Foregut, Midgut And Hindgut Flashcards
What does the dorsal mesentery contain?
Neurovascular supply & lymph drainage to the gut
What organs are retroperitoneal?
S: suprarenal (adrenal) gland A: aorta/IVC D: duodenum (second and third part) P: pancreas (except tail) U: ureters C: colon (ascending and descending) K: kidneys E: (o)esophagus R: rectum (inferiorly)
How do organs become retroperitoneal?
- Gut tube moves + rotates during developing pushing sections of gut tube against the posterior abdominal wall
- Mesenteric zygosis: mesenteries fuse + disappear
- Gut tube stuck to posterior abdominal wall helping keep the GI tract in position + not falling down with gravity
Where are the paracolic gutters (PCG)?
Right lateral PCG
Left lateral PCG
Right medical PCG
Left medial PCG
(all relating to the position of the colon)
What is the clinical relevance of paracolic gutters (PCG)?
Act as routes for fluid movement + infection spread (infections can spread from under diaphragm to pelvic cavity)
What paracolic gutter (PCG) can cause diaphragmatic infection? Why?
Lateral PCGs as the medial PCGs are in the middle of the colon so the transverse colon mesentery prevents fluid from moving further upward than this whereas fluid can move around the sides of the colon with the lateral PCGs
Where does the oesophagus extend from and to?
Begins at distal laryngopharynx (C6/7 at upper oesophageal sphincter)
~25cm long
Ends at stomach
Describe the structure of the oesophagus.
Skeletal voluntary muscle forms the upper 1/3
Smooth autonomic muscle forms the lower 2/3’s
What does the oesophagus run posterior to?
Trachea
Tracheal bifurcation
Left atrium of heart
Where does the anterior vagal trunk (CN X) pass?
Through diaphragm: anterior (left vagus) or posterior (right vagus) to the oesophagus
What is a sphincter?
A ring of muscle fibres that can contract or relax, closing the diameter of the circle, sealing off tubes + pipes
Does the vagus nerve (CN X) supply sympathetic or parasympathetic fibres to the gut?
Parasympathetic
Where does the lower oesophageal sphincter sit? Why is it not a true physiological sphincter?
T11-12 (T10 sometimes)
Right diaphragmatic crus is a loop of muscle that forms + closes this sphincter when it contracts by producing a kink in the muscle
At what vertebral level does the aorta pass through the diaphragm?
T12 just L of midline
At what vertebral level does the aorta bifurcate?
L4
Describe the structure of the stomach.
Oesophagus -> fundus -> body -> pyloric antrum -> pyloric canal -> pyloric sphincter -> duodenum (part 1)
Lesser curvature + greater curvature
Folds of mucosa inside the stomach = rugae
Why can you see the fundus on radiographs?
As it often contains air so it will be black when the patient is standing up
What are mesenteries?
Double-layered folds on peritoneum attaching the gut tube to the body wall
What type of mesentery to the foregut, midgut and hindgut have?
Foregut: ventral + dorsal
Midgut + hindgut: dorsal ONLY
What is the function of rugae?
Assist with mechanical digestion of food
What is the lesser omentum? What is found within its free lower edge?
The peritoneum between the stomach’s lesser curvature + liver
Common bile duct, hepatic artery + portal vein
What is the greater omentum?
Large flap/loop of 4 layers of peritoneum OR 2 layers of dorsal mesentery that hangs off the greater curvature of the stomach
What is a beneficial feature of the greater omentum?
Sticks down to sites of inflammation/infection helping to stop the spread but this is not done purposefully
How does the greater omentum form?
Because rapid peritoneal growth forms a fold
What is the blood supply to the stomach?
Coeliac trunk
Branches: Oesophageal L gastric R gastric Short gastric L gastro-omental R gastro-omental Gastroduodenal Common hepatic Splenic artery Superior pancreaticoduodenal
Where does the gastroduodenal artery pass?
Behind duodenum (part 1)
Where does the blood supply to the stomach run mainly?
In the greater + lesser curvatures
What is the structure of the duodenum?
Pyloric sphincter opens into part 1
2, 3 + 4
Duodenal-jejunal flexure
Extends from L1-3 + is C-shaped
What is the clinical significance of the ligament of Treitz?
Landmark of duodeno-jejunal flexure that attaches to the diaphragm supporting a junction point
It is a marker point as it helps define upper GI bleeds proximal to it + lower GI bleeds distal to it
Where does the duodenal arterial supply come from?
Coeliac trunk
SMA via pancreaticoduodenal arteries
Branches:
- Gastroduodenal + superior pancreaticoduodenal (coeliac)
- Inferior panreaticoduodenal (SMA)
Duodenal ulcers may occur in the 1st part of the duodenum. What problems might occur?
An ulcer can erode through the wall perforating the gastroduodenal artery that runs behind causing a patient to bleed out
What are the anatomical relations of the stomach and duodenum?
IVC, aorta, SMA + SMV emerge behind them + the pancreas
Kidneys + pancreas are in close proximity too
What is the structure of the small intestine?
4-9m
Jejunum (proximal): wider tube, thicker wall, bigger but straighter vessels + blood supply shorter
Ileum (distal): smaller tube, thinner wall but longer + more branched blood supply
Where is the caecum?
R iliac fossa
What are the significant structural features of the caecum?
Teniae Coli: longitudinal muscle of colon sitting in 3 bands that begin at caecum (appendix) + end by covering the rectum
Haustrum/sacculation: pouch of LI
Ileocaecal junction
Mesoappendix: mesentery connecting ileum to appendix
What is the appendix? What is its blood supply?
Blind-ended sac in the retrocaecal position
Via appendiceal branch of ileocolic artery
How can you tell the difference between the jejunum, ileum and colon in barium contract meal/enema imaging?
Jejunum lining is frilly whilst ileum lining is smooth
Colon shows larger sacculations
What is the classic pain pattern of appendicitis?
Early visceral pain refers from the T10 dermatome to the umbilical region
Later stage somatic pain refers to the R iliac fossa so it localises because the parietal peritoneum becomes irritated
What are the different parts of the large intestine?
Ascending colon Transverse colon Descending colon Sigmoid colon Rectum
Where does the ascending colon refer pain?
Umbilical region
Where does the descending colon, sigmoid, rectum refer pain?
Pubic region
How does the state of faeces change as it moves through the large intestine?
Faeces is more liquid in the ascending colon where there is a wider diameter and becomes firmer in the descending colon when the diameter is narrower
How can the transverse colon vary between individuals?
Can droop very low into the umbilical or pubic region
Why will tumours in the ascending colon present much later than in the descending colon?
The ascending colon is much wider so is harder to block than the descending colon
What are the branches of the SMA that supply the midgut-derives structures?
Middle colic Inferior pancreatico-duodenal Jejunal R colic Ileal Ileocolic Appendicular
What are the branches of the IMA that supply the hindgut-derived structures?
L colic
Sigmoidal
Superior rectal
What is special about the large intestinal blood supply?
SMA and IMA join at the midgut-hindgut boundary to the marginal artery forming an anastomoses (must avoid this in surgery as a lot of bleeding would take place!)
What is the lymph drainage of the GI tract?
Travels alongside arterial supply along 3 major blood vessels back towards the aorta + joins at the cisterna chyli to track back to the thoracic duct
What is the cisterna chyli?
Saccular area of dilatation of lymph to the immediate right of the abdominal aorta
What is the thoracic duct?
Main vessel of lymphatic system passing in front of spine upwards + draining into the L subclavian vein near the base of the neck near the supraclavicular lymph nodes (why GI cancers can spread here causing a left supraclavicular lymphadenopathy)
What is the blood supply to the rectum?
IMA
Internal iliac arteries
(has significant implications for venous drainage of the region as some drain up + some down)
Internal venous plexus
External venous plexus
Where are transverse rectal folds?
2 on L
1 on R
What are the origin points for classic and external haemorrhoids?
Classic = internal venous plexus
External = external venous plexus
Explain the term pelvic pain line.
Pelvic organs covered in peritoneum has visceral sensory nerves that travel along sympathetic nerves originating from T11-L2 referring pain to here
Pelvic organ not covered in peritoneum have visceral sensory nerves travelling along parasympathetic nerves originating from S2-4 so refer pain to here
What are the classically reported positions for haemorrhoids to arise in?
3, 7 + 11 O’clock around the anus if looking at patient from feet up
What is the defecation reflex?
Reflex at the level at the sacral spinal cord level that we develop higher-level CNS control over
What connects the rectum to the outside world?
Anal canal
What nerves control the defecation reflex? What do they do?
Sympathetic nerves stops defecation by contracting the internal anal sphincter
Somatic (pudendal nerves S2-4) stop defecation by causing tonic contraction of external anal sphincter
Parasympathetic (pelvic splanchnic nerves S2-4) cause defecation by relaxing internal anal sphincter + causing rectal wall contraction
What is the innervation of the internal and external anal sphincters?
Internal: autonomic motor innervation from sympathetic + parasympathetic
External: somatic motor innervation
Explain the defecation reflex.
- Mass movement of faeces towards rectum
- Distension activates stretch receptors
- Rectal + sigmoid colon contraction, relaxation of IAS + contraction of EAS
- Increased rectal pressure causes either delay or defecation
5a. Delay causes contraction of EAS + reverse peristalsis in rectum
5b. Defecation relaxes EAS + forward peristalsis in rectum along with Valsalva maneuver
