Duodenum, jejunum and ileum

Question 1

What are the lengths of the duodenum, jejunum and ileum?

Answer 1

25 cm

2. 5 m
3. 75 m

Question 2

How tall are villi?

Answer 2

1 mm

Question 3

What are the invaginations formed by the folding of the submucosa called?

Answer 3

Crypts of Lieberkuhn

Question 4

What are the three types of cells that constitute the mucosa (excluding the crypts of Lieberkuhn)?

Answer 4

Enterocytes
Goblet Cells
Enteroendocrine Cells

Question 5

What are the two main cells types found within the crypts of Lieberkuhn?

Answer 5

Stem Cells and Paneth Cells

Question 6

What is the role of enterocytes and what is their lifespan?

Answer 6

Enterocytes are primary involved in the absorption of substances - they have a life span of around 36 hours (1-6 days)

Question 7

What is the length of a microvillus?

Answer 7

0.5-1.5 micrometers

Question 8

What is the layer that covers the microvilli?

Answer 8

Glycocalyx

Question 9

What are the properties of the glycocalyx?

Answer 9

Rich in carbohydrates and protects the epithelia from the digestional lumen but is still allows absorption. It traps a layer of water and mucus called the ‘unstirred layer’, which regulates the rate of absorption from the intestinal lumen.

Question 10

The folds, villi and microvilli increase the surface area of the small intestine by what factor?

Answer 10

500 fold

Question 11

How does goblet cell distribution change across the intestines?

Answer 11

Goblet cells increase in number as you go down the small intestine - this is because the intestinal contents become more and more solid meaning that you require more lubrication to facilitate movement.

Question 12

What is the role of enteroendocrine cells?

Answer 12

Hormone secreting cells - most often found in the lower parts of the crypts (they are columnar epithelial cells)
They produce hormones that influence gut motility

Question 13

What do paneth cells produce and what is their role?

Answer 13

They are ONLY found in the bases of crypts and they contain large acidophilic granules containing antibacterial enzyme (LYSOZYME), glycoproteins and zinc. They regulate intestinal flora and protect stem cells.

Question 14

Describe the arrangement of paneth cells and stem cells in the crypts.

Answer 14

Paneth cells are found right at the bottom of the crypts of Lieberkuhn and on either side of them you find stem cells.

Question 15

Why is there such a rapid turnover of enterocytes?

Answer 15

Enterocytes are the first line of defence against GI pathogens and they may be directly affected by toxic substances in the diet. Rapid turnover means that any lesions are short lived and that any interference with host cell function will be diminished.

Question 16

How does the cholera endotoxin affect the gut?

Answer 16

Results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water.

Question 17

Describe a characteristic feature of the duodenum.

Answer 17

Brunner’s Glands - submucosal coiled tubular mucus glands secreting alkaline fluid.
Alkaline fluid neutralises acidic chyme from the stomach, protecting the proximal small intestine and it helps optimise the pH for pancreatic enzymes.

Question 18

Describe a characteristic feature of the jejunum.

Answer 18

Plicae circulares (valves of Kerckring) - numerous large folds in the submucosa 
These are present in the duodenum and ileum but in the jejunum they are taller, thinner and more frequent.

Question 19

Describe a characteristic feature of the ileum.

Answer 19

Peyer’s patches - primes the immune response against intestinal bacteria
They are well positioned to prevent bacteria from the colon
migrating into the small intestine

Question 20

Describe segmentation and the point of it.

Answer 20

Occurs by the stationary contraction of circular muscles at intervals.
It mixes the contents of the lumen (pancreatic enzymes and bile mix with the chyme)

Question 21

Describe peristalsis.

Answer 21

Involves sequential contraction of adjacent circular rings of smooth muscle. It propels chyme towards the colon.

Question 22

What is the migrating motor cortex?

Answer 22

Cycles of smooth muscle contraction. Begins in the stomach, through the small intestine and stops at the start of the colon.
Purpose: SCREEN the gut and keep it clear
More ordered and frequent when fasting

Question 23

Describe the digestion of carbohydrates.

Answer 23

Digestion begins in the mouth with the action of salivary alpha-amylase
Most digestion takes place in the duodenum by the action of pancreatic alpha-amylase
Digestion of amylase products and simple carbohydrates occurs at the membrane

Question 24

What conditions does pancreatic alpha-amylase need to function properly?

Answer 24

Needs CHLORIDE for optimum activity

Also needs SLIGHTLY ALKALINE pH

Question 25

Once the pancreatic enzymes have acted on the carbohydrates, where does the rest of the digestion take place?

Answer 25

Digestion of amylase products and simple carbohydrates takes place at the membrane

Question 26

Describe the absorption of carbohydrates.

Answer 26

Absorption of glucose and galactose is by secondary active transport via SGLT-1. Glucose and galactose then move into the plasma by facilitated diffusion through GLUT-2.

Question 27

How does fructose absorption differ from glucose and galactose absorption?

Answer 27

Fructose has a lower concentration inside cells so it can move in by facilitated diffusion via GLUT-5

Question 28

What is the main pancreatic protease that is required to activate other proteases and how is it activated?

Answer 28

TRYPSIN
Trypsin is secreted by the pancreas as an inactive precursor.
It is activated by ENTEROKINASE (an enzyme found in the duodenal brush border).
Trypsin can then activate other enzymes.

Question 29

How are amino acids absorbed by the cells?

Answer 29

Brush border peptidases break down the larger peptides before absorption.
Amino acids are then absorbed by facilitated diffusion and secondary active transport.
Cytoplasmic peptidases will break down the di and tripeptides before they cross the basolateral membrane

Question 30

State the four step procedure by which lipids are digested.

Answer 30

Secretion of bile and lipases
Emulsification
Enzymatic hydrolysis of ester linkages
Solubilisation of lipid products in bile salt micelles

Question 31

What is the point of emulsification?

Answer 31

Increases the surface area for digestion

Question 32

What is the point of bile salt micelles?

Answer 32

Allows pancreatic lipase to break down the triglycerides

Question 33

How does lipase break down triglycerides?

Answer 33

Triglycerides are broken down to 2 free fatty acids and a monoglyceride

Question 34

How are trypsin and lipase different in the way that they are activated?

Answer 34

Trypsin is secreted by the pancreas as an inactive precursor that is converted to its active form by enterokinase.
Pancreatic lipase is secreted in its active form but it must bind to a cofactor (Colipase - which is secreted in an inactive form) to become active.

Question 35

State two important enzymes involved in lipid hydrolysis.

Answer 35

Phospholipase A2 - hydrolyses fatty acids in the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids
Pancreatic Cholesterol Esterase - hydrolyses cholesterol ester to free cholesterol and fatty acid

Question 36

What is the role of micelles?

Answer 36

They are shuttles that take lipids to the surface of the epithelium (across the unstirred layer) and present the fatty acids and monoglycerides to the brush border.

Question 37

Where are bile salts reabsorbed?

Answer 37

Ileum

NOTE: lipid absorption is usually complete by the middle of the jejunum

Question 38

What are the two pathways of resynthesis of triglycerides? Describe them.

Answer 38

Monoglyeride Acylation (MAIN one) - acylates the monoglyceride to produce a triglyceride
Phosphatidic Acid Pathway - triglycerides are synthesised from CoA fatty acid and alpha-glycerophosphate

Question 39

How are TAGs packaged and released from the basolateral membrane?

Answer 39

TAGs are packaged into chylomicrons which then enter the lacteals (they are too big to enter the capillaries)