Small Intestine Physiology

Question 1

Where is mucus produced in the small intestine?

Answer 1

Brunner’s Gland in the duodenal mucosa

Viscous, alkaline rich fluid

Question 2

Where is water secreted from in the small intestine?

Answer 2

Intestinal epithelium secretes Na+, Cl- and HCO3+ into the lumen, water follows by osmosis.

The hypotonicity of chyme encourages water to cross the small intestine by osmosis

Question 3

What hormones are secreted by the small intestine?

Answer 3

• Secretin
• Cholecystokinin (CKK)
• Motilin
• Vasoactive intestinal peptide (VIP)
• Gastric Inhibitory Peptide (GIP)

Question 4

What does motilin do?

Answer 4

stimulates migrating motor complexes (MMCs) via both enteric and autonomic nervous system.

Question 5

What do migrating motor complexes (MMCs) do?

Answer 5

MMCs act between periods of digestive motility. They sweept contents of the GI tract that cannot be digested out.

Responsible for the rumbling tummy in between meals

Involved in opening of sphincters and strong peristaltic movements

Question 6

What does Vasoactive Intestinal Peptide (VIP) do?

Answer 6

Increase blood flow to the GI tract

Question 7

What does Gastric Inhibitory peptide do? (GIP)

Answer 7

Glucose dependent insulinotropic peptide

• Inhibits gastric secretion
• Stimulates insulin secretion

Question 8

What does Cholecystokinin (CKK) do?

Answer 8

• Acts on exocrine pancreas to stimulate production of pancreatic enzymes.
• Acts on gallbladder causing contraction and release of contents into small intestine

Question 9

What does Secretin do?

Answer 9

• Acts on the exocrine pancreas, stimulating bicarbonate production.
• Stimulates liver to produce bile.
• Inhibits gastric motility

Question 10

Where are pancreatic enzymes secreted from?

Answer 10

Acinar Cells

Question 11

What do the Acinar cells of the pancreas secrete?

Answer 11

secrete Enzymes into the duct, they leave the pancreas via the common bile duct

Question 12

Where is alkali secreted from in the pancreas?

Answer 12

Duct cells in

Question 13

What do the duct cells of the pancreas secrete?

Answer 13

Alkali

Question 14

What is the function of the alkali secreted by pancreatic duct cells?

Answer 14

• An isotonic solution rich in bicarbonate ions

- Neutralises duodenal contents - alkaline bile from the liver also helps

Question 15

Name the digestive enzymes produced by the pancreas?

Answer 15

Proteases:

• Trypsin
• Chymotrypsin
• Carboxypeptidase

Pancreatic amylase (breaks down starch)

Lipases (breaks down fats)

Question 16

How are Pancreatic enzymes produced?

Answer 16

-Produced as ZYMOGENS (inactive enzymes)

E.g. Pancreas secretes inactive Trypsinogen

-Cleaved by membrane bound enterokinase to active peptidase TRYPSIN

Trypsin then acts on other inactive enzymes e.g. chymotrypsin & carboxypeptidase, releasing the active form of these enzymes.

Question 17

Outline Bicarbonate production in the Duct Cells

Answer 17

• Similar to HCl prod in parietal cells but ion movements are reversed
• Bicarb ions are secreted into the duct lumen
• H+ ions are pumped out of duct cells and released into the blood
• Reaction between CO2 + H2O is catalysed by Carbonic Anhydrase

Question 18

How is enzyme secretion in the pancreas controlled?

Answer 18

• By CCK

- By Ach from parasympathetic postganglionic neurons of Vagus

Question 19

How is the alkali secretion of the pancreas stimulated?

Answer 19

• Secretin

- Potentiated by CCK & Ach (vagal tone)

Question 20

Where does digestion of Fats, Starch and proteins occur?

Answer 20

• Lumen of the duodenum and the proximal jejunum

Question 21

How are fats digested?

Answer 21

• Fats (triglyceride) are digested by pancreatic lipase to monoglyceride and fatty acids

(lumen of duodenum and proximal jejunum)

Question 22

How is starch digested?

Answer 22

• Starts in the mouth by salivary amylase, completed by pancreatic amylase
• Disaccharides are broken down into monosaccharides by brush border enzymes on the luminal membrane of the small intestine

Question 23

How are proteins digested?

Answer 23

• Protein digestion begind in the stomach by Pepsin
• Broken down into peptide fragments by Trypsin and chymotrypsin
• Peptide fragments are digested to free amino acids by carboxypeptidase (pancreas) and aminopeptidase (luminal membranes of small intestine epithelial cells)

Question 24

What is the role of pancreatic lipase in digestion?

Answer 24

• Digest fats (triglyceride) into monoglyceride and fatty acids

Question 25

What is the role of pancreatic amylase in digestion?

Answer 25

- Complete digestion of starch after it has been started by salivary amylase. - Into disaccharides

Question 26

How are disaccharides taken in from the diet digested?

Answer 26

-Broken down into monosaccharides by brush border enzymes in the luminal membrane of the small intestine

Question 27

What is the role of brush border enzymes in the luminal membrane of the luminal membrane of the small intestine?

Answer 27

- Brush border enzumes are embedded in the membrane with the catalytic domain facing the lumen - Break down disaccharides into monosaccharides

Question 28

What is the role of trypsin and chymotrypsin?

Answer 28

- Break proteins down into peptide fragments

Question 29

What is the role of carboxypeptidase and aminopeptidase?

Answer 29

Digest peptide fragments into free amino acids after digestion by trypsin and chymotrypsin

Question 30

Where is carboxypeptidase secreted?

Answer 30

Protease secreted from pancreas

Question 31

Where is aminopeptidase secreted from?

Answer 31

Luminal membranes of small intestine epithelial cells

Question 32

What is the first step in Absorption of fats?

Answer 32

- Emulsification of fats by bile salts and phospholipids: - Fats delivered at a controlled rate from duodenum - Bile salts surround fats, hold them in suspension... presents surface to pancreatic lipase for digestion into monoglyceride + fatty acid

Question 33

After emulsification, what is the next step in fat absorption?

Answer 33

- Fats are digested by pancreatic lipase, the products are held in micelles combined with bile salts and phospholipids - Fat soluble Vitamines D,A,K,E also carried in micelles.

Question 34

After fats are digested and the products are suspended in micelles, what happens next?

Answer 34

- Micelles diffuse into unstirred layer next to surface of epithelial cells. Fatty acids and monoglycerides diffuse into epithelial cells. - Once inside, they are reassembled into fats. - Triglyceride droplets are packaged into chylomicrons - Chylomicrons are exported across the basolateral membrane and leave via lymph vessel.

Question 35

What are chylomicrons and how do they travel through the body?

Answer 35

- After digestion and entry into the epithelial cell, Triglyceride droplets are packaged into chylomicrons - Chylomicrons are exported across the basolateral membrane and leave via lymph vessel.

Question 36

After digestion by brush border enzymes, how are monosaccharides transported across the epithelium into the hepatic portal vein? REDO ANSWER

Answer 36

Fructose moves across the membrane via facilitated diffusion, down its concentration gradient - Glucose or galactose is transported via SGLT - secondary active transport, uses energy generated by movement of Na+ ions out of the cell, brings glucose or galactose with it across basolateral membrane

Question 37

What do aminopeptidases do?

Answer 37

Break down tri/dipeptides into amino acids

Question 38

What enzyme breaks tri/dipeptides down into amino acids?

Answer 38

Aminopeptidases

Question 39

How are amino acids transported into the portal vein from the epithelial cell?

Answer 39

Via co-transporters linked with sodium. Different structures are needed depending on the character of the A As