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Anatomy of the Gastrointestinal SystemDR CHRIS MOORE

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System overview

Broadly divided into 2 categories

GI Tract

Oral cavity Oesophagus

StomachSmall

intestine

Large intestine

Rectum and Anus

Accessory organs

Liver Gallbladder

Pancreas Appendix

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GI System in situ

Stomach

Liver

Gallbladder

Small Intestine

Large Intestine

Rectum & Anus

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Reveal viscera

Abdominal cavity

Peritoneal cavity contains serous fluid – lies between:

Parietal peritoneum - lines body wall

Visceral peritoneum - lines external surface of most GI organs

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What kind of

space does the

peritoneal

cavity have?

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From the beginning – Oral cavity

Cheeks – cont with lips, reflect onto gums

Lined by mucous membrane

Two palates

Anterior – Hard

Posterior - Soft

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Teeth - mechanical 6

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Tooth surfaces 7

Lingual

Buccal

Distal

MesialOcclusal

Incisal

Labial

Enamel

Dentin

Gingival sulcus

Gingiva (gum)

Pulp in pulp

cavity

Cementum

Root canal

Alveolar bone

Periodontal

ligament

Apical foramen

Nerve

Blood supply

Sagittal section of a mandibular (lower) molar

CROWN

NECK

ROOT

Sagittal

plane

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Salivary Glands - chemical

Parotid – watery saliva with enzymes

Parotid ducts open

opposite buccal side of

upper 2nd molars

Submandibular – enzyme and mucous

Sublingual – mucous saliva

Each drained by ducts of

Rivinus in floor of mouth

(10-20 ducts)

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Parotid

Submandibular

Sublingual

Parotid duct

Opening of parotid duct

(near second maxillary molar)

Second maxillary molar tooth

Tongue (raised in mouth)

Lingual frenulum

Submandibular duct

Mylohyoid muscle

SUBMANDIBULAR GLAND

Zygomatic arch

PAROTID GLAND

Lesser sublingual duct

SUBLINGUAL GLAND

(a) Location of salivary glands

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Parotid duct

SUBMANDIBULAR

GLAND

PAROTID

GLAND

(b) Right lateral view

Zygomatic arch

Masseter muscle

Oesophagus

Long, thin, muscular tube

25cm (approx)

Topographically related to trachea

Protects

C-shape

Ant Post

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Oesophagus

Penetrates

diaphragm at

oesophageal hilus

Two valves -

Oesophageal

sphincters

Upper and Lower

LOS (or LES in U.S) -GERD

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Inner structure

2 Muscle types

Circular (inner) and

longitudinal (outer)

Muscular layer

differs according

to position

U 1/3 – striated (voluntary)

M 1/3 – striated and

smooth

L 1/3 – smooth

(involuntary)

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Mucosa

Submucosa

Muscularis

(circular layer)

Muscularis

(longitudinal layer

Vagus nerve

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Stomach

Lies beneath liver and diaphragm in epigastric and left hypocondrium regions

Alters size and position in response to eating

A sphincter at each end regulates passage

LES, Pyloric

Three major parts

Pylorus Body

Fundus

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Identify the

three main

regions of the

stomach

Internal anatomy

Tri-directional muscle layers

Folds – Ruggae

Five layers of tissue (common across GIT)

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Duodenum – small intestine 1

First section of small intestine (SI)

25cm long (approx)

Four parts – Superior, Descending, Horizontal, Ascending

Receives secretions from pancreatic duct and common bile duct (into descending via major duodenal papilla)

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Jejunum – small intestine 2

Middle section of SI – 2.5m (approx)

Jejunum and Ileum - no clear division

Villi – site of absorption

Gives velvety appearance

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Villi and Microvilli

Each Villus (pl. Villi) has arteriole, venule, lymph vessel

Villi are lined on the surface with Epithelial cells

Microvilli on surface of epithelial cells create brush border

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Intestinal support and blood supply

Mesentery – pair of membranes

Support the intestines

Adhere them to the posterior abdominal wall

Provides degree of mobility

Huge network of blood vessels

Supply blood to villi

and intestines

Lymph nodes and ducts

present (defence)

Superior

Mesenteric artery and

vein

Ileocoelic

artery and vein

Inferior

Mesenteric vein

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Ileum – small intestine 3

Not to be confused with Ilium!

Around 2-4m long

Absorbs bile salts and vitamins

Anything missed by Jejunum

really

Toggle in situ

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Aren’t they the same...

More fat in Mesentery

Ileum has more vascular arcades

Jejunum 2, Ileum 5-6

Plicae circulars more numerous in Jejunum

Absorption greater in Jejunum

Why are there

more circulars?

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Aren’t they the same...

Ileum has Peyer’s patches

Collections of lymphoid tissue related to immune system

Specialised epithelium containing “M” cells

Sample antigens and present for “analysis”

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Caecum

Blind pouch – breakdown of plant material

Significant species variation

?

Vermiform appendix

Ileocaecal valve

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Colon

Ascending

Transverse

Descending

Sigmoid

Note descending and sigmoid are situated more posteriorly

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Surface features of colon

Teniae Coli

Longitudinal smooth muscle

bands

Slightly shorter than colon

Haustra

Sacculations caused by the

teniae

Teniae Coli

Haustra

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Rectum and Anus

Separate structures

Rectal folds help regulate flow

Internal anal sphincter

Smooth muscle

External anal sphincter

Striated muscle

Rectum

Anus

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Physiology of

DigestionGASTROINTESTINAL SYSTEM PART 2

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A multi-stage process

Ingestion

Propulsion/motility

Mechanical digestion

Chemical digestion

Absorption

Defecation/evacuation/egestion

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The Enteric nervous system

GI wall has complex network of neurons

Myenteric plexus

Submucosal plexus

Connected to each other, the GI muscles, and the CNS

Together they form the

ENTERIC nervous system

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Physiology Overview

Digestive activities of the gastrointestinal tract occur in three

overlapping phases:

1. The cephalic phase

2. The gastric phase

3. The intestinal phase

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Physiology Overview

During the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem to prepare for digestion.

The brain stem activates the facial (CN VII) and glossopharyngeal (CN IX) nerves to stimulate secretion of saliva, while the vagus nerves (CN X) stimulate secretion of gastric juice.

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Physiology Overview

Once food reaches the stomach, the gastric

phase of digestion begins. Neural and hormonal mechanisms (the hormone gastrin is a key player)

promote secretion of gastric juice and increase gastric motility.

The intestinal phase of digestion begins once food enters the

small intestine. Neural and hormonal responses promote the continued digestion of foods

that have reached the small intestine.

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The Large Intestine

The gastroileal reflex causes relaxation of the ileocecal valve, intensifies

peristalsis in the ileum, and forces any chyme into the cecum.

The gastrocolic reflex intensifies strong peristaltic waves that begin at

about the middle of the transverse colon and quickly drive the contents

of the colon into the rectum.

This mass peristalsis takes place three or four times a day during or

immediately after a meal, and may lead to defecation.

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Clinical

Complications

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Common conditions

Motility and Excretion related

Diarrhoea

Increased fluid caused by increased

gut motility and decreased intestinal

absorption

Constipation

Decreased gut motility leading to

further water absorption in colon,

dryness of faecal matter

Periodontal disease (often confused with peridontitis)

Inflammatory and Blockage

Peptic ulcers

Appendicitis

Pancreatitis and Cholecystitis

Gallstone, pancreatic stones

Inflammatory bowel disease

Two forms: Crohn’s disease and Ulcerative

colitis

Crohn’s – infl of distal ileum/proximal colon

(serosa through mucosa)

UC – infl of mucosa of distal colon/rectum

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Omphaloceles

Bowel development outside of abdominal cavity

Midgut fails to return back inside the abdominal cavity during

10th week of

embryologic

development

Easily corrected with surgery

following birth

Click here for clinical image. Warning, this is a paediatric image and so may disturb. Please click the image to hide it if you

wish.

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