8. Small Intestine

Question 0

What are intestinal crypts (intestinal glands)?

Answer 0

Glands lining the intestinal epithelia which usually sit at base of villi.

Secrete various enzymes & play vital role in cell turnover of gut.

Question 1

Describe 2 factors that encourage absorption in the intestines

Answer 1

Mucosa folded into villi

Villi have micro-villi (brush border)

Question 2

Describe the role that intestinal crypts/glands play in renewing intestinal epithelia

Answer 2

Base of glands have cells that multiply & migrate up to tips of villi, where are shed.
As cells migrate, they mature & develop ability to absorb.
This allows mucosa to be continually renewed.
The glands are affected by IBD, which can cause ‘cryptitis’

Question 3

What is the ‘unstirred layer’

Describe its function

Answer 3

Layer of water & mucus which lies adjacent to intestinal wall.
Enzymes secreted by intestinal epithelia trapped in this layer.

Functions to slow diffusion of molecules, allowing them to be further broken down by trapped enzymes & therefore completing digestion

Question 4

What are polysaccharides?

Answer 4

Polysaccharides = long chains of monosaccharides (e.g. Starch, glycogen).
Long chains can be cleaved into shorter chains by actions if various enzymes:

When chain is 2 sugars long = disaccharide (e.g. Maltose)

When chain is 1 sugar long = monosaccharide (e.g. Glucose, fructose, galactose)

Question 5

What is starch made of?

What are the breakdown products of starch?

Answer 5

Starch = glucose monomers:
amylose (straight chains)
amylopectin (branched polypeptide)

Breakdown of 1,4 bonds in amylose by amylase = glucose + maltose
Amylase cant break down 1,6 bonds in amylopectin

Isomaltase can break down 1,6 linkages = dextrins

Question 6

What enzymes are involved in breaking down polysaccharides & where are they released in the GIT?

Answer 6

Examples include:
Amylases (from saliva & pancreatic secretions)
Maltases
Sucrases
Lactases (all secreted by intestinal epithelia)

Question 7

Describe the lath that glucose takes as it is absorbed from the intestinal lumen into capillary blood

Answer 7

Na/K ATPase pump: sits on basolateral side of epithelial cell & pumps Na out of cell into ECF (in exchange for K+).
Sets up an osmotic gradient which allows Na to move into cell from luminal side down conc gradient.
Glucose uses this conc gradient & is co-transported with Na into cell from lumen via SGLT1 transporter protein via secondary active transport.
Glucose then moves out of basolateral membrane via facilitated diffusion through GLUT2 transporter, where then enters capillary blood

Question 8

Why do oral rehydration therapies contain glucose & salt?

Answer 8

Presence of glucose & Na in solution = greater absorption of water

Glucose & Na co-transported into enterocyte via Na gradient created by Na/K ATPase pump.
In diseased states, if just put water/water & salt into gut, will pass through with limited absorption.

If add glucose to solution, gut will absorb this, even in disease state.
As glucose is co-transported with Na via SGLT1, there is net movement of both molecules.
Movement of Na sets up movement of water, so water moves into enterocyte.

Question 9

List the main proteases present in the GIT & where they are released from

Answer 9

Pepsin: released as pepsinogen from Chief cells in stomach.
Begins breakdown of proteins

Pancreatic proteases (e.g. trypsin, chymotrypsin): released as inactive precursors (trypsinogen, chymotrypsinogen).
Trypsinogen activated by enterokinase (present in intestinal mucosa)
Trypsin activates chymotrypsin

Question 10

Briefly describe the breakdown pathway for proteins

Answer 10

For example:
Proteins in stomach broken down into oligopeptides (2-20 aa’s) by Pepsin.
Oligopeptides then broken down by pancreatic proteases (e.g. trypsin, chymotrypsin)
Broken down further into polypeptides & then to amino acids

Question 11

What is meant by ‘segmentation’ & what is its purposes?

Answer 11

Contraction of the intestinal smooth muscle whereby the intestinal contents undergo gentle agitation & mixing by back & forth mvmt (rather than just propelled in 1 direction).
Contraction of muscle produces small ‘segments’ along length of involved gut

Question 12

How does segmentation differ from peristalsis?

Answer 12

Peristalsis = Coordinated contraction of smooth muscle of gut with aim of propelling luminal contents in one direction. Movement is primary purpose.

Segmentation = purpose more in agitation & mixing of luminal contents.
Due to intestinal pacemakers firing more rapidly @ proximal locations, segmentation does create slow caudal progression of intestinal contents

Question 13

What are the longitudinal muscles in the large intestines known as?

Answer 13

Taenia Coli

Question 14

What is meant by the term ‘mass movement’?

Answer 14

Peristaltic movements (occurring once or twice a day), present in the transverse & descending colon, which rapidly forces faeces into rectum.
Resulting stretch in rectum stimulates urge to defecate

Question 15

What is the gastro-colic reflex?

Answer 15

Stretching of stomach following meal initiates peristalsis within gut.
Can include mass movement

Question 16

What are the difference between the internal & external anal sphincters?

Answer 16

Internal: smooth muscle, therefore under parasympathetic control

External: striated muscle, largely under voluntary control

Question 17

Describe classical pernicious anaemia

Answer 17

Vit B12 deficiency related specifically to lack of produced intrinsic factor.
If there is reduced absorption of Cobalamine (Vit B12), can = ineffective erythropoiesis (result of thymidine synthase dysfunction) & megaloblastic anaemia

Question 18

Describe the normal pathway involved in Vit B12 (Cobalamine) absorption & how this is interrupted in pernicious anaemia

Answer 18

Vit B12 an organo-metalic compound, obtained mainly thru dietary meat & milk ingestion
Intrinsic factor released by Parietal cells in stomach
Binds to VitB12 & the complex travels into terminal ileum where is absorbed.
VitB12 released & enters plasma

In pernicious anaemia: lack of intrinsic factor & hence VitB12 cant be transported & absorbed

Question 19

What is thought to cause Pernicious anaemia?

What clinical signs may be present if the disease is advanced?

Answer 19

Atrophic gastritis fuelled by auto-immune disease
Gastritis = destruction in Parietal cells, therefore reduction in amount of intrinsic factor

Low grade fever, tachycardia, glossitis, cognitive impairment, unsteady gait, sensory loss, parasthesia

Question 20

What treatment options are available for Pernicious anaemia?

Answer 20

Adequate dose of Cobalamine (VitB12) for life

Usually by injection & later by oral dose

Question 21

List 3 possible causes of:

Ongoing diarrhoea, lower abdo pain, no perianal disease, fresh blood in rectum

Answer 21

Ulcerative colitis (most likely)
Crohn’s
Infective agents (e.g. salmonella)
Colorectal cancer

Question 22

What might be seen on a colonoscopy for a patient with ulcerative colitis?

Answer 22

Diffuse contiguous (not skip lesions) & circumferential inflammation

Severe cases: mucosa bleeds easily with contact, ulcers merge into larger linear patterns

Question 23

Differentiate Ulcerative Colitis from Crohn’s

Answer 23

UC: 
bloody diarrhoea, 
diffuse continuous inflammation of large bowel & ulceration, 
mild fibrosis & lymphoid reaction, 
no granulomas, 
rectum (almost always) colon & occasionally terminal ileum affected, 
no skip lesions, 
fistulae rare, 
crypt abscesses,

CD:
Abdo pain, perianal disease, abdo mass,
Interrupted inflammation (skip lesions),
Fibrosis, marked lymphoid reaction,
Granulomas common,
Can occur anywhere along GIT
Fistulae about 10%
Crypt abscesses unusual
Occasionally rectal involvement but terminal ileum commonly involved

Question 24

Is there a potential cure for Ulcerative Colitis?

Answer 24

Total colectomy