Week 2: Gross Anatomy of the Small and Large Intestines

Question 1

Small Intestine: Functions

Answer 1

Mechanical Digestion and mixing chyme from the stomach with various secretions

Secretions include: water, ions, mucous.

The Small Intestine also receives pancreatic secretions

SI performs chemical digestion of nearly all nutrients - via enzymes secreted by the SI, received from the pancreas, and bile

Absorption of nearly all nutrients, most water and ions

Peristaltic movement of chyme to large intestine

Question 2

Describe the gross anatomical structure of the SI

Answer 2

Lumen of the SI has circular infoldings of the mucosa and submucosa which are macroscopically visible - Plicae circulares

Villi are probably just visible macroscopically

(Villi also have microvilli, forming a brush border, which are present on the epithelial cells and are only visible microscopically)

Question 3

What are the parts of the SI?

Answer 3

Duodenum = first 20-25cm (apparently 12 fnger-lengths)

Jejenum = Proximal 2/5 of small intestine after the duodenum

Ileum = Distal 3/5 of small intestine after the duodenum

Question 4

Describe the anatomy of the Duodenum

Answer 4

The duodenum extends from the pyloric sphincter to the duodenojejunal flexure: ~20-25cm long

The duodenojejunal flexure occurs at the end of the ascending duodenum, where is turns downwards abruptly, thus becoming the jejunum

4 parts: superior, descending, horizontal and ascending

The descending part receives the bile duct from the liver, and the pancreatic duct. These two structures perforate the medial side of the descending duodenum, forming a structure called the Major Duodenal Papilla

The minority of people also have an accessory pancreatic duct, which enters the duodenal lumen via the minor duodenal papilla, which is slightly higher up than the major duodenal papilla.

Question 5

Functions of the Duodenum

Answer 5

Receives Chyme from stomach, which is highly acidic

Acidic Chyme is neutralised by Duodenal gland secretions, bile from liver and gallbladder, and pancreatic juices

Duodenum is involved in regulating gastric (stomach) emptying, gallbladder contraction, and secretions from pancreas and lover - via endocrine and neural control

Also involved in digestion and absorption

Question 6

Functions of the Jejunum

Answer 6

Consists of the proximal 2/5 of the SI, beyond the duodenum

Jejunem engages in segmentation movements, which mixes the chyme with bile, pancreatic juices and SI secretions

Also engages in peristaltic movements to move contents through to Ileum

Most chemical digestion and absorption occurs in Jejunum

Question 7

Functions of the Ileum

Answer 7

Ileum = distal 3/5 of the SI

Segmentation (mechanical mixing) and Peristalsis (movement through)

Some chemical digestion and absorption occurs here

Question 8

Surface Anatomy of the Duodenum: which quadrants and regions?

Answer 8

Mainly in Right upper quadrant, and a little in the left upper.
Umbilical and epigastric regions

Question 9

Surface anatomy of jejunum and Ileum: which quadrants and regions?

Answer 9

Fills most of abdominal cavity

Present in all 4 quadrants

Mainly present in the umbilical and hypogastric regions, although sneaks into the middle edges of all other regions as well

Question 10

Describe the ‘segmentation’ movement of the SI

Answer 10

Segmentation serves to mix digestive contents

Involves contraction of short lengths (1-5cm) of inner circular muscle of the jejunum/ileum at a time

Involves slow wave potentials of depolarisation/repolarisation: 12-15 cycles per minute in duodenum, and 6-9 cycles per minute in ileum. Not all waves of potentials cause contractions.

Segmentation is controlled by the enteric nervous system (ENS)

Question 11

Describe the peristalsis of the small intestine

Answer 11

Peristalsis = waves of relaxation and contraction of the inner circular layer of smooth muscle, with a leading wave of contraction of the outer longitudinal smooth muscle

Each individual wave occurs over 10-70cm of SI at a time

Propels contents along length of SI

Migrating Motility Complex (regulated by motilin) also occurs during fasting periods = Intense peristaltic contractions that progress from stomach to duodenum, to ileocaecal junction, once overy ~1.5 hours. Serves to move any residual chyme/contents through.

Question 12

Describe the regulation of motility of the SI

Answer 12

Local mechanical and chemical (including hormonal) stimuli stimulate the smooth muscle to contract locally. E.g. distension, low pH, digestion products, etc.

Typical transit time through SI = 3-5 hours

Question 13

What is the Ileocaecal Sphincter?

Answer 13

Ileocaecal sphincter is at the junction of the ileum and caecum of large intestine

Periodically relaxes to allow chyme from ileum to caecum

Is tonically contracted to prevent reflex, and engages in even stronger contraction as the caecum distends due to filling from ileum

Question 14

Large Intestine: Functions:

Answer 14

Primarily secretes mucous - has protective function due to lots of bacteria, also helps to lubricate passage of dehydrated stools through LI

Absorbs most remaining water and ions, and some nutrients

Lots of bacteria present, which perform some digestion

Performs compaction of remaining luminal contents into faecal stools

Peristalsis moves faeces along

Storage and defecation of faeces

Question 15

What are the components of the Large Intestine?

Answer 15

Caecum (receives chyme from SI via Ileocaecal sphincter), the appendix, colon, rectum and anal canal

Question 16

What are the anatomical components of the Colon?

Answer 16

Ascending colon, with the Right (Hepatic) Colic Flexure, the Transverse colon, the Descending colon, with the Left (Splenic) Colic Flexure, the Sigmoid colon, which leads into the rectum

Taeniae coli: 3 discontinuous bands of longitudinal muscle, the contraction of which form the haustra (sacculations) of the colon.

Omental Appendices may also be seen (adipose tissue contained within peritoneum, situated along the length of the colon).

Everything before 2/3 of the way through the transverse colon = midgut derived

Everything beyond that point = hindgut derived

Question 17

Describe the position of the caecum

Answer 17

First part of the large intestine, recieves chyme from SI via Ileocaecal sphincter.
Usually anterior to iliacus muscle
Completely covered by peritoneum. The caecum lacks a mesentery, however, and is less mobile than the SI because of this.

Question 18

Describe the position of the Root of Appendix

Answer 18

The location of the appendix can vary: may sit anterior, posterior, medial or lateral to the caecum, but most commonly hangs below the caecum

The root of the appendix is located 2cm inferior to the ileocaecal valve, at the intersection of taeniae coli

The appendix is intraperitoneal

Question 19

Describe the structure and function of the rectum

Answer 19

Rectum is the distal part of the LI

It has no haustra and no teniae coli - it has a complete layer of longitudinal muscle instead

Lacks mesentery

Stores faeces

The sling-like muscle, puborectalis, pulls the retroanal junction anteriorly, creating the Perineal Flexure (which is ~80 degrees). This contributes to faecal continence:

Transverse rectal folds are incomplete valve-like infoldings of the lumen

Question 20

Are the following intraperitoneal or retroperitoneal etc…

Duodenum, Jejunum, Ileum, Ascending, descending colon, Transverse and Sigmoid colon,

Answer 20

Duodenum = secondarily retroperitoneal (had mesentery during development, but later became positioned against posterior abdominal wall)

Jejunum & Ileum = Intraperitoneal (have mesentery)

Ascending & Descending Colon: Secondarily Retroperiotoneal

Transverse & Sigmoid Colon: Intraperitoneal (have a mesentery - mesocolon)

The transverse mesocolon divides the greater sac of the peritoneal cavity into supracolic and infracolic compartments

Question 21

What are the paracolic gutters and what is thie clinical significance?

Answer 21

The paracolic gutters are left and right peritoneal recesses on the posterior abdominal wall, lying alongside the ascending and descending colons. The main paracolic gutters lie laterally to the asc/desc colons, although a medical paracolic gutter may occur medially to the colon, especially on the right side (ascending colon).

Significant because these gutters serve as a passage for the spread of infective contents between different abdominal organs - e.g. infective contents from appendix could spread up the right paracolic gutter to the hepatorenal recess, and cause infection in either of those organs

Question 22

What are the organ contents of the infra- and supra-colic cavities of the greater sac of the peritoneal cavity?

Answer 22

The infra- and supra-colic compartments are separated by the transverse mesocolon

Infracolic compartment = Distal portion of the duodenum, the jejunum, ileum, caecum and appendix, and colon

The supracolic compartment = distal portion of oesophagus, stomach, proximal portion of the duodenum, liver & spleen??

Question 23

What is the transverse mesocolon?

Answer 23

A derivative of dorsal mesentery

It is two layers of peritoneum that connect the transverse colon to the posterior abdominal wall.

It is also continuous with the 2 posterior layers of the greater omentum

*In fact, the mesocolon and transverse colon often fuse to the greater omentum

The mesocolon also separates the greater sac of the peritoneal cavity into the infra- and supra-colic compartments

Question 24

What is the greater omentum?

Answer 24

A double layer of peritoneum which hangs from the greater curvature of the stomach, descends anteriorly in front of the SI, then reflects back upon itself, reaching the transverse colon, and then continuing to the posterior abdominal wall

Although it is a double-layer f peritoneum, it becomes 4 layers of peritoneum anteriorly, as it doubles back upon itself, anteriorly to the SI

Question 25

Describe the surface anatomy of the different compartments of the large intestine: in terms of the 9 regions of the abdomen

Answer 25

Caecum = Right Flank

Root of appendix = Usually at McBurney’s point: 1/3 of the way from ASIS (Anterior superior iliac spine) to the umbilicus

Ascending colon = Right Flanks

Transverse Colon = R Hypochondriac, to umbilicus, back up to L Hypochondriac

Descending colon = L Hypochondriac to L Flank

Sigmoid Colon = L groin to Hypogastric region

Question 26

Why is the left colic flexure more superior than the right colic flexure?

Answer 26

Because the Liver is located on the right, and tends to push the colic flexure inferiorly

Question 27

Describe the blood supply of the proximal duodenum (foregut)

Answer 27

Branches of the coeliac artery.

The Supraduodenal artery and Superior Pancreaticoduodenal artery…. details of origin**