Alimentary Systems 3 - Small Intestine

Question 1

What is the function of the small intestine?

Answer 1

To absorb nutrients salt and water

Question 2

List the dimentions of the small intestine

Answer 2

• 6m long, 3.5cm diameter
• Duodenum 25cm
• Jejunum 2.5m
• Ileum 3.75m
• No sudden transition between the 3 parts

Question 3

What is the function of mesentry in the small intestine?

Answer 3

• Throws the small intestine into folds

- Supports the blood supply

Question 4

Describe the layout of the epithelium of the small intestine

Answer 4

• External wall has longitudinal and circular muscles
• Internal mucosa arranged in circular folds
• Mucosa covered in villi
• Invaginations (crypts of Lieberkuhn)

Question 5

List the characteristics of villi

Answer 5

• Only present in the small intestine to increase the SA
• Motile with rich blood supply and lymph drainage for absorption of digested nutrients
• Good innervation from the submucosal plexus.
• Simple epitelium dominated by enterocytes

Question 6

List the cell types in the mucosa

Answer 6

• Simple columnar epithelium
• Primarily enterocytes (absorptive)
• Scattered goblet cells
• Enteroendocrine cells

Question 7

List the cell types in the crypts of Lieberkuhn

Answer 7

• Paneth cells

- Stem cells

Question 8

Where are villi located on the enterocytes?

Answer 8

• Apical membrane

- Blood at the basolateral membrane

Question 9

What are microvilli

Answer 9

• They make up the brush border, present on villi.

- Have glycocalyx, a carboydrate layer

Question 10

What are the functions of glycocalyx

Answer 10

• Allows for absorption while protecting from the digestional lumen.
• Also traps a layer of water and mucous called the unstirred layer which regulates the rate of absorption from the intestinal lumen

Question 11

What is the function of goblet cells?

Answer 11

• Secrete mucous containing granules

- There are more the further down the digestive tract

Question 12

What is the function of enteroendocrine cells in the small intestine? Give examples

Answer 12

• Found in the lower parts of the crypts of Lieberkuhn
• Secrete hormones to influence gut motility
• G-cells secrete gastrin
• I-cells secrete cholecystokinin
• S-cells secrete secretin
• D-cells secrete somatostatin

Question 13

What is the function of paneth cells in the SI?

Answer 13

• Found only in the bases of the crypts
• Contain acidophilic granules containing lysozyme and glycoproteins
• Engulf some bacteria and protozoa

Question 14

Why is the cell turnover in the epithelium of the gut so high?

Answer 14

• Due to damage by toxins
• Lesions will be sort lived
• Allows people to recover from cholera if hydrated for 1-2 days

Question 15

How is the duodenum distinguished?

Answer 15

• The presence of brunners glands
• These glands are submucosal coiled tubular mucous glands that secrete alkaline fluid
• Open into the base of the crypts
• Alkaline secretions neutralise chyme from the stomach, and optimise pH for pancreatic digestion enzymes

Question 16

How is the jejunum distinguished?

Answer 16

• Presence of numerous large folds in the submucosa called plicae circulares (valves of Kerckring)
• Though present in all the small intestine, they are taller, thinner and more frequent here

Question 17

How is the ileum distinguished?

Answer 17

• Lots of peyers patches (lymph nodules in the submucosa)

- Prevents bacteria from the colon migrating into the SI

Question 18

How are things moved through the small intestine?

Answer 18

• Segmentation mixes contents of the lumen by contraction of circular muscles
• Peristalsis moves chyme towards the colon
• Migrating motor complex

Question 19

What is the migrating motor complex?

Answer 19

• Seen in fasting

- Cycles of smooth muscle contraction to clean the intestine and move bacteria away from the ileum

Question 20

Where does digestion occur in the duodenum?

Answer 20

• In the lumen and in contact with the membrane

Question 21

What is the difference between primary and secondary active transport?

Answer 21

• Primary involves hydrolysis of ATP

- Secondary involves electrochemical gradients

Question 22

Describe the process of carbohydrate digestion

Answer 22

• Starts with salivary amylase in the mouth, destroyed by the stomach pH
• In the duodenum pancreatic a-amylase is secreted in response to a meal. Needs an alkaline pH and Cl-.
• a-amylase acts on the lumen to break down starch and glycogen to smaller carbohydrates
• Amylase products and simple carbohydrates are digested at the membrane

Question 23

How does absorption of carbohydrates occur in the SI?

Answer 23

• Secondary active transport of glucose and galactose by SGLT-1
• Fructose by facilitated diffusion using GLUT-5
• GLUT-2 facilitates exit at the basolateral membrane

Question 24

Describe the digestion of proteins in the SI

Answer 24

• Begins in the stomach by pepsin which is inactivated in the duodenum due to pH
• Trypsin is activated by enterokinase at the duodenal brush border, and then activates other proteases
• Brush border peptides break down larger peptides

Question 25

How are amino acids and di/tri peptides absorbed at the brush border?

Answer 25

- Absorption of amino acids occurs by facilitated diffusion and secondary active transport - Di/tri-peptides require specific carrier proteins - Cytoplasmic peptidases break down di/tri-peptides before they cross the basolateral membrane

Question 26

How are lipids digested?

Answer 26

- Secretion of bile and lipases - Emulsification (increase SA for digestion) - Enzymatic hydrolysis and ester linkages (lipase + colipase) - Solubilization of lipolytic products in bile salt micelles

Question 27

What is the function of colipase?

Answer 27

Prevents bile salts from displacing lipase from the fat droplet

Question 28

What are the other enzymes, not icluding lipase, important for lipid digestion?

Answer 28

- Phospholipase A2 hydrolyses fatty acids at 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 29

Describe the structure of a bile salt

Answer 29

- Steroid nucleus planar- two faces. - Amphipathic. - Hydrophobic (nucleus and methyl) face dissolves in fat - Hydrophilic (hydroxyl and carboxyl) face dissolves in water

Question 30

What are bile salt micelles?

Answer 30

- Hydrophilic head in contact with solvent, hydropobic tails in the middle with a lipid core - Water insolble in SI

Question 31

How are lipids absorbed?

Answer 31

- Micelles important in absorption - Allow transport across the unstirred layer, present the fatty acids and monoglycerides to the brush border - Bile absorbed at the ileum, lipids by the middle of the jejunum

Question 32

How are lipids resynthesised into triglycerides?

Answer 32

- Monoglyceride acylation (major), | - Phosphatidic acid pathway (minor)

Question 33

Describe the process of monoglyceride acylation

Answer 33

- Fatty acids bind to the apical membrane. - Fatty acid binding proteins (FABP) facilitate transfer of fatty acids from apical membrane to the smooth ER. - In the smooth ER - fatty acids esterified into diglycerides and triglycerides.

Question 34

Describe the process that occurs in the phosphatidic acid pathway

Answer 34

Triglycerides synthesised from CoA fatty acid and a-glycerophosphate

Question 35

Describe the pathway taken by chylomicrons

Answer 35

- Chylomicrons transported to the golgi and secreted across the basement membrane by exocytosis - Too big to enter blood capillaries of villi, so travel through lymph channels instead

Question 36

What is the function of the ileocaecal sphincter

Answer 36

- Separates the ileum from the colon - Relaxation and contraction controls the passage of material into the colon - Prevents backflow of bacteria into the ileum