20. The anatomy, histology and development of the small intestine.

20. The anatomy, histology and development of the small intestine.

* Is made of foregut/midgut, and almost entirely intraperitoneal, except for part of duodenum

Anatomy of Small Intestine
General Info:
Consisting of 3 parts: duodenum, jejunum, + ileum
Function: Is the primary site for absorbtion of nutrients from ingested material,
Extends from the pylorus to the iliocecal junction, where the the ileum joins the cecum.
pyloric part of the stomach empties into the duodenum, regulated by the pyloric sphincter

Duodenum in Situ

Duodenum:

• First and shortest part of the small intestine, is also the widest+ most fixed part.
• Duodenum makes a C- shaped around the head of the pancreas.
• Duodenum begins at the pylorus on the right side+ ends at the duodenojejunal junction on the left side.
• Junction: app. At the level of the L2 vertebra, 2-3 cm to the Lof the midline, the duodenojejunal flexure.
• Most of the duodenum is fixed by peritoneum to structures on the posterior abdominal wall+ is considered partially retroperitoneal.

Topography of Duodenum:

Topography of Duodenum

Parts and Surface Projection of Duodenum

Duodenum is divisible in 4 parts:
Sup(first) part: only part that is intraperitoneal

• SP =  lies anterolat. to the body of the L1 vertebra (2cm R of it)
• Topography = liver, GB, SVC, Aorta, portal v, right crus of diaphragm
• connected to liver via hepatoduodenal lig
• first 2 cm of the sup. part of the duodenum,
• immediately distal to the pylorus
• has a mesentery,  is mobile.& called ampulla

Descending(second) part: retroperitoneal

• SP =  descends along the right sides of the L1- L3 vertebrae, 1 finger med to R mid inguinal line
• Topography = R kidney, transverse mesocolon, common bile duct, SI loops
• location of major duodenal papilla of Vater, entrance of bile duct and major pancreatic duct
  • papilla is closed off by Sphincter of Oddi
  • division between FOREGUT & MIDGUT
  • acc pancreatic duct can open online here, or in separate opening = minor duodenal papilla

Horizontal(Third) part: 6-8 cm long

• SP = crosses the L3 vertebrae.
• Topography = IVC, aorta, sup mesenteric a/v, SI loops, R crus of diaphragm
• Sup. To it is the head of the pancreas and its uncinate process.
• Sup mesenteric a/v cross in front, w/ aorta & IVC behind = area of crossing = pars tecta

Ascending(fourth) part: short(5 cm

• SP = begins at the left of the L3 vertebrae+ rises sup. As far as the sup. Border of the L2 vertebrae.
• Topography = Aorta, left crus diaphragm, loops of SI
• terminates @ duodenal-jejunal jxn –> fixed by suspensory lig (of Trietz) to R crus of diaphragm
  • if the musc by the ligament contracts, this lifts the flexure and widens the angle of it, make it easier for contents to move thru it

Blood Supply:

• B/c the duodenum arises from FOREGUT, & MIDGUT –> will be supplied by 1st, celiac trunk, then 2nd sup mesenteric a
• FOREGUT PART ( b4 the major duod papilla): sup pancreaticoduodenal a (gastroduodenal a of celiac trunk)
• MIDGUT PART (after papilla): inf pancreaticoduodenal a (sup mesenteric a)
• both anastomose w/ each other
• Venous drainage: duodenal v –> portal v

Blood Supply of duodenum

CLINICAL NOTE: Ileus – Ileus is a partial or complete non-mechanical blockage of the small and/or large intestine, caused by a manual obstruction, either by twisting or large strucutre stuck in bowel, or by dec blood flow to area

Lymph Drainage:

• ant lymph vessels –> pancreaticoduodenal l.n. –> pyloric l.n –>cysterna chyli
• post lymph vessels –> run behind head of pancreas –> sup mediastinal l.n.
• can also go to celiac l.n

Nerve Supply:

• from CN X (PNS), SNS nerves via celiac/sup mesenteric plexuses – around the arteries

Jejunum & Ileum

*Intrapertioneal – mesentarium proper connect them w/ dorsal body wall –> root of mesentery

Function: complete digestion and absorption of H2O, electrolytes, nutrients and minerals

Jejunum

• prox 2/5 of SI
• Emptier, larger in diameter, and thicker wall than ileum
• has plicae circulares (circular folds), tall and closely packed
• NO Payer’s patches
• Translucent area = windows –> b/w blood vessels of mesentery
• Less prominent arterial arcades (than ileum)
• longer vasa recta  (straight a ) compared w/ ileum

Ileum

• distal 3/5 of SI
• occupies false  pelvis in R lower quadrant of abdomen
• has Peyer’s patches
• shorter plicae circulares, more mesenteric fat, and arterial arcades than jejunum

Intestines are considered to have 2 edges:

1. Mesenteric
   • line of attachment of mesenterium proper,
   • a/v, lymph and nerves sent and leave here
   • The mesentery is a fan- shaped fold of peritoneum that attaches the jejunum+ ileum to the posterior abdminal wall.
   • The root of the mesentery is directly oblique, inf.+ to the right.
     • It extends from the duodenojejunal junction, left of L2 –> to the ileocolic junction by  R sacroiliac joint.
       The avarage length of the mesentery from its root to the intestinal border is 20 cm.
     • Root of Mesentery crosses:
       • R crus of diaphragm
       • R psoas major m, R ureter
       • R genitofemoral n, R SNS trunk
       • R gonadal vessels
       • IVC, Aorta
       • 3rd part of duodenum

   W/in attachment line: sup mesenteric a/v, mesenteric plexus (ANS), Lymph network of SI

2. AntiMesenteric
   • surgery is done on this side in order not to disrupt those structures

Blood Supply:

• all MIDGUT = sup mesenteric a
  • make arterial arcades that run w/in the mesentery of Jejuno-Ileum
  • from the arcades come the vasa recta – straight arteries
• drained by sup mesenteric v
  • joins splenic v, behind the jxn b/w pancreas head/neck, to form portal v
  • inf mesenteric v can drain into sup mesenteric v, splenic v, or jxn b.w the 2

Lymph Drainage:

• lymph vessels run w/in mesentery
  • l.n located along arteries, close to the wall, and around the prox part of sup mesenteric a
  • –>  sup mesenteric l.n. (prox), ileocolic l.n. (dist)

Nerve Supply:

• have a perivascular nerve plexus, run w/ arteries
• SNS = from T5-9 –> celiac plexus via SNS trunk & splanchnic n
  • dec motility and secretion, VC
• PNS = synpase on celiac/sup mesenteric ggl –> post ggl fibers run in myenteric/submucosal plexuses of SI wall
  • inc motility and secretion, VD

Histology of Small Intestine

Anim = structure of the small intestine.

Comparison of Histology of SI v LI

Embryology: development of the small intestine.

Anim = Development of the Stomach, Omenta and Duodenum

Anim2 = Rotation of the Intestine

• Primitive gut consists of foregut, midgut, hindgut
• Foregut = Esophagus, Stomach, pancreas, liver, spleen, biliary tree, GB, and prox duodenum
• Midgut = distal duodenum, SI, and LI until distal 1/3 of transverse colon = cranial limb of intestinal loop
  
• For a while, due to rapid growth of abdominal organs, loops of SI herniate into umbilical region
• @ abt wk 8, it rotates 270 degrees around the axis of sup mesenteric a — continues to grow
  
• cranial limb, now on R side of body
• as body grows and relative size of liver/kidney decrease, midgut returns into ab cavity fully @ wk 10
• As it is coming out, it rotates another 180 degrees around sup mesenteric a
  
• cranial limb will emerge 1st and settle on L side of body
  
• mesentery will change in size as SI loops move