Week 2: Gross Anatomy of the Small and Large Intestines Flashcards
Small Intestine: Functions
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
Describe the gross anatomical structure of the SI
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)
What are the parts of the SI?
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
Describe the anatomy of the Duodenum
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.
Functions of the Duodenum
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
Functions of the Jejunum
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
Functions of the Ileum
Ileum = distal 3/5 of the SI
Segmentation (mechanical mixing) and Peristalsis (movement through)
Some chemical digestion and absorption occurs here
Surface Anatomy of the Duodenum: which quadrants and regions?
Mainly in Right upper quadrant, and a little in the left upper.
Umbilical and epigastric regions
Surface anatomy of jejunum and Ileum: which quadrants and regions?
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
Describe the ‘segmentation’ movement of the SI
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)
Describe the peristalsis of the small intestine
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.
Describe the regulation of motility of the SI
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
What is the Ileocaecal Sphincter?
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
Large Intestine: Functions:
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
What are the components of the Large Intestine?
Caecum (receives chyme from SI via Ileocaecal sphincter), the appendix, colon, rectum and anal canal
What are the anatomical components of the Colon?
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
Describe the position of the caecum
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.
Describe the position of the Root of Appendix
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
Describe the structure and function of the rectum
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
Are the following intraperitoneal or retroperitoneal etc…
Duodenum, Jejunum, Ileum, Ascending, descending colon, Transverse and Sigmoid colon,
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
What are the paracolic gutters and what is thie clinical significance?
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
What are the organ contents of the infra- and supra-colic cavities of the greater sac of the peritoneal cavity?
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??
What is the transverse mesocolon?
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
What is the greater omentum?
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
Describe the surface anatomy of the different compartments of the large intestine: in terms of the 9 regions of the abdomen
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
Why is the left colic flexure more superior than the right colic flexure?
Because the Liver is located on the right, and tends to push the colic flexure inferiorly
Describe the blood supply of the proximal duodenum (foregut)
Branches of the coeliac artery.
The Supraduodenal artery and Superior Pancreaticoduodenal artery…. details of origin**
