GI Tract Secretion

Question 1

What is GI function co-ordinated by?

Answer 1

integration of neural and hormonal signals

Question 2

What type of signals stimulate GI function?

Answer 2

physical (stretch receptors)

chemical (sights and presence of food)

Question 3

What are the 3 main phases that regulate GI function?

Answer 3

• cephalic
• gastric
• intestinal

Question 4

Draw a diagram showing the links between the three phases. Highlight both the feed forward and feed backward mechanisms

Answer 4

Question 5

What is the cephalic phase intiated by?

Answer 5

taste, thought, smell of food

Question 6

What is the most important regulator of the cephalic phase and what is it triggered by?

Answer 6

Brain and higher iontegrating centres increase vagal activity which intiates the parasympathteic branch of the ANS

Question 7

What are the effects of the PSNS on the cephalic phase

Answer 7

• increase salivary secretion
• increase secretion in the stomach
  
  • HCl
  • Mucus
  • Pepsin
  • Gastrin
• Stimulation of bile ducts, hepatocytes and pancreatic acinar cells

Question 8

What occurs as a result of increased secretion in the stomach?

Answer 8

increased blood flow to stomach

blood factors (e.g. insulin and glucagon)

Question 9

What is the gastric phase initiated by?

Answer 9

food entering the stomach causing distension and release of gastrin - especially coffee, alcohol, calcium

Question 10

What are the 4 main inputs into G cells of the stomach during the gastric phase?

Answer 10

• Parasympathetic pathways
• Decreased acidity to stomach due to buffering of food
• distension of antrum
• proteins, peptides and amino acids

Question 11

What is released by G cells of the stomach and what are their primary targets?

Answer 11

Gastrin

• Gastric motility
• Parietal cells
• Trophic maintenance of GI epithelium (cell turn over)

Question 12

What do parietal cells release that act as a negative feedback mechanism

Answer 12

HCl

Question 13

What is the intestinal phase intiated by?

Answer 13

presence of food in the duodenum

Question 14

What is the requirement for the release of gastrin?

Answer 14

If pH >3, duodenal peptides/amino acids cause gastrin release

Question 15

What is the requirement for gastric inhibition and intestinal stimulation?

Answer 15

If pH < 2

Question 16

What causes release of GIP and CCK?

Answer 16

duodenal fats and breakdown products

Question 17

What causes secretin release

Answer 17

acid entering the intestine

Question 18

What is secretion?

Answer 18

Addition of substances (fluids, enzymes, ions etc) into the lumen of the gastrointestinal tract

Question 19

What are the main producers of secretions?

Answer 19

• Produced by salivary glands (saliva)
• Cells of the gastric mucosa (gastric secretion)
• Exocrine cells of the pancreas (pancreatic secretion)
• Liver (bile)

Question 20

What are the main characterisitics of saliva?

Answer 20

• High HCO₃^-
• High K⁺
• Hypotonic
• α-amylase and lingual lipase

Question 21

Factors that increase the secretion of saliva

Answer 21

PSNS (primary) and SNS

Question 22

What factors decrease the secretion of saliva?

Answer 22

• sleep
• dehydration
• atropine

Question 23

What are the main functions of saliva?

Answer 23

• Initial digestion of starches and lipids (very little)
• Dilution and buffering of ingested foods
• Protection of teeth and gums
• Lubrication of ingested foods with mucous (mucin)

Question 24

Describe the location and cell type of sublingual glands

Answer 24

deep in floor of mouth, underneath tongue, mostly mucous cells

Question 25

Describe the location and cell type of subandibular glands

Answer 25

under lower edge of mandible, mixed glands containing serous and mucous cells

Question 26

Describe the location and cell type of parotid glands

Answer 26

below ear and over the masseter (jaw muscle), serous cells secreting an aqueous fluid composed of water, ions and enzymes

Question 27

Describe stage 1 of salivation

Answer 27

* Acini cells secrete 1° secretion (isotonic) * Na+, Cl-, K+, HCO3- * Amylase and mucin production

Question 28

Describe stage 2 of salivation

Answer 28

Myoepithelial cells stimulated by neural input

Question 29

Describe stage 3 of salivation

Answer 29

* Duct cells - 2° medication * Reabsorb Na+ and Cl- and add K+ * HCO3- concentration is altered depending on flow rate

Question 30

Affect of high flow rate on saliva?

Answer 30

increased HCO₃^-

Question 31

Affect of low flow rate on saliva?

Answer 31

more HCO₃^- extracted

Question 32

Describe saliva in the mouth in comparison to the plasma?

Answer 32

hypotonic (more dilute)

Question 33

Draw a diagram indicating how salivary secretion is regulated

Answer 33

Question 34

What is the main function of oesophgeal secretion?

Answer 34

secretes mucous for lubrication/protection (no real digestive function)

Question 35

What is the action of simple mucous glands in the main body of the oesphagus?

Answer 35

protects against mechanical damage

Question 36

What is the function of compound mucous glands at the gastric end of the oesophagus?

Answer 36

protect agaonst chemical damage

Question 37

What are the 4 main types of gastric cells in the stomach and give their exact location

Answer 37

* parietal cells - body * chief cells - body * G cells - antrum * mucous cells - antrum

Question 38

Action of HCl

Answer 38

Protein digestion (a bit) – mostly for pepsinogen activation at acid pH, creates pH~2

Question 39

Action of pepsinogen

Answer 39

protein digestion (a bit)

Question 40

Action of intrinsic factor

Answer 40

vitamin B12 absorption (in the ileum) – really important

Question 41

action of mucus

Answer 41

protection and lubrication

Question 42

What are the three main chemicals of gastric secretion

Answer 42

* HCl * Pepsinogen * Intrinsic Factor

Question 43

What factors increase gastric secretion?

Answer 43

* Gastrin * Acetylcholine * Histamine * Parasympathetic

Question 44

What factors decrease gastric secretion?

Answer 44

* H+ in stomach * Chyme in duodenum * Somatostatin * Atropine * Cimetidine * Omeprazole

Question 45

Describe HCl secretion on the apical surface

Answer 45

* H+ is secreted into the lumen via the H+-K+ ATPase * Cl- follows by diffusion through an apical channel

Question 46

Describe HCl secretion on the basolateral surface

Answer 46

* HCO3- is exchanged for Cl- via the chloride-bicarbonate exchanger (alkaline tide) * Eventually HCO3- is secreted back into the GI tract in pancreatic secretions

Question 47

How much does the cephalic phase contribute to secretion? and what is it triggered by?

Answer 47

30% of secretion smell, taste, chewing, swallowing, conditioned reflex in anticipation of food

Question 48

What is responsible for stimulation of the cephalic phase?

Answer 48

* Direct stimulation of the parietal cells by the vagus * Indirect stimulation of the parietal cells by gastrin * Vagal gastrin releasing peptide (GRP) stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl

Question 49

What causes the gastric phase and what percentage does this contribute to secretion?

Answer 49

60% secretion Distension of the stomach and presence of breakdown products of proteins, amino acids and small peptides in the stomach

Question 50

What does distension of the stomach cause?

Answer 50

o direct vagal stimulation of parietal cells o indirect stimulation via gastrin o local reflexes in the antrum that stimulate gastrin release

Question 51

What are the direct effects of amino acids and small peptides on G cells?

Answer 51

stimulates gastrin release

Question 52

What percentage is the intestinal phase of secretion and what is it triggered by?

Answer 52

10% of secretion presence of breakdown products in the duodenum

Question 53

When is HCl secretion inhibited?

Answer 53

when HCl is no longer needed to convert pepsinogen to pepsin. This occurs after the chyme moves into the small intestine and the H+ buffering capacity of the food is no longer a factor

Question 54

What is the direct pathway of somatostatin?

Answer 54

bindstoreceptorsonparietalcells (and inhibits adenylate cyclase via Gi protein so inhibits HCl release)

Question 55

What is the indirect pathway of somatostatin?

Answer 55

inhibitshistaminereleasefrom stomach and gastrin release from G cells so HCl inhibited

Question 56

What is responsible for pepsinogen secretion?

Answer 56

secreted by chief mucous cells in the oxyntic glands in response to vagal stimulation H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen

Question 57

draw a diagram detailing the conversion of pepsinogen to pepsin

Answer 57

Question 58

How much does the exocrine pancreas secrete into the duodenum per day?

Answer 58

~1L of fluid

Question 59

What is the composition of the fluid secreted into the duodenum by the exocrine pancreas

Answer 59

Composed of an aqueous solution containing enzymes and high HCO-3

Question 60

What is the function of HCO₃^-

Answer 60

to neutralise stomach H⁺

Question 61

Characterisitcs of pancreatic secretion

Answer 61

* High HCO-3 (isotonic) * Pancreatic lipase, amylase and proteases

Question 62

Factors that increase pancreatic secretion

Answer 62

* Secretin * Cholecytokinin (CCK) (potentiates secretin) * Parasympathetic supply

Question 63

What is the innervation of the exocrine pancreas?

Answer 63

* PSNS = vagus stimulates secretion * SNS = inhibits secretion

Question 64

How are pancreatic enzymes stored?

Answer 64

Stored in condensed zymogen granules until release

Question 65

Where is pancreatic secretion released from and describe the composition of this secretion

Answer 65

Aqueous component of secretion released from centroacinar cells and ductal cells * Pancreatic fluid is an isotonic fluid containing Na+, K+ Cl-and HCO-3 * Modification of the (ion) composition by the ductal cells results in a fluid secretion rich in HCO-3

Question 66

Where is the enzymatic component of pancreatic secretion released from and what is its composition?

Answer 66

* Pancreatic amylase and lipases are secreted as active enzymes * Pancreatic proteases are secreted in an inactive form and activated in the duodenum

Question 67

Draw a diagram detailing what is released from different cells in pancreatic cells and the ion exchnage that takes place

Answer 67

Question 68

What are the three types of pancreatic enzymes?

Answer 68

1. pancreatic amylase 2. pancreatic lipase 3. proteolytic

Question 69

action of pancreatic amylase

Answer 69

polysaccharide --\> disaccharide much more important than salivary amylase

Question 70

action of pancreatic lipase

Answer 70

triglycerides --\> monoglycerides and fatty acids very important source of lipase

Question 71

Action of proteolytic enzymes

Answer 71

cleave proteins at different sites --\> amino acids and small peptides

Question 72

What are the three main types of pancreatic proteolytic enzymes?

Answer 72

* Tripsinogen --\> trypsin * Chymotrypsingogen --\> Chymotrypsin * Procaboxypeptidase --\> carboxypeptidase All inactive to prevent self digestion

Question 73

Draw a diagram showing the activation of proteolytic enzymes

Answer 73

Question 74

Describe the regulation of pancreatic secretion in the cephalic phase of digestion

Answer 74

mediated by the vagus nerve ( mainly enzymatic secretion)

Question 75

Describe the regulation of pancreatic secretion in the gastric phase of digestion

Answer 75

initiated by distension of the stomach and mediated by the vagus nerve ( mainly enzymatic secretion)

Question 76

Describe the regulation of pancreatic secretion in the intestinal phase of digestion

Answer 76

accounts for 80% of pancreatic secretion and both enzymatic and aqueous secretions are stimulated

Question 77

Pancreatic Response of acinar cells in the intestinal phase of digestion

Answer 77

* Duodenal I cells secrete CCK in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen * Vagal release of ACh potentiates CCK action * Triggers acinar cells to produce enzymes

Question 78

Pancreatic Response of ductal cells in the intestinal phase of digestion

Answer 78

* Secretin released by the S cells of the duodenum is the major stimulus for aqueous rich HCO-3 secretion * Secretin release is triggered by the arrival of acidic chyme in the duodenum * ACh and CCK potentiate secretin action * Triggers ductal cells to produce Na+, K+ Cl- and HCO-3

Question 79

What are the 3 main functions of the gall bladder?

Answer 79

* Stores bile which is continuously produced by the hepatocytes and flows to the gallbladder through the bile ducts * Concentrates bile * Epithelial cells lining the gallbladder absorb ions and water iso-osmotically * Ejects bile * Begins ~30 minutes after a meal. The major stimulus for ejection is the release of cholecystokinin from the I cells in the duodenum and jejunum

Question 80

What is bile?

Answer 80

* Essential for the digestion and absorption of lipids * Bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into ‘packets’ called micelles * Mixture of bile salts, bile pigments and cholesterol Bile is not enzymic

Question 81

Describe the regulation of bile secretion

Answer 81

* • In between meals * Bile salts recirculated to liver via the enterohepatic system more bile produced * Bile stored / concentrated in gall bladder * Cephalic phase (just before a meal) * Neural (parasym) stimulation via vagus nerve to ↑ bile flow * During/after a meal * Chyme in duodenum stimulates release of CCK & secretin

Question 82

Action of CCK?

Answer 82

* triggers release of stored bile * gallbladder contraction * sphincter of oddi relaxation

Question 83

Action of secretin

Answer 83

* triggers bile secretion * especially NaHCO₃^-

Question 84

Draw a diagram indicating the effect of chyme in the duodenum on bile secretion

Answer 84

Question 85

Outline the secretions of the small intestine

Answer 85

* Crypts of Lieberkühn * Located over the surface of the SI * Crypts lie between intestinal villi * Crypts and villi covered with two types of cells * goblet cells – secrete mucus * enterocytes – in crypts secrete water and electrolytes (~1800ml/day) * in villi absorb water and electrolytes (along with products of digestion) * Exact mechanism for secretion still not known –involves active secretion of Cl- and HCO3- into the crypts at apical membrane. Electrical gradient causes Na+ to be drawn though and together, the osmotically active particles draw water though by osmosis. * Regulated largely by distension and tactile or irritative stimuli from chyme

Question 86

Outline the secretions of the large intestine

Answer 86

* Alkaline mucus * High[K+]&[HCO3-] * No digestive enzymes * Functions: * protection & lubrication * neutralisation of H+ produced by gut bacteria * Greatest trigger is distension / mechanical stimulation of walls * ↑ secretion triggered by acetyl choline & vasoactive intestinal peptide (VIP) * ↓ secreVon triggered by adrenaline & somatostaVn * Some parasymp input - Extreme parasymp stimulationexcess mucus - defection (e.g., stress, nervousness)

Question 87

Main stimulus for bile secretion

Answer 87

* secretin * CCK (gall bladder contraction: relaxation of sphincter of Oddi) * Some central and enteric nerve input