GI Tract Secretions

Question 1

Name the three different regulatory phases of the GI function

Answer 1

1) Cephalic,
2) Gastric,
3) Intestinal

Question 2

Describe the primary regulatory events in the cephalic phase

Answer 2

It is initiated by taste/smell of food. Vagal activity triggers increase in salivary secretion, an increase in the secretions in the stomach (HCL, mucus, pepsin and gastrin) It also stimulates bile ducts, hepatocytes and pancreatic acinar cells.

Question 3

Describe the primary regulatory events in the gastric phase

Answer 3

• Initiated by food entering stomach causing distension and release of gastrin. G cells are triggered by parasympathetic pathways, HCL, decreased acidity in stomach, distension of antrum and protein, peptides and amino acids. G cells release gastrin which causes gastric motility, parietal cells and trophic maintenance of GI epithelium.

Question 4

Describe the primary regulatory events in the intestinal phase

Answer 4

• Initiated by presence of food in duodenum.
• If pH is below 2 then is causes gastric inhibition and intestinal stimulation. if above 3 then release of gastrin. Acid causes release of secretin

Question 5

What are some factors that increase and decrease saliva secretion

Answer 5

Increased - PSNS (primarily) and SNS.

Decrease - Sleep, dehydration and atropine.

Question 6

Describe the three stages in saliva release.

Answer 6

1) Acinar cells secrete primary secretions containing Na, Cl, K and bicarbonate as well as amylase and mucin.
2) Myoepithelial cells stimulated to eject saliva.
3) Ductal cells cause secondary modification, the bicarbonate conc is dependent on flow rate

Question 7

Describe how flow rate impacts the bicarbonate concentration of saliva

Answer 7

High flow rate means saliva has a high bicarbonate conc, this means that after a meal the bicarbonate can protect teeth from bacteria. Low flow rate = low conc of bicarbonate

Question 8

Describe the different salivation reflexes

Answer 8

Simple - Pressure receptors in mouth activated in presence of food.

Acquired - Smell, see, hear prep of meal. Both lead to increased production of saliva

Question 9

Describe how the different subsections of the autonomic NS effect the make up of saliva

Answer 9

PSNS - Large volume of watery, enzyme rich saliva.

SNS - Small volume, thick and mucousy

Question 10

Describe features of oesophageal secretion

Answer 10

Only secretes mucous for lubrcation/protection. The main body is lined by simple mucous glands for mechanical damage protection. The gastric end has compound mucous glands to protect against chemical damage

Question 11

Where are the different ccell types located in the stomach?

Answer 11

Body - Parietal and cheif cells,

Antrum - G cells and mucous cells

Question 12

Name the different gastric secretions and their function.

Answer 12

• Hydrocholric acid (small protein digestion but mainly activation of pepsinogen)
• Pepsinogen (protein digestion)
• Intrinsic factor ( Vit B12 absorption)
• Mucous (protection and lubrication

Question 13

What increases the surface area in the stomach?

Answer 13

Gastric pits which contain mucous cells, parietal and chief cells

Question 14

Describe the process of HCl secretion which occurs in order to protect the oarietal cells

Answer 14

• At the basolateral membrane, bicarbonate ions are exchanged for chloride ions.
• At the apical membrane H+ ions are secreted into the lumen via the H/K-ATPase and chloride ions then diffuse across the membrane. In the lumen they form hydrochloric acid

Question 15

Describe the proportion of secretions that occur at each of the main gastric stages

Answer 15

Cephalic - 30% which occurs via direct stimulation of parietal cells via vagus or indirect via gastrin.

Gastric - 60% Distension causes direct vagal stimulation, indirect release via gastrin and local reflexes that cause gastrin release.

Intestinal - 10% due to presence of breakdown products

Question 16

Describe the inhibition of HCl secretion

Answer 16

Occurs when HCl is no longer needed to convert pepsinogen into pepsin. Inhibiton occurs via somatostatin which directly inhibits release via binding to receptors on parietal cells or indirectly by inhibiting histamine and gastrin release

Question 17

Describe features of pepsinogen secretion

Answer 17

• Secreted by cheif and mucous cells in oxyntic (gastric) glands response to vagal stimulation.
• H+ ions trigger local reflexes which stimulate secretion.

Question 18

Describe the composition and functions of exocrine pancreatic secretions

Answer 18

Composed of an aquesous solution containing enzymes and high bicarbonate ions.

The bicarbonate ions neutralise stomach acid and the enzymes digest carbs, proteins and lipids.
Secretion is increased by secretin, CCK and PSNS

Question 19

Describe how the PSNS and SNS effect pancreatic secretions and where the enzymes are stored.

Answer 19

PSNS - Stimulates secretion via vagus.
SNS - Inhibits secretion.
Enzymes - stored in zymogen cells

Question 20

Describe the formation of pancreatic secretions

Answer 20

• Aqueous component released from centroacinar and ductal cells. Ductal cells cause secretion rich in bicarbonate. Enzymes are released from acinar cells. Amylase and lipase are secreted as active by protease are inactive and therefore need activated in the duodenum.

Question 21

Name the three types of pancreatic enzymes

Answer 21

• Pancreatic amylase (polysaccharide to disaccharide)..
• Pancreatic lipase (Triglycarides to monoglycerides and fatty acids)
• Proteolytic (Cleaves proteins into small peptides and amino acids

Question 22

Name the 3 major proteolytic enzymes

Answer 22

Trypsinogen, chymotrypsinogen and procarboxypeptidase. All are inactive in zymogens to prevent self digestion

Question 23

Describe the activation of the pancreatic proteolytic enzymes

Answer 23

Trypsinogen is activated by enterokinase/enteropeptidase to form trypsin. Trypsin then goes on to activate more trypsinogen, chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase

Question 24

Describe the regulation of the pancreatic secretions in the cephalic and gastric phases of digestion

Answer 24

• Cephalic phase (via vagus),

- Gastric phase (via distenstion of stomach and vagus)

Question 25

Describe the pancreatic response to the intestinal phase of digestion

Answer 25

• Duodenal I cells secrete CCK, vagal release of ACh potentiates CCK action and this triggers acinar cells to produce enzymes.
• Secretin is major stimulus for rich bicarbonate secretion. ACh and CCK potentiate secretin action and this triggers ductal cells to produce Na, K, Cl and HCO

Question 26

Describe the function of the gall bladder

Answer 26

Stores bile which is produced by hepatocytes. It also concentrates bile. Major stimulus for bile ejection is cholecystokinin (CCK) which is released form I cells in duodenum and jejunum.

Question 27

Describe the regulation of bile secretion

Answer 27

In between meals - Bile salts recirculate to the liver so more bile produced, it is then stored in gall bladder.

Cephalic phase - Vagal stimulation increases bile flow.

After meal - Chyme in duodenum stimulates release of CCK and Secretin (trigger release of bile)

Question 28

Describe the secretions of the small intestone

Answer 28

• Crypts of leiberkuhn and villi are covered in goblet cells (secrete mucus) and enterocytes that secrete water and electrolytes in the cryptes and absorb water and electrolytes in villi. Secretions are regulated by distention and irritative/tactile stimuli from chyme.

Question 29

Describe the secretions of the large intestines

Answer 29

• Alkaline mucus which is high in potassium and bicarbonate. This protects and lubricates as well as neutralising H+ ions produced from gut bacteria. Secretion is stimulated by distention and some parasympathetic input