GI secretions (Part 2) - Intestine, pancreas, liver and gallbladder

Question 1

Function of the duodenum

Answer 1

Receives stomach contents, pancreatic juice and bile, neutralises stomach acids, emulsifies fats, pepsin inactivated by ph increase, pancreatic enzymes

Question 2

Function of the jejunum

Answer 2

Most nutrient absorption occurs here

Question 3

What are pever’s patches?

Answer 3

Aggregated lymphoid nodules which are found throughout the ileum region of the small intestine

They also form an important part of the immune system by monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria in the intestine

Question 4

What is the product of the SI called?

Answer 4

Succus entericus

Question 5

What are villi?

Answer 5

Fingerlike projections 1mm tall which contain blood vessels and lymphatics(lacteal) nutrient absorption

Question 6

What are microvilli?

Answer 6

1 micron tall; cover surface brush border on cells. Brush border enzymes for final stages of digestion

Question 7

What is the crypt-villus unit?

Answer 7

The functional unit of the small intestine

Stem cell division produces immature cells in crypts of lieberkuhn which secrete fluid; mature cells at the villus tip absorb nutrients, electrolytes and fluid

Question 8

What is there at the villus tip?

Answer 8

• At the villus tip, enterocytes are fully differentiated and undertake the absorption of nutrients, electrolytes, and fluid
• After 3-4 days, the cells are sloughed off the villus tip as a defense mechanism against insults from the luminal contents

Question 9

What is the maturation zone?

Answer 9

The maturation zone is an intermediate zone where cells are moving toward the tip of the villus and are beginning to express enzymes and absorptive membrane transport proteins

Question 10

What is the crypt?

Answer 10

• Crypt contains rapidly dividing stem cells that force migration of cells up the side of the villus
• The cells initially produced in the intestinal crypts are immature and do not express enzymes or membrane transporters for nutrient absorption

Question 11

How can you identify the duodenum in histology?

Answer 11

• Brunner glands, which empty into the intestinal glands, secrete an alkaline fluid which exerts a phsyiologic anti-acid function by coating the duodenal epithelium, therefore protecting it from the acid chyme of the stomach
• Brunner’s glands in submucosa

Question 12

How can you identify the jejunum in histology?

Answer 12

• Extensive intestinal folds

- Main site of absorption

Question 13

How can you identify the ileum in histology?

Answer 13

• Contains peyer’s patches (intestinal immune system)

Question 14

What is celiac sprue

Answer 14

A malabsorption syndrome caused by hypersensitivity to wheat gluten and gliadin, resulting in immune-mediated destruction and denudation of the small intestinal villi

Question 15

What are the effects of celiac sprue

Answer 15

• Malabsorption of nutrients causing diarrhoea and steatorrhoea(excess fecal fat) with associated abdominal bloating and flatulence

Question 16

What is the function of pancreas in digestion?

Answer 16

• Secretes about 1.5l of juices
• Secretes an alkaline fluid that neutralises the acidic chyme that enters the small intestine from the stomach. This fluid is necessary because pancreatic enzymes have a neutral pH optimum
• To secrete the enzymes that break down the macromolecules in food and to produce smaller nutrient molecules for intestinal absorption
• Has a separate endocrine function to secrete insulin and glucagon involved in metabolic regulation

Question 17

What do the exocrine glands of the pancreas secrete?

Answer 17

• Secretions delivered to the duodenum via the large pancreatic duct
• 1-2L of pancreatic juice is secreted per day
• Consisting of a mixture of secretions from the acini and ducts
• The exocrine glands deliver digestive enzymes and an isotonic HCO3- ion rich secretion into the intestinal lumen

Question 18

Describe the endocrine glands of the pancreas

Answer 18

• Consist of 4 types of islet cells that releases hormones

- Secretions delivered to blood stream

Question 19

What is the composition of pancreatic juices?

Answer 19

• Proteolytic enzymes
• Amylolytic enzymes
• Nucleases
• Lipolytic enzymes

Question 20

What are the enzyme secretions of the pancreas?

Answer 20

Ach - binds muscarinic receptor on acinar cells, Ach/vasoactive intestinal peptide increase blood flow, and gastrin

CCK - The major agonist released from duodenal I cells when food enters duodenum in response to fats, monoglycerides, fatty acids, AAs

Question 21

What are the alkaline secretions of the pancreas?

Answer 21

Secretin - Released by S cells in duodenum. S cells stimulated by low pH as food enters, secretin enters blood - duct cells

pH rarely low enough to stimulate high levels of secretin. Duct cells become hypersensitive to low levels of hormone - CCK(and Ach in early phases) potentiate actions of secretin

Question 22

What is the mechanism of enzyme secretions by acinar cells?

Answer 22

• Appropriate signal leads to fusion and exocytosis of zymogen granules
• Zymogen granules move to apical membrane, fuse with plasma membrane and discharge their content into luminal space by the process of exocytosis

– Upon stimulation by agonists(such as CCK, Ach, or secretin), release of content into the lumen via intra

Question 23

Prevention of pancreatic autodigestion

Answer 23

• Most enzymes are produces as inactive precursors called zymogens
• Enzymes are sequestered in membrane-limited vesicles throughout synthesis to the point of exocytosis, avoiding contact with the acinar cell cytoplasm
• Activation of zymogens occurs in the small intestine , this process depends on the conversion of the proenzyme trypsinogen to the active protelytic enzyme trypsin

Question 24

Which enzyme cleaves trypsinogen

Answer 24

Enterokinase, which is bound to the apical cell membranes of enterocytes lining the small intestine

Question 25

What does trypsin do once activated?

Answer 25

It cleaves and activates all other zymogens

Question 26

What is the trypsin inhibitor for?

Answer 26

The trypsin inhibitor is produced by the pancreas to prevent activation of zymogens within the pancreas if trypsin is inappropriately activated inside the gland

Question 27

What is pancreatitis?

Answer 27

Occurs when pancreatic enzymes are activated within the pancreas, resulting in autodigestion of the tissues

Question 28

What are the most causes of pancreatitis?

Answer 28

Gender specific and include gallstones in women and alcohol use in men

Question 29

What is pain from pancreatitis like?

Answer 29

Classically described as epigastric pain radiating from the epigastrium to the back and is often relieved by leaning forward

Question 30

Mechanism of isotonic NaCl primary secretion by acinar cells

Answer 30

• The Na-K pump creates the inwardly directed Na+ gradient across the basolateral membrane
• The Na/K/Cl cotransporter produces the net Cl- uptake, driven by the Na+ gradient, which is generated by the Na-K pump
• The rise in intracellular [K+] that results from the activity of the pump and cotransporter is shunted by basolateral K+ channels that provides an exit pathway for K+
• The intracellular accumulation of Cl- establishes the electrochemical gradient that drives Cl- secretion into the acinar lumen through apical membrane Cl- channels

Question 31

What are potent stimulators of Cl- secretion?

Answer 31

The hormone CCK and the cholinergic neurotransmitter acetylcholine

Question 32

What are the secretions of the pancreatic duct cells?

Answer 32

• Duct cells secrete 1-2l of alkaline(HCO3- rich) fluid into the duodenum per day which

Provides optimum pH for pancreatic enzyme function

Protects the mucosa from erosion by acid

Question 33

What is the mechanism of HCO3- secretion by pancreatic ductal cells

Answer 33

• HCO3- secretion from the cell cytoplasm into the lumen occurs via the cl-/HCO3- exchange in the luminal cell membrane
• To supply enough intracellular Cl- to sustain the rate of Cl-/HCO3- exchange Cl- is recycled from the lumen into the cell via the cystic fibrosis transmembrane conductance regulator(CFTR) Cl- channel
• Na+ is secreted into the duct lumen following HCO3- secretion; water follows by osmosis to produce fluid secretion

Question 34

Relation between rate of secretion and concentrations of HCO3- and Cl- levels

Answer 34

The greater the rate of secretion, the higher the HCO3- levels and the lower Cl- levels

Question 35

What is the flow rate at an unstimulated state

Answer 35

Low and electrolyte composition is similar to that of plasma

Question 36

What is the flow rate at a stimulated state

Answer 36

Flow rate increases and the rise in [HCO3-] ion in the secondary secretion is matched by reciproacal decline in Cl- ion

Question 37

Purpose of bile

Answer 37

• Provides alkali to neutralise acid
• Provides bile salts to facilitates absorption of fats
• Acts as a vehicle for excretion of breakdown of blood cell components(provides a pathway to excrete hydrophobic molecules that may not be readily excreted by the kidney)

Question 38

Composition of bile

Answer 38

• Water
• Ions
• Bilirubin and biliverdin
• Bile salts

Question 39

Production and functions of bile salts

Answer 39

• Synthesized from cholesterol

- Required for emulsification of and absorption of lipids, cholesterol and phospholipids

Question 40

What is enterohepatic circulation

Answer 40

Recycling of bile acids from ileum

Question 41

What is urobilinogen?

Answer 41

Intestinal bacteria convert bilirubin pigment from haemoglobin breakdown

Question 42

What does the gall bladder do to the bile?

Answer 42

Concentrates bile salts

Question 43

What is bile acid-dependent fraction

Answer 43

Made by hepatocytes when sufficient bile acids available

Question 44

What is bile acid-independent fraction

Answer 44

produced by duct epithelium - by secretion of water and electrolytes (HCO3- rich)

Question 45

What acids are produced from primary bile salts

Answer 45

From cholesterol metabolism - generates cholic acid and chenodeoxycholic acid

Question 46

Which acids are produced from secondary bile acids?

Answer 46

• Small amount of secondary bile acids formed by bacteria converting small amount of primary acids (deoxycholic and lithocholic acid)