowest one.
Student doctor, please work your way down the left side of the rectum, beginning just under the rectosigmoid junction and describe the contour of the rectum on the left side and correspond it to the structures.
- First, there will be a notch, just below the rectosigmoid junction and this corresponds with the superior rectal valve. Then there will be a bulge to the left and the apex of this bulge to the left corresponds to the level of the notch that’s on the right-hand side, which corresponds of course, to the middle rectal valve inside the lumen. And then at the bottom of this bulge on the left that we’re talking about, there is another notch, of course, and this notch corresponds to the inferior rectal valve.
Below this notch on the left is another bulge to the left and this goes all the way down to?
- The pelvic floor.
Now looking at the right-hand side of the rectum.
The single notch that is on the right-hand side of the rectum corresponds of course with what structure inside the lumen?
- The middle rectal valve.
And the notch that’s on the right corresponds with what contour on the left?
- It corresponds with the apex of the first bulge on the left which begins just below the rectosigmoid junction.
Between which two rectal valves is the level where the peritoneum reflects off of the rectum, leaving the extraperitoneal rectum below?
- The peritoneum reflects off the outside of the rectum at about the level between the middle and inferior rectal valves, or between the notch on the right and the lower notch on the left.
Now when the rectum is empty, what do the rectal valves do?
- They interlock.
And when the rectum is full, what happens to the rectal valves?
- They are flattened out by the distension of the rectal walls. Now the middle two-thirds of the rectum, from the superior rectal valve to the perineal flexure at the pelvic floor is quite distensible and as it fills up and expands the feeling of the need to defecate becomes conscious.
What is the name of this distensible middle two-thirds?
- The rectal ampulla.
When the feeling of the need to defecate becomes conscious due to filling of the rectal ampulla and the stretching of both the rectal walls and the rectal valves, what general type and plexus of nerves carries this afferent feeling back to consciousness?
- The parasympathetic pelvic splanchnics.
What reflex action then happens in the rectum in an attempt to defecate?
- Peristaltic contractions of the rectum.
And what general type and plexus of nerves carries this efferent command to the rectum to initiate peristaltic contractions?
- Again, the parasympathetic pelvic splanchnics.
Which of the three rectal valves we discussed above is much the largest?
- The middle rectal valve.
And which side is it on?
- The right side of the rectum.
And what is the other name for the middle rectal valve on the right?
- Kohlrausch’s Fold.
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