The Large Intestine

Question 1

Describe the cellular structure of the large intesine and what the luminal surface looks like

Answer 1

Large intestine has a simple columnar epithelium with numerous crypts of Lieberkuhn

Luminal surface does not have any villi but instead has haustra (folds) formed by the contraction of teniae coli - external longitudinal muscle spilt into three bands

Question 2

How does the colonic mucosa obtain its nutrients

Answer 2

Majority of colonic nutrients come from fermentation of dietary fibre

Fermentation produces short chain fatty acids which are uesd by the mucosa

Question 3

What is the arterial supply to the colon

Answer 3

SMA gives off:

• Ileo-coli - caecum
• Right colic - ascending colon
• Middle colic - proximal 2/3 transverse colon

IMA gives off:

• Left colic - descending and distal 1/3 transverse colon
• Sigmoid - descending colon
• Superior rectal - upper 1/3 rectum

Question 4

What is the marginal artery

Answer 4

Continuous arterial arcade along the inner border of the colon formed by anastomoses between the terminal branches of the superior and inferior mesenteric arteries

Question 5

What veins drain the colon

Answer 5

Superior mesenteric vein drains caecum, ascending and proximal 2/3 transverse colon

Inferior mesenteric vein drains sigmoid, distal 1/3 transverse and descending colon

Question 6

What are the functions of the colon

Answer 6

Water reabsorption

Temporary storage

Final electrolyte absorption and some bile salt absorption

Question 7

What is the gastrocolic reflex

Answer 7

A relfex generate by eating that causes movement in the GI tract

Stimulates contractions in the large intestine that causes bowel movement

Question 8

What are the functions of the bacteria in the colon

Answer 8

Synthesis of vitamins

Breakdown of primary to secondary bile acids

Conversion of bilirubin to non-pigmented metabolites

Question 9

How is water reabsorbed in the colon and what adaptations aid water reabsorption

Answer 9

Water reabsorption is facilitated by ENaC - create an osmotic gradient for water to follow

Tight junctions between cells aids water reabsorption as it allows for a large ion gradient to form and prevents diffusion of ions back into the lumen

Question 10

What is inflammatory bowel disease

Answer 10

Group of conditions characterised by idiopathic inflammation of the GI tract

Typically affects younger pepole and those aged 40-60

Question 11

What are some extra-intestinal probelms that IBD can cause

Answer 11

MSK pain - arthritis

Skin - psoriasis, erythema nodosum, pyoderma gangrenosum

Liver/billary tree pathology - PSC

Eye problems

Question 12

What is Crohn’s disease

Answer 12

Inflammation of the colon

Affects anywhere in the colon but normally affects ileum, typically terminal ileum

CD is transmural inflammation -> goes through the bowel

Has skip lesions -> has patches of inflammation and patches of normal bowel

Question 13

How can a patient with Crohn’s disease present

Answer 13

History of loose stools - non-bloody

Weight loss

Right lower quadrant pain

Tender mass in RLQ

Some joint pain

Smoker

Mild perianal inflammation/ulceration - may have fistula

Low grade fever

Mildly anaemic

Question 14

What gross pathology can be seen in Crohn’s disease

Answer 14

Skip lesions

Hyperaemia

Mucosal oedema

Superficial and/or deep ulcers

Transmural inflammation causing thickening of bowel wall and narrowing of the lumen

Cobblestone appearance

Fistulae - between bowel and bowel/bladder/vaagina/skin

Question 15

What investigations can be done with Crohn’s disease

Answer 15

Bloods - anaemia and raised CRP

CT/MRI - wall thickening, obstruction, extramural problems

Barium enema/follow through

Colonoscopy

Radiology using barium follow through - look for strictures and/or narrowing

Question 16

What is ulcerative colitis

Answer 16

Inflammation of the colon that begins in the rectum -> extends proximally to involve entire colon

Has continuous pattern

Only causes mucosal inflammation -> superficial

Can cause backwash ileitis if it spreads to the terminal ileum

Question 17

How can a patient with ulcerative colitis present

Answer 17

History of bloody stools

Weight loss

Mild lower abdominal pain/cramping

Painful red eye

Mildly tender abdomen

No perianal disease

Normal temperature

Question 18

What pathological changes occur in ulcerative colitis

Answer 18

Chronic inflammatory infiltrate of lamina propria

Crypt abscesses - neutrophilic exudate in crypts

Crypt distortion - due to repeated inflammation and healing

Reduced goblet cell number and reduce bacteria

Pseudopolyps - after repeated episodes of inflammation and healing

Loss of haustra

Friable mucosa - easy bleeding

Question 19

What investigations can be performed for a patient with suspected ulcerative colitis

Answer 19

Bloods - anaemia and serum markers

Stool culture

Colonoscopy

Plain abdominal radiograph - can show loss of haustra, mucosal inflammation (appears granulated)

Barium enema

CT/MRI - used to look for toxic megacolon

Question 20

What are the treatments for IBD

Answer 20

Stepwise approach used with pharmacological treatment:

• Aminosalicytes - flares and remission.
• Corticosteroids for flares
• Immunomodulators for fistulas and long term control

Surgery:

• Crohn’s - remove affected parts of the bowel
• UC - remove whole bowel

Question 21

What features can be seen under the microscope in Crohn’s Disease

Answer 21

Granulomas
Organised collection of epithelioid macrophages