10. SMALL INTESTINE: A brief description of the anatomy of the GIT

It explain in brief details the structure and function of small intestine

1. Small intestine
Dr. D.A Ofusori

2. •The part of the gastrointestinal tract (GI tract) that
extends from the pyloric sphincter to the ileocecal valve,
where it empties into the large intestine.
•In vertebrates, the small intestine is the part of the
gastrointestinal tract (gut) following the stomach and
followed by the large intestine, and is where the vast
majority of digestion and absorption of food takes place.
•The small intestine finishes the process of digestion,
absorbs the nutrients, and passes the residue on to the
large intestine.
•The liver, gallbladder, and pancreasare accessory
organs of the digestive system that are closely associated
with the small intestine.

3. The small intestine is divided into the duodenum,
jejunum, and ileum.
The small intestine follows the general structure of the
GI tract in that the wall has a mucosa with simple
columnar epithelium, submucosa, smooth muscle with
inner circular and outer longitudinal layers, and serosa.
The absorptive surface area of the small intestine is
increased by plicae circulares, villi, and microvilli.
Exocrine cells in the mucosa of the small intestine
secrete mucus, peptidase, sucrase, maltase, lactase,
lipase, and enterokinase. Endocrine cells secrete
cholecystokinin and secretin.

4. •The most important factor for regulating
secretions in the small intestine is the
presence of chyme.
•This is largely a local reflex action in response
to chemical and mechanical irritation from
the chyme and in response to distention of
the intestinal wall. This is a direct reflex
action, thus the greater the amount of
chyme, the greater the secretion.

5. Size, location, and structure of the
small intestine
•2.5cm wide and 6m long
•Fills most of the abdomen
•Begins at the pyloric sphincter and ends with
its connection to the large intestine at the
ileocaecal valve
•3 parts: Duodenum, jejunum and ileum

6. Duodenum:
•The duodenum is largely responsible for the breakdown of
food in the small intestine, using enzymes. Brunner's
glands, which secrete mucus, are found in the duodenum.
•The duodenum wall is composed of a very thin layer of
cells that form the muscularismucosae. The duodenum is
almost entirely retroperitoneal.
•The duodenum also regulates the rate of emptying of the
stomach via hormonal pathways. Secretinand
cholecystokininare released from cells in the duodenal
epithelium in response to acidic and fatty stimuli present
there when the pylorus opens and releases gastric chyme
into the duodenum for further digestion.

7. PARTS OF THE DUODENUM
•The duodenum is divided into four sections for the purposes of description. The
first three sections curve in a "C"-loop concavity in which the head of the
pancreas lies.
•Only the first 2 cm of the superior part is mobile (covered by peritoneum) –the
distal 3cm of the first part along with the rest of the duodenum is retroperitoneal
(immobile).

8. First part
•The first (superior) part begins as a continuation of the duodenal end of the
pylorus. From here it passes laterally (right), superiorly and posteriorly, for
approximately 5 cm, before
•making a sharp curve inferiorly into the superior duodenal flexure (the end of the
superior part). It is intraperitoneal.
Relations
•· Anterior
•o Peritoneum
•o Gallbladder
•o Quadrate lobe of liver
•· Posterior
•o Bile duct
•o Gastroduodenal artery
•o Portal vein
•o Inferior vena cava
Superior
o Neck of gallbladder
o Hepatoduodenal ligament (lesser
omentum)
· Inferior
o Neck of pancreas
o Greater omentum

10. Second part
•The second (descending) part of the duodenum
begins at the superior duodenal flexure.
•It passes inferiorly to the lower border of vertebral
body L3, before making a sharp turn medially into the
inferior duodenal flexure (the end of the descending
part).
•The pancreatic duct and common bile duct enter the
descending duodenum, commonly known together as
the hepatopancreaticduct
•This part of the duodenum also contains the minor
duodenal papilla, the entrance for the accessory
pancreatic duct.
•The junction between the embryological foregut and
midgutlies just below the major duodenal papilla.

12. Third part
•The third (inferior/horizontal) part
of the duodenum begins at the
inferior duodenal flexure and
•passes transverselyto the left,
•crossingthe right ureter, right
testicular/ovarian vessels, inferior
vena cava, abdominal aorta,
superior mesenteric artery and the
vertebral column.

14. Fourth part
•The fourth (ascending) part passes superiorly,
either anterior to, or to the right of, the aorta,
until it reaches the inferior border of the body
of the pancreas. Then, it curves anteriorly and
terminates at the duodenojejunal flexure
where it joins the jejunum.
•The duodenojejunal flexure is surrounded by a
peritoneal fold containing muscle fibres: the
ligament of Treitz.

16. BloodSupply
Theduodenumreceivesarterialbloodfromtwodifferentsources.
Thetransitionbetweenthesesourcesisimportantasitdemarcates
theforegutfromthemidgut.
Proximaltothe2ndpartoftheduodenum(approximatelyatthe
majorduodenalpapilla–wherethebileductenters)thearterial
supplyisfromthe
(1)gastroduodenalarteryanditsbranchthesuperior
pancreaticoduodenalartery.
Distaltothispoint(themidgut)thearterialsupplyisfromthe
(2)superiormesentericartery,anditsbranchtheinferior
pancreaticoduodenalarterysuppliesthe3rdand4thparts.
Thevenousdrainageoftheduodenumfollowsthearteries.
Ultimatelytheseveinsdrainintotheportalsystem,eitherdirectly
orindirectlythroughthesplenicorsuperior
mesentericvein.

17. •Lymphatic drainage
The lymphatic vessels follow the arteries in a
retrograde fashion.
The anterior lymphatic vessels drain into the
pancreatoduodenallymph nodes located along the
superior and inferior pancreatoduodenalarteries and then
into the pyloric lymph nodes (along the gastroduodenal
artery).
The posterior lymphatic vesselspass posterior to the head
of the pancreas and drain into the superior mesenteric
lymph nodes.
•Efferent lymphatic vessels from the duodenal lymph
nodes ultimately pass into the CELIAC LYMPH
NODES.

18. •The inner surface of the jejunum, its mucous
membrane, is covered in projections called
villi, which increase the surface area of tissue
available to absorb nutrients from the gut
contents. The epithelial cells which line these
villi possess even larger numbers of microvilli.
•The villi in the jejunum are much longer than
in the duodenum or ileum.
Jejunum

19. •The jejunum contains very few Brunner's
glands (found in the duodenum) or Peyer‘s
patches (found in the ileum). Instead, it has
many large circular folds in its submucosa
called plicae circulares which increase the
surface area for nutrient absorption.

20. ILEUM
•The ileum is the final section of the small intestine in most higher
vertebrates. In humans, the ileum is about 2-4 m long, and the pH is
usually between 7 and 8 (neutral or slightly alkaline).
Function
•The function of the ileum is mainly to absorb vitamin B12and bile salts
and whatever products of digestion that were not absorbed by the
jejunum.
•The wall itself is made up of folds, each of which has many tiny finger-
like projections known as villi on its surface. In turn, the epithelial cells
that line these villi possess even larger numbers of microvilli.
•The villi contain large numbers of capillariesthat take the amino acids
and glucose produced by digestion to the hepatic portal vein and the
liver. Lacteals are small lymph vessels, and are present in villi. They
absorb fatty acid and glycerol, the products of fat digestion. Layers of
circular and longitudinal smooth muscle enable the digested food to
be pushed along the ileum by waves of muscle contractions called
peristalsis.

21. Function of the small intestine
•Forward propulsion of contents
•Major site of digestion
•90% absorption
•Protection against infection
•Hormone secretion
•Intestinal juice secretion