Small intestine

Question 1

What is the function of the small intestine?

Answer 1

To absorb water, nutrients and salts

Question 2

What are the three main regions of the small intestine and their lengths?

Answer 2

duodenum (25cm)
jejunum (2.5m)
ileum (3.75m)

Question 3

What is the small intestine lined in to increase surface area?

Answer 3

The small intestine is lined on the inside by fan-shaped mesentery which increases the surface area of the intestine with folds and supports the blood supply. The mesentery then has villi covering it.

Question 4

What kind of muscles are in the external wall of the small intestine?

Answer 4

Longitudinal and circular muscles which are important for motility.

Question 5

Is there a transition between the duodenum, jejunum and ileum?

Answer 5

No, all have the same basic histological organisation

Question 6

What are the invaginations into the intestine known as?

Answer 6

Crypts of Lieberkuhn

Question 7

What is the villi made of (cells) and what does it have to help improve its function?

Answer 7

• Have a good blood and lymph supply
• Have a good innervation from the submucosal plexus
• Has simple columnar epithelium: primarily enterocytes (absorptive cells), scattered goblet cells and enteroendocrine cells

Question 8

What do the Crypts of Lieberkühn comprise of?

Answer 8

Stem cells - replace high turnover

Paneth cells - base of crypt, secrete acidophilic granules, lysozyme

Question 9

Enterocytes

Their features, abundance, their lifespan and cell shape

Answer 9

• most abundant in small intestine
• tall columnar cells with microvilli
• have a basal nucleus
• specialised for absorption and transport
• 1-6 days lifespan

Question 10

What are the microvilli on enterocytes covered in and why?

Answer 10

Glycocalyx - rich carbohydrate layer on apical membrane that serves as a protection from the digestional lumen yet allows absorption

It traps a layer of water and mucus (unstirred layer) to regulate rate of absorption and to protect surface of microvilli from pH of lumen

Question 11

How is the surface area of the small intestine increased?

Answer 11

Cylindrical internal surface area of the small intestine is 0.4m2 and the folds, villi and microvilli increase it to 200m2

Question 12

Goblet cells

Abundance, what do they do, how and distribution?

Answer 12

• 2nd most abundant
• produces mucus which facilitate passage of material through the bowel
• mucus containing granules accumulate at the apical end causing the goblet shape
• the abundances increases as you move through the bowel as more lubrication is required

Question 13

Enteroendocrine cells

Shape, distribution and function?

Answer 13

• columnar epithelial cells
• scattered among absorptive cells
• found in lower parts of the crypts
• secrete hormones e.g. histamine to influence gut motility
• aka chromaffin cells

Question 14

Paneth cells

Where are they found, what do they contain and their functions?

Answer 14

• Found only in the base of the crypts
• Contain large, acidophilic granules that contain:
  lysozyme (antibacterial to protect stem cells), glycoproteins (to protect the enzymes from themselves) and zinc (to act as a co-factor to many of the enzymes)
• Can have phagocytic roles
• May regulate intestinal flora.

Question 15

Epithelial cell turnover

Answer 15

• Cell proliferation, differentiation and death are continuous
• Enterocytes and goblet cells have lifespan of around 36 hrs
• Replaced by dividing stem cells in the crypt
• Stem cells are pluripotent

Question 16

How do stem cells move from the crypt to the tip?

Answer 16

• Dividing stem cells in the crypt are shunted up and at the tip the cells become senescent (old)
• Apaptosed cells are sloughed into lumen and digested/rebabsorbed by intestine

Question 17

Why does the intestine have such fast turnover?

Answer 17

Enterocytes are the first line of defence so they may be directly affected by toxic substances in the diet -> interference in cell function is not dangerous as any lesions are short lived

Question 18

What may cause intestinal dysfunction?

Answer 18

If the escalator transit of enterocytes is interrupted through impaired production of new cells e.g. radiation

Question 19

What does cholera enterotoxin do?

Answer 19

Causes prolonged opening of chloride channels in the small intestine leading to uncontrolled secretion of water
Can result in massive dehydration and death

Question 20

What are the differences between the duodenum, jejunum and ileum?

Answer 20

Have some general differences

Duodenum: distinguished by the presence of Brunner’s gland (submucosal gland found in the crypt base secreting alkaline fluid into chyme - neutralises acid to protect the SI and optimise the pH for enzymes)

Jejunum: Presence of numerous large folds in the submucosa called plicae circulares (or valves of kerckring). They are found in the other two areas but here they are thinner/taller and more frequent

Ileum: Major immune defence - has lots of Peyer’s patches which are large cluster of lymph nodules in the submucosa. Immune defences against intestinal bacteria. Other defence mechanism = paneth cells and rapid cell turnover

Question 21

What are the 3 functions of small intestine motility?

Answer 21

• mix food with enzymes
• facilitate contact between contents and mucosa
• propel intestinal contents along alimentary tract

Question 22

What is the migrating motor complex and its purpose?

Answer 22

• Waves of peristalsis
• Most prevalent in the fasting state
• Occurs between meals
• Also occur in the fed state but less ordered and less frequent
• Each cycle - contraction of adjacent segments of small intestine
• Begins in the stomach and migrates through small intestine towards colon
• On reaching terminal ileum next contraction starts in the duodenum
• Prevents migration of colonic bacteria into the ileum and may clean the intestine of residual food

Question 23

Digestion in the duodenum

Answer 23

• In the small intestine digestion occurs in an alkaline environment
• Digestive enzymes and bile enter the duodenum from the pancreatic duct and bile duct
• The duodenal epithelium also produces its own digestive enzymes
• Digestion occurs in the lumen and in contact with the membrane

Question 24

How does absorption occur - methods?

Answer 24

• passive
• facilitated diffusion
• primary active transport
• secondary active transport

Question 25

Digestion of carbohydrates

Answer 25

- digestion begins in the mouth by salivary alpha amylase but is destroyed in the stomach (pH) - most of the digestion of carbs occurs in the small intestine

Question 26

How may carbohydrates exist?

Answer 26

- simple | - complex

Question 27

Pancreatic alpha amylase

Answer 27

- secreted into the duodenum in response to a meal - continues digestion of starch and glycogen in the small intestine (started by salivary amylase) - needs Cl- for optimum activity and neutral/slightly alkaline pH (Brunner's gland in duodenum) - acts mainly in the lumen and some is also absorbed at the brush border

Question 28

How are carbohydrates digested?

Answer 28

complex to disaccharides by pancreatic amylase and then become monosaccharides when they come into contact with the brush border which has membrane bound disaccharidases - they enter the enterocyte

Question 29

How are carbohydrates absorbed?

Answer 29

- Absorption of glucose and galactose via secondary active transport symporter or antiporter) - carrier protein is SGLT-1 on the apical membrane (SGLT-1 transports one carbohydrate for one glucose) - Absorption of fructose via facilitated diffusion and carrier protein is GLUT5 on the apical membrane - All the monosaccharides diffuse down their concentration gradient through the basolateral membrane using GLTU2 - Sodium is pumped out of the basolateral surface by a sodium potassium ATPase to ensure the sodium gradient.

Question 30

What does GLUT2 do?

Answer 30

facilitates exit at the basolateral membrane

Question 31

Protein digestion - stomach, pancreas and enzyme activation

Answer 31

Major regulator is trypsin - Digestion begins in the stomach by pepsin but is inactivated in the alkaline duodenum - Pancreas secretes zymogen proteases - This is activated by duodenal membrane bound enterokinase into trypsin - Trypsin then activates other proteases

Question 32

How are proteins absorbed?

Answer 32

- Brush border peptidases break down the larger peptides before absorption - Amino acids are absorbed by facilitated diffusion and secondary active transport - Di and tripeptides are absorbed using carrier proteins different to amino acids and cytoplasmic peptidases break down most of the di and tripeptides before thy can cross the basolateral membrane - Uses AA/Na+ transporter for amino acids at apical and FD at basolateral - Dipeptides and tri use AA/H+ transporter at apical

Question 33

Digestion of lipids

Answer 33

``` Lipids are poorly soluble in water Four processes occur: - secretion of bile and lipases - emulsification - enzymatic hydrolysis of ester linkage - soluble bile salt micelles are made Trigylcerides become 2 fatty acids and a monoglyceride ```

Question 34

Bile salt molecule

Answer 34

- Steroid - Planar with two faces - One face is hydrophobic (methyl) and other is hydrophilic (OH-) - The hydrophilic head regions are is contact with the solvent and hydrophobic towards the centre of the micelle

Question 35

What is the role of enzymes in lipid digestion?

Answer 35

- Lipases break down triglycerides into monoglycerides and free fatty acids - Pancreatic lipases complexes with colipase - Colipase prevents the bile salts from displacing lipase from the fat droplet - Phospholipase A2 hydrolyses datty acids at the 2 position in many phospholipids resulting in lysophospholipids and free fatty acids - Pancreatic cholesterol esterase hydrolyses cholesterol ester into free cholesterol and fatty acid

Question 36

How are lipids absorbed? | Is whole micelle absorbed together?

Answer 36

- Micelles are absorbed much more quickly than emulsions - The whole micelle isn't absorbed together, the bile salts are absorbed in the ileum, lipids absorbed by middle jejunum - Bile salts are transported back to the liver (enterohepatic circulation)

Question 37

What are the two ways in which monoglycerides and free fatty acids are resynthesised into triglycerides in enterocytes?

Answer 37

- Monoclyceride acylation (major) | - Phsophatidic acid pathway (minor)

Question 38

What is the monoclyceride acylation?

Answer 38

Fatty acids bind to the apical membrane and then fatty acids binding protein moves the fatty acid to the smooth ER and the fatty acids is esterified into the diglyceride and triglycerides

Question 39

What is the phosphatidic acid pathway?

Answer 39

Triglycerides are synthesised form CoA fatty acids and alpha glycerophosphate

Question 40

How are chylomicrons formed and what is their composition? | Transport pathway?

Answer 40

Lipoprotein particles are synthesised in enterocytes as an emulsion - 80-90% triglyceride, 8-9% phospholipid, 2% cholesterol and 2% protein and trace carbohydrate - They are transported from the golgi and secreted via exocytosis into the lacteals

Question 41

How is the ileum seperated from the colon? | How does this control movement through the colon?

Answer 41

- By the ileocaecal sphincter - Relaxation and contraction controls the passage of material into the colon - Also prevents the back flow of bacteria into the ileum.

Question 42

What are the three types of intestinal motility?

Answer 42

segmentation peristalsis migrating motor complex

Question 43

What two kinds of enzymes are found in the small intestine?

Answer 43

Enzymes that cleave big nutrients found in the gut lumen (usually released from glandular organs) Enzymes that cleave dimeric nutrients into monomers for absorption are in the brush border of the enterocytes (usually released from epithelium)

Question 44

Give examples of enzymes found in the brush border made by epithelial cells (protein)

Answer 44

- Tripeptidases and dipeptidases | - Specifically, endopeptidase, dipeptidase, aminopolypeptidase and carboxypeptidase

Question 45

Give examples of pancreatic enzymes (protein)

Answer 45

trypsin, chymotrypsin and carboxypeptidase