The small intestine. Gestational tract

1. The small intestine

2. Introduction
• The small intestine is the longest part of
the digestive system.
• It extends from the stomach (pylorus) to
the large intestine (cecum) and
• consists of three
parts: duodenum, jejunum and ileum.
• The main functions of the small intestine are
to complete digestion of food and to absorb
nutrients.

3. Contd
• Dysfunction of the small intestine can bring
you some uneasy experiences such as
diarrhoea while travelling or worse, on a date.

4. Key facts about the small intestine
Definition A part of the
alimentary tract
which extends from
the stomach (pyloric
orifice) to the large
intestine (ileal orifice)
Parts Duodenum, jejunum,
ileum
Blood supply Arteries: celiac trunk,
superior mesenteric artery
Veins: hepatic portal
vein, superior mesenteric
vein
Innervation Parasympathetic: vagus
nerve (CN X) (through the
submucosal (Meissner’s) and
myenteric (Auerbach’s)
nervous plexuses)
Sympathetic: Thoracic
splanchnic nerves

5. Duodenum
• The duodenum by definition is the first part of
the small intestine.
• It extends from the pyloric sphincter of
the stomach, wraps around the head of
the pancreas in a C-shape and ends
at duodenojejunal flexure.
• This flexure is attached to the posterior
abdominal wall by a peritoneal fold called the
suspensory muscle (ligament) of duodenum,
also called the ligament of Treitz.

6. For details on the ligament of Trietz
https://www.earthslab.com/anatomy/ligament
-of-treitz-suspensory-muscle-of-duodenum/

8. Parts of the duodenum
• The duodenum has four parts:
– superior (duodenal bulb/ampulla),
– descending,
– horizontal and
– ascending.

9. • Among several features of the duodenum, we’ll
list the two most important:
– The superior part (duodenal bulb/ampulla) is the
only intraperitoneal part, as the hepatoduodenal
ligament and greater omentum attach to it.
– The descending part of the duodenum has an opening
called the major duodenal papilla (tubercle of Vater).
The papilla contains the hepatopancreatic sphincter
(sphincter of Oddi, Glissons’ sphincter) which
regulates the emptying of the bile from the
hepatopancreatic ampulla.

10. Jejunum
• The jejunum is the second part of the small
intestine.
• It begins at the duodenojejunal flexure and is
found in the upper left quadrant of
the abdomen.
• The jejunum is entirely intraperitoneal as the
mesentery proper attaches it to the posterior
abdominal wall.

11. • There is no clear line of demarcation between
the jejunum and ileum,
• But there are some anatomical and
histological differences that distinguish them:

13. Some few features
• The jejunum represents the proximal two-
fifths of the jejunum-ileum continuum
• The wall of the jejunum is thicker and its
lumen is wider than in ileum
• The jejunum contains more prominent circular
folds of Kerckring
• NB: Assign. Generate a list of ALL gross
anatomical and histological features to
differentiate jejunum from ileum

14. Ileum
• The ileum is the last and longest part of the
small intestine.
• It is found in the lower right quadrant of the
abdomen, although the terminal ileum can
extend into the pelvic cavity.
• The ileum terminates at the ileal orifice
(ileocecal junction) where the cecum of
the large intestine begins.

15. • At the ileocecal junction, the lamina
muscularis of the ileum protrudes into the
lumen of the cecum forming a structure called
the ileocecal fold.
• These muscular fibers form a muscular ring
within the fold called the ileocecal sphincter
which controls the emptying of ileal content
into the large intestine.

16. Histology
• Histologically, the small intestine has four
layers. From internal to external, they are:
– mucosa,
– submucosa,
– muscularis externa, and
– serosa.
 These layers are easy to remember using
the mnemonic M.S.M.S.

17. • There are several unique features in the small
intestine, which act to significantly increase its
absorptive surface: these include:
• Circular folds (valves of Kerckring, plicae circulares)
are the transverse folds of mucosa found
predominantly in the distal duodenum and proximal
jejunum

19. • Name the abbreviated parts in the diagram
above
• B= Brunner’s glands

23. Contd
• Intestinal villi are fingerlike extensions of
intestinal mucosa which project into the
lumen of the small intestine. Between the villi
are intestinal glands (crypts of Lieberkuhn)
which secrete intestinal juice rich in digestive
enzymes.
• Microvilli are projections found on the apical
surface of each intestinal cell (enterocyte)

24. • There are also features of the small intestine
which are segment-specific:
• Peyer's patches are part of gastrointestinal
associated lymphoid tissue (GALT). They are
found in ileum.
• Brunner glands are found in the submucosa of
the duodenum. They produce mucus rich in
alkalines which protects the duodenum from
the corrosive effects of gastric acid.

25. Functions of the small intestine
• The main functions of the small intestine
are secretion and absorption.
• The epithelial cells of the small intestine
secrete enzymes which digest chyme into the
smallest particles, making them available for
absorption.
• Concurrently the duodenum functions to mix
food with bile and pancreatic enzymes to
continue the digestion of carbohydrates, fats,
and proteins.

26. • Concerning absorption, carbohydrates and
proteins are absorbed in the duodenum and
jejunum respectively.
• The jejunum also functions to absorb most
fats. The ileum function involves absorption of
vitamin B12, bile salts and all digestion
products which were not absorbed in
duodenum and jejunum.
• All three small intestine segments absorb
water and electrolytes.

27. Blood supply
• The duodenum is supplied by the branches of
the celiac trunk and superior mesenteric artery (SMA);
the superior, middle and inferior pancreaticoduodenal
arteries.
• The jejunum and ileum are supplied by 15-18 branches
of the SMA called the jejunal and ileal arteries. They
anastomose with each other to form arterial
arcades which send numerous straight arteries (vasa
recta) to the jejunum and ileum.
• The small intestines drain into the hepatic portal vein.

28. Lymphatic drainage
• The lymph of the small intestine is drained
into the superior mesenteric lymph nodes.

30. Nerve supply
• The small intestine is innervated by branches of
the vagus nerve (CN X) and thoracic splanchnic nerves.
Their nerve branches extend throughout the entire
length of the small intestine in the form of two
plexuses:
• Submucosal plexus (of Meissner) found in the
submucosa of the small intestine and contains
only parasympathetic input from the vagus nerve (CN
X)
• Myenteric plexus (of Auerbach) located in the
muscularis externa of the small intestine, contains
both sympathetic and parasympathetic nerve fibers

32. • Learn to differentiate between these layers really
easily using a mnemonic! Just memorise
'SMP & MAPS', which stands for:
• Submucosal
• Meissner's
• Parasympathetic
• Myenteric
• Auerbach's
• Parasympathetic
• Sympathetic

33. Clinical correlations
• Diarrhea is the frequent passage of unformed stool. In
most cases, the diarrhea is caused by microorganisms
such as Escherichia coli, Salmonella and Shigella.
• Obstructive disorders including paralytic ileus, a hernia
or volvulus are common but can become complicated.
• Infectious diseases such as tapeworm, tropical sprue or
giardiasis are rare but severe when left untreated.
More common infections such as the adenovirus or
salmonella are seen in the waste.
• Neoplastic growths may include gastrointestinal
stromal tumors (GIST), lymphomas and sarcomas.

34. • Developmental, congenital or genetic
conditions include pyloric stenosis, duodenal
atresia and gastroschisis.
• Other conditions or a miscellaneous mixture
of diseases that can affect the small intestine
include Crohn’s disease, coeliac disease,
gastric dumping syndrome and irritable bowel
syndrome to name but a few.

35. • An ulcer is a defect of intestinal mucosa.
Ulcers can appear in the stomach and/or
duodenum, usually due to a bacterial infection
of the pylorus of the stomach
with Helicobacter pylori combined with the
corrosive effect of gastric acid and pepsin.

36. • Duodenal peptic ulcers are usually found in the
ampulla of duodenum. They appear more often
within the younger population, contrasting with
gastric ulcers which are more prevalent in the
older population. The most common symptom of
duodenal ulcers is the presence of burning pain in
the epigastric region of the abdomen around 2-3
hours after a meal. Depending on the severity of
ulceration, pain can be followed by nausea and
vomiting.

37. References:
• Drake, R. L., Vogl, A. W., & Mitchell, A. W. M. (2015).
Gray’s Anatomy for Students (3rd ed.). Philadelphia,
PA: Churchill Livingstone.
• Kasper, D. L., Hauser, S. L., Jameson, J. L., et al. (2015).
Harrison’s Principles of Internal Medicine (19th ed.).
New York, NY: McGraw-Hill Education.
• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014).
Clinically Oriented Anatomy (7th ed.). Philadelphia, PA:
Lippincott Williams & Wilkins.
• Ross, M. H., Pawlina, W. (2011). Histology (6th edition).
Philadelphia, PA: Lippincott Williams & Wilkins.

38. Contd
• Young, B., Woodford, P., & O’Dowd G. N
(2021) Wheater’s Functional Histology: A Text
and Colour Atlas (6th edition). Elsevier
Churchill Livingstone
• https://www.earthslab.com/anatomy/ligamen
t-of-treitz-suspensory-muscle-of-duodenum/