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Flashcards in Small Bowel Deck (90)
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Week 4 GI embryo

Primitive gut tube (made from endoderm) begins to develop into foregut, midgut, and hindgut

Endoderm becomes intestinal epithelium and glands

Mesoderm become connective tissue, muscle, and wall of intestine


Week 5 GI embryo

Intestine elongates and midgut loop herniates through umbilical ring

Midgut loop continues to lengthen extracoelomically until about week 10


Week 10 GI embryo

Midgut loop rotates 270 degrees counterclockwise and returns back into the abdominal cavity

This is around axis of SMA


What is the SI derived from?

All of the SI is from midgut EXCEPT proximal duodenum (foregut)

Junction btw foregut and midgut is just distal to opening of common bile duct


Vitelline duct

Initially, the primitive gut tube communicates with the yolk sac. This narrows by week 6 to form the vitelline duct.

If duct fails to obliterate by end of gestation, it persists as a Meckel's diverticulum (2% of population)


Small bowel relation to peritoneum

Duodenum: First 2 cm is intraperitoneal. Rest is retro.

Jejunum and ileum: intraperitoneal


3 parts of SI

Duodenum (about 25cm)

Jejunum (100-110 cm)

Ileum (150-160 cm)

Total is 5-10m (6 avg)


Duodenum anatomy

from pylorus to dudodenojejunal junction

4 parts

1 = superior - duodenal bulb: 5cm long. Site of most ulcers

2 = descending - 10cm. Curves around head of pancreas

3 = Transverse - 10cm. Crosses anterior to aorta and IVC and posterior to SMA and

4 = Ascending - 5cm. Ascends past left side of aorta, then curves anteriorly to meet with jejunum, forming the duodenojejunal junction, which is suspended by ligament of Treitz

Jejunum begins at Treitz

Since duodenum is retroperitoneal, it is tethered to posterior abdominal wall and has no mesentery at its posterior aspect

Plicae circulares (transverse mucosal folds in lumen) are most prominant in prox small bowel (duodenum and jejunum) than in distal small bowel


Blood supply of duodenum

1) Prox (up to ampulla of Vader) = gastroduodenal (first branch of proper hepatic) bifurcates into anterior and posterior superior pancreaticoduodenals

2) Distal = inferior pancreaticoduodenal (first branch of SMA) bifurcates into anterior and posterior inferior pancreaticoduodenals

1) Anterior and posterior pancreaticoduodenal veins drain into SMV. Joins splenic vein behind neck of pancreas to form portal vein
2) Prepyloric vein of Mayo is landmark of pylorus


Jejunum and ileum anatomy

No anatomic boundary between the 2

Jejunum is prox 40% of SI distal to ligament of treitz while ileum is distal 60%

Combined length is 5-10m.

Mesentery tethers the jejunum and ileum to posterior abdominal wall.


Blood supply to jejunum and ileum

1) Both jejunum and ileum supplied by branches of SMA which runs in mesentery
2) Arteries loop to form arcades that give rise to straight arteries - vasa recta

SMV drains both


Lymphatics of SI

Bowel wall - mesenteric nodes - lymph vessels parallel the corresponding arteries - cisterna chyli (retroperitoneal structure btw aorta and IVC) - thoracic duct - L subclavian vein

Participate in absorption of fat


Anterior vs posterior ulcer rupture

Anterior = peritonitis. Leakage of duodenal contents into peritoneal cavity

Posterior = bleeding. Penetrated gastroduodenal artery.



1) Parasympathetic:
Fibers originate from vagus and celiac ganglia
- enhances bowel secretion, motility, and other digestive processes

2) Sympathetic:
Fibers from ganglion cells that reside in plexus at base of SMA
- opposes effects of para system on bowel

3) Enteric:
Consists of Meissner plexus at base of submucosa and Auerbach plexus between inner circumferential and outer longitudinal layers of muscle wall


Peristalsis rate

1-2 cm/sec


SMA Syndrome

Since 3rd part of duodenum runs behind SMA, compression of SMA on duodenum can lead to SBO.

These pts are thin and have lost the fat pad between SMA and duodenum leading to recurrent symptoms of SBO


Which part of the small bowel is not supplied by branches of the SMA?

proximal duodenum (branches of celiac trunk)


Intestinal immune function

Largest immune organ in body

IgA is most common type of Ig in lumen of GI tract

Lymphoid nodules, mucosal lymphocytes, and isolated lymphoid follicles in appendix and mesenteric lymph nodes together constitute the mucosa-associated lymphoid tissue (MALT)


SBO definition

Cessation, impairment or reversal of physiologic transit of intestinal contents secondary to a mechanical or functional cause

#1 cause = adhesions from prior abdominal surgery (#2 = hernia)


SBO etiologies

1) Mechanical
Adhesions, hernia, cancer, abscess, congenital

Gallstone ileus, foreign body, intussusception

Crohn's, lymphona, radiation enteritis

2) Functional (paralytic ileus)

Lyte issues (hypoK)


Meds (opiates, anticholinergics)

Hemoperitoneum/ retroperitoneal hematoma



From: Antrum

Action: Gastric acid secretion and cell growth

Stimulated by: Vagus, Food in antrum, gastric distention, calcium

Inhibited by: Antral pH



From: Duodenum

Action: GB contraction stimulates pancreatic acinar cell growth. Inhibits gastric emptying

Stim: Polypeptides, AAs, Fat, HCl

Inhib: Chymotrypsin, trypsin



From: Duodenum

Action: Stimulates pancreatic secretion of H2O and HCO3. Bile secretion of HCO3. Pepsin secretion. Inhibits gastric acid secretion

Stim by: Low pH, intraluminal duodenal fat

Inhib by: High duodenal pH



From: Pancreas

Action: Increases SI absorption of H2O and lytes. Inhibits cell growth, GI motility, GB contraction, pancreatic/biliary/enteric secretion of gastric acid, secretion/action of all GI hormones

Stimulated by: Intraluminal fat, gastric and duodenal mucosa, catecholamines

Inhibited by: ACh release


Pancreatic polypeptide

From: Pancreas

Action: Clinical usefulness of pancreatic polypeptide is limited to being a marker for other endocrine tumors of the pancreas

Stimulated by: Cephalic - vagus. Gastric - reflexes. Intestinal - food in small bowel



From: SI, Colon

Action: Pancreatic secretion, vasodilation, inhibits gastric acid secretion

Stim by: Fat


Peptide Y

From: SI, Colon

Action: Inhibits gastric acid secretion, pancreatic exocrine secretion, and migrating myoelectric complexes



From: SI, Colon

Action: Increases glycogenolysis, lipolysis, gluconeogenesis

Stim by: Lowe serum glucose

Inhib by: Somatostatin



Action: Inhibits MMCs, increases gastric emptying, increases pepsin secretion alkaline enviornment

Stim by: Vagus, fat, intraduodenal

Inhib by: Pancreatic polypeptide


How do you differentiate btw jejunum and ileum?

Jejunum has larger diameter, thicker wall, more prominent plicae circulares

Jejunum has few arcades (1-2) with long vasa recta

Ileum has many arcades with short vasa recta

Ileum has fatty mesentery