USMLE Step 1 Review 04 04 Anatomy Large Intestine

USMLE Step 1 Review 04 04 Anatomy Large Intestine


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Play USMLE Audio MP3 04 04 Anatomy Large Intestine Below


Begin 04 04 Anatomy Large Intestine Transcription

Alright, we’re going to move on to the colon now.

What is the major function of the colon?

  • Reabsorption of water and electrolytes. Now the chyme is thickened in the colon by the water reabsorption.

What two processes however transform it into feces?

  • Fermentation and decomposition. Some of the water and electrolytes come from what was ingested but much of it arises from other sources.

What are the other sources?

  • Secretions of the stomach, the gallbladder, the pancreas, and the small intestines.

About how much water is reabsorbed by the colon in a day?

  • About one liter.

And normally, how much water is lost through defecation in a day?

  • Less than one hundred ccs.

What happens if substantially more than one liter of water enters the colon during a day?

  • Diarrhea. The colon is not able to absorb amounts greatly in excess of one liter.

About how long is the large intestine?

  • One and a half to 1.8 meters.

What ends the small intestines at the end of the ileum and begins the large intestines?

  • The ileocecal valve or colic valve.

In what quadrant of the abdomen is the ileocecal valve?

  • The lower right quadrant.

The ileocecal valve empties into the top of what pouch?

  • The cecum.

And what hangs from the cecum, originating inferior to the ileocecal valve?

  • The vermiform appendix.

And what is the function of the appendix?

  • Immune system monitoring of output from the small intestines.

What immune structures are present in both the submucosa and particularly the mucosa of the appendix?

  • Lymph follicles.

What aggregation of lymph follicles were present in the ileum which served the same purpose?

  • Peyer’s patches.

Now when the lumen of the appendix has become constricted due to reaction to contents, what process and structure is responsible for the constriction?

  • Hypertrophy of the lymph follicles.

What feature does the mucous membrane of the appendix share with the entire large intestines?

  • Crypts.

What structure present in the small intestines is absent in the mucosa of the appendix and large intestines?

  • Villi.

What are the six parts of the large intestine in order after the cecum?

  • Ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus.

Now what area marks the transition from the ascending colon to the transverse colon?

  • The right colic flexure.

And the right colic flexure is inferior to what organ?

  • The liver.

What is the area of transition from transverse to descending colon?

  • The left colic flexure.

And the left colic flexure lies anterior to what structure?

  • The lower pole of the spleen.

Which colic flexure is much more superior?

  • The left.

Which three parts of the colon are anterior to various parts of the small intestine?

  • Cecum, ascending colon, transverse colon.

Which four parts of the large intestines then are generally posterior to small intestines?

  • Appendix, descending colon, sigmoid colon, rectum.

Which of these four parts may have a portion not covered by small intestine and against the anterior abdominal wall?

  • The first half of the sigmoid colon.

Now which part of the large intestine presents the largest resistance to the passage of contents, requiring more active peristalsis to move bowel contents past it?

  • The left colic flexure.

What part of the large intestine has the widest diameter?

  • The cecum.

In what space is the cecum located?

  • The right iliac fossa.

And in what space is the sigmoid colon located?

  • The left iliac fossa.

And what is the general shape of the sigmoid colon?

  • S-shaped.

In what section of colon does colon cancer tend to grow the largest before being detected?

  • The ascending colon.

What are the three reasons that colon cancer located in the ascending colon tends to be discovered later than in other locations?

  • First, the lumen is larger. Second, the content’s more liquid and third, the wall’s thinner and more easily expanded. So the result of these three is that the growing tumor takes longer to cause obstruction.

Which structure of the large intestines has the most anatomic variability in position?

  • Vermiform appendix.

And what is the most common position of the appendix?

  • The retrocecal position, 65% of the time.

How does the appendix lie and run in the retrocecal position?

  • It lies posterior to the cecum and runs superiorly.

What is the name of the location for attempting to palpate the origin of the appendix?

  • McBurney’s point.

And where is McBurney’s point?

  • About two-thirds lateral on a line from the umbilicus to the right ASIS.

And what does ASIS stand for?

  • Anterior superior iliac spine. Now in about 30% of the time the appendix descends from the cecum.

When the appendix descends, where is the tip palpated?

  • About two-thirds toward the right on a line between both ASIS’s.

And what is the name of this point?

  • Lanz’s point.



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