Small Intestine | Digestive Function, Structure & Division

by DR. HAMID HUSSAIN
Small Intestine Digestive Function, Structure & Division

Introduction-

  • The small intestine is the longest part of the alimentary canal.
  • It extends from the pylorus of the stomach to the ileocaecal junction.
  • The greater part of digestion and food absorption takes place in the small intestine.

Size :-

  • It is about 6 meter long.
  • The length is greater in males than in females.
  • Greater in cadavers (due to loss of tone) than in the living.

Division of Small Intestine

  • It is divided into :-
  • (A) An upper part :- Fixed part, called the duodenum, which measures about 25 cm. in length.
  • (B) A lower part- Mobile part, forming a very long convoluted tube.
    • The upper 2/5 of the mobile intestine are known as the jejunum and
    • Lower part 3/5 are known as the ilieum.
  • Upper part :- Fixed part- Duodenum- 25cm.
  • lower part :- Mobile part –
    • (2) Jejunum-Upper 2/5=230 cm.
    • (3) Ilium – lower 3/5=345cm.
Small Intestine  Digestive Function, Structure & Division

Duodenum – 1st Part of Small Intestine

Introduction-

  • The duodenum is ac shaped tube, about 10 inches that joins the stomach to the jejunum.
  • The duodenum curves around the head of the pancreas.
  • It receives the openings of the bile and pancreatic ducts
  • It is mostly retroperitoneal.

Location-

  • The duodenum lies above the level of the umbilicus opposite to vertebrae L1-L2-L3.
  • Duodenum is situated in the epigastrium and umbilical region.

Parts of the duodenum-

  • It have four parts :-
    • First part of the duodenum- Superior part-2 inch or 5 cm.
    • Second part of the duodenum- Descending part-3 inch or 7.5 ch.
    • Third part of the duodenum- Horizontal part- 4 inch or 10cm
    • Fourth part of the duodenum- Ascending part-1 inch or 2.5 cm. (10 inch or 25 cm).
First part of the duodenum
  • The first part of the duodenum is 2 inch long and begins the pylorus and runs upward and backward on the right side of the vertebrae L1. It lies on the transpyloric plane.

Relations

  • Anteriorly :- Quadrate lobe of liver, gall bladder.
  • Posteriorly :- Lesser sac, gastroduodenal artery, bile duct, portal vein, inferior venacava.
  • Superiorly :- Lesser sac.
  • Inferiorly :- Head and Neck of pancreas.

Second part of the duodenum-

  • It is 3 inch long. It begins at the superior duodenal flexure, and runs vertically downward infront of the hilum of the right kidney on the right side of the vertebrae L2-L3.
  • About halfway down its medial border, the bile duct and the main pancreatic duct pierce the duodenal wall.

Relations

  • Anteriorly :- Funds of gall bladder, right lobe of liver, transverse colon, coils of small intestine.
  • Posteriorly :- Hilum of right kidney, right ureter, inferior
  • Laterally :- Ascending colon, right colic flexure. venacava.
  • Medially :- Head of pancreas, bile duct, main pancreatic duct.

Third part of the duodenum-

  • It is 4 inch long. It begins at the inferior duodenal flexure on the right side of the lower border of vertebrae L3.
  • It passes almost horizontally and slightly upwards in front of the inferior venacava and ends by joining the 4th part in front of the abdominal aorta.

Relations-

  • Anteriorly :- Superior mesenteric vessels, root of mesentery.
  • Posteriorly :- Right ureter, right psoas major muscle, inferior venacava, abdominal aorta, right testicular and ovarian vessels.
  • Superiorly ;- Head of pancreas with uncinate process.
  • Inferiorly :- Coils of jejunum.
Fourth part of duodenum-
  • It is 1 inch long. It runs upward and up to the upper border of the vertebrae L2. Where it joins with duodenojejunal flexure.

Relations-

  • Anteriorly :- Transverse colon, transverse meso colon, stoma
  • Posteriorly :- Left psoas major, left lar vessels.
  • Right :- Root of mesentery.
  • Left :- Left kidney , left ureter.
  • Superiorly :- Body of pancreas.

Arterial supply of duodenum-

  • The upper 1/2 is supplied by the- Superior pancreatico duodenal artery.
  • The lower 1/3 is supplied by the- Inferior pancreatico duodenal artery.
Venous drainage-
  • Splenic vein.
  • Superior mesenteric vein.
  • Portal vein.
Lymphatic drainage-
  • Pancreaticoduodenal nodes.
  • Hepatic nodes.
  • Superior mesenteric vein.
  • Pyloric nodes.
  • Coeliac nodes.
Nerve supply-
  • Sympathetic nerve- T9 -T10.
  • Parasympathetic nerve- Vagus.
Applied aspect-
  • P.U.D.- Peptic ulcer disease- Most occur in the pylorus first part of duodenum. Where they are called duodenal ulcer.
  • Duodenal ulcers- First part of duodenum.
  • Duodenal cap.
  • Trauma to the duodenum.
  • Obstruction of the second part of duodenum.

Jejunum and Ileum – 2nd & 3rd Part of Small Intestine

Introduction-
  • The upper 2/5 part of mobile intestine are known as the jejunum.
  • The lower 3/5 part are known as the ileum.
  • The total length about 6 meter long.
  • The jejunum begins at the duodenojejunal flexure.
  • The ileum ends at the ileocaecal junction.
  • The coils of jejunum and ilium are freely mobile and are attached to the posterior abdominal wall by a fan shaped fold of peritoneum known as the mesentery of the small intestine. The mesentery extends downwards and to the right from the left side of the vertebrae L2, to the region of the right sacroiliac joint.
Interior of the intestine-
  • It has four layers :-
    • Mucosa
    • Submucosa
    • Muscularis
    • serosa.
  • Large surface area :- For absorption of digested food a very large surface area is required.
    (A) The great length of intestine- Absorption
    (B) The presence of circular folds of mucous membrane, villi and microvilli.
  • Intestinal glands :- They secrete digestive enzymes and mucus. The duodenal glands (Brunner’s glands) lie in the
    submucosa.
  • Lymphatic follicles :- Solitary lymphatic follicle.
    • Aggregated lymphatic follicles (Peyer’s patches).

Differences B/w Jejunum and Ileum

FeaturesJejunumIleum
LocationAround the umbilicus region.
Occupies upper and
left parts of the intestinal area.
Hypogastric and pelvic region.
Occupies Lower and right part of the intestinal area.
WallThicker and more vascular.Thinner and vascular.
LumenWider and often empty.Narrower and often loaded.
MesenteryWindows present.
Fat less abundant
Arterial arcades, 1 or 2.
Vasa recta longer fewer.
no Windows.
Fat more abundant.
Arterial arcades, 3 or 6.
Vasa recta shorter and
fewer.
Valve of kerkringLarger and more closely set.Smaller and sparse.
VilliLarge, thick and more abundant.Shorter, thinner and less abundant.
Solitary Iymphatic follicles (S.LF)fewer.more numerous.
Aggregated lymphatic follicles (S.L.F) Absent.Present.

Arterial supply :-

  • Branches from the superior mesentric artery.

Venous drainage :-

  • Superior mesenteric vein.

Lymphatic drainage :-

  • Superior mesenteric nodes.
  • Aortic nodes.

Nerve supply

  • Sympathetic nerve- T9-T11
  • Parasympathe tic nerve- Vagus nerve.

Applied aspect

FeaturesSmall intestine Large intestine
Appendices epiploicaeAbsentpresent
Taeniae coliAbsentpresent
Sacculations (Haustrations)Absentpresent
Calibre (Diameter)LargerSmaller
FixityGreater part is freely mobileGreater part is fixed
Villipresent Absent
Transverse mucosal fold PermanentObliterated when logitudinal muscle coat relaxes.
Peyer’s patchespresent in ileumAbsent

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