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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 02 Anatomy Small Intestine 7 Below
Begin 04 02 Anatomy Small Intestine 7 Transcription
Now except for the part of the duodenum we’ve already discussed, the superior mesenteric artery supplies all of the small intestines and it also supplies parts of the colon which we’ll discuss later when we talk about the colon. For now, we’ll just deal with the branches from the superior mesenteric artery that supply the small intestines. First just a little review.
Which part of the duodenum is intraperitoneal?
- The first portion.
Within what part of peritoneum?
- The hepatoduodenal ligament.
What is the relationship of the descending, horizontal, and ascending portions of the duodenum to the peritoneum?
- The rest of the duodenum is retroperitoneal.
And what is the relationship of the jejunum and ileum after the duodenojejunal flexure to the peritoneum?
- The jejunum and ileum are both intraperitoneal.
What structure hanging from the greater curvature of the stomach and from the transverse colon overlays as a free-hanging curtain the small intestines as they are convoluted within the abdominal space?
- The greater omentum.
Are the small intestines suspended within the greater omentum?
What is the name for the pouch of peritoneum which suspends the entire small intestines, that is the jejunum and ileum?
- The mesentery.
Now this pouch of peritoneum which suspends the small intestines is what type of peritoneum?
- Visceral peritoneum.
Its two layers reflect from the posterior abdominal wall joined together to form the two-layered mesentery and then extend out to the convolutions of small bowel. Mesentery is fixed at its root to the posterior abdominal wall where it reflects.
What is the name for this root of the mesentery?
- The radix mesenterii.
How long is the root of the mesentery, the radix mesenterii?
- Fifteen to eighteen centimeters long.
However, the mesentery, when it attaches to the small intestines, what is the total length of that pouch-like enfoldment if the small intestines were to be removed and spread out?
- Over four meters.
The root of peritoneum, where peritoneum from the right and peritoneum from the left meet and reflect outward to form the mesentery, forms a line which moves obliquely downward across the posterior abdominal wall and to the right.
Where does the root of the mesentery begin, at the top or most superior aspect?
- At the place where the superior mesenteric vessels, the superior mesentery artery on the left and the superior mesenteric vein on the right, both of which are just to the right of the duodenojejunal flexure.
The root of the mesentery then moves down following the course of the superior mesenteric artery and vein as they pass down over the horizontal portion of the duodenum, and then it moves obliquely down and to the right across what structure?
- The inferior vena cava.
And it moves further down obliquely to the right across what structure?
- Psoas major muscle.
And then where does it end?
- At the ileocecal junction.
Now as the root of the mesentery cuts down obliquely to the right, across psoas major muscle, there are five structures running vertically that it cuts across.
What are they in order from left to right?
- First, the right ureter, then one of the ovarian or testicular veins, then the right ovarian or testicular artery, and then the right ovarian or testicular vein again, (there’s two of them and they’re parallel on either side of the right ovarian or testicular artery) and then lastly, although it’s underneath a pad of fat, is the right genitofemoral nerve. Now we’ve been talking about the root of the mesentery cutting across all of these structures. However, the root of the mesentery is actually following the course of a particular structure, and it is this particular structure that’s actually cutting across all of these others.
What structure is the root of the mesentery following from the duojejunal all the way to the ileocecal junction?
- The course of the superior mesenteric artery. Now as the superior mesenteric artery courses downward and to the right, it is going to give off several branches that are going to the colon on the right hand side.
What is the relationship of these colic branches from the superior mesenteric artery to the peritoneum?
- These branches going to the right are retroperitoneal.
Now the superior mesenteric artery gives off many branches to the left and to below, going to the various parts of the small intestines, after the duodojejeunal flexure, what is the relationship of all of these branches to the peritoneum?They are all intraperitoneal and in fact within what part of the peritoneum?
- They are all within the mesentery, between the two layers of peritoneum that have reflected from the posterior abdominal wall and joined together to run out to the small intestines.
What is the first artery given off to the left from the superior mesenteric artery running in the mesentery to parts of the small intestine?
- The first jejunal artery.
Now the rest of the arteries given off to the left into the mesentery, to supply the jejunum and ileum are called either?
- Jejunal or ileal arteries.
Now the division between the jejunum and the ileum is not distinct. One way of dividing them is to say that one fraction of the intraperitoneal section of the small intestine is jejunum and then the lower fraction is ileum.
What upper fraction of the intraperitoneal small intestine is usually termed jejunum?
- The upper two-fifths.
In that case, what is the lower fraction?
- The lower three-fifths is the ileum.
Now these many jejunal and ileal arteries coming from the superior mesenteric artery are joined together by what type of small sections of artery?
- Anastomotic loops. Then, many short arteries run from these anastomotic loops to the sections of small intestine.
What are these short connecting arteries called?
- The straight arteries.
What is the general architectural term for this network of anastomotic loops and straight arteries?
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