Secretions Of The GI Tract

Question 1

What are the functions of the salivary glands?

Answer 1

Initial digestion of starches and lipids
Dilution and buffering of ingested food
Lubrication of ingested food with mucus

Question 2

Describe the parotid gland

Answer 2

Composed of serous cells
Secrete fluids composed of water, ions and enzymes (rich in amylase which digests starches)
Secretes 25% of the daily output of saliva

Question 3

Describe sub axillary and sublingual glands

Answer 3

Mixed glands composed of both serous and mucous cells
Secrete aqueous fluid and mucin glycoprotein for lubrication
Secrete the remainder 75% of daily output saliva

Question 4

The acinar cells located in an acinus of a salivary gland secrete what?

Answer 4

The initial saliva

Question 5

What are myoepithelial cells?

Answer 5

Located in salivary glands
Have motile extensions
When stimulated by neural input they contract to eject saliva into the mouth

Question 6

Describe the intercalated duct of a salivary gland

Answer 6

Saliva in the intercalated duct is similar in ionic composition to plasma

Question 7

Describe the striated duct of a salivary gland

Answer 7

Lined by columnar epithelial cells (ductal cells)
Ductal cells modify the initial saliva to produce the final saliva (hypotonic)
Ductal cells alter the concentration of various electrolytes

Question 8

What is saliva composed of?

Answer 8

H2O, electrolytes, alpha-amylase (sensitive to pH and is deactivated in the stomach), lingual lipase (begins lipids digestion), kallikrein (proteases and vasodilator that increases blood flow to the salivary gland and increase production of saliva) and mucous

Question 9

Compared to plasma, saliva is what?

Answer 9

Hypotonic
Greater K+ and HCO3- concentrations
Lower Na and Cl concentrations

Question 10

Describe the parasympathetic innervation of salivary glands

Answer 10

Presynaptic nerves originate at facial and glossopharyngeal nerves
Postsynaptic fibers in autonomic ganglia innervate individual glands

Question 11

Describe the sympathetic innervation of salivary glands

Answer 11

Preganglionic nerves originate at the cervical ganglion whose postganglionic fibers extend to the glands in the periarterial spaces

Question 12

What are two unusual features in the regulation of salivary secretion?

Answer 12

Exclusively under the control of the ANS

Salivary secretion is increased by both parasympathetic and sympathetic stimulation

Question 13

What are the main components of gastric juice?

Answer 13

HCl, pepsinogen, mucosa and intrinsic factor (and H2O)

Question 14

What is the role of HCl in gastric juice?

Answer 14

Necessary for the conversion of pepsinogen to pepsin

Kills a large number of bacteria that enter the stomach

Question 15

What is the role of pepsinogen in gastric juice?

Answer 15

Inactive precursor

Gets cleaved into pepsin and works with HCl to initiate protein digestion

Question 16

What is the role of mucus in gastric juice?

Answer 16

Lines the wall of the stomach and protects it from damage
Lubricant
Together with HCO3 it neutralizes acid and maintains the surface of the mucosa at neutral pH

Question 17

What is the role of intrinsic factor in gastric juice?

Answer 17

Required for the absorption of vitamin B12 in the ileum

Question 18

What is the role of water in gastric juice?

Answer 18

Medium for the action of HCl and enzymes

Solubilizes much of the ingested material

Question 19

What are the two types of glands in gastric mucosa?

Answer 19

Oxyntic and pyloric

Question 20

What are oxynitc glands?

Answer 20

Located in the proximal 80% of the stomach (body and fundus)
Secretes acid
Contains parietal cells, enterochromaffin like cells, chief cells and mucous cells

Question 21

What are pyloric glands?

Answer 21

Located in the distal 20% of the stomach (Antrum)
Synthesizes and releases gastrin into the circulation
Contain D cells, G cells (secrete gastrin) and enterochromaffin cells

Question 22

What do parietal cells secrete?

Answer 22

HCl and intrinsic factor
Located in the body
Contains lots of mitochondria (needed for energy to secrete HCl)

Question 23

What do chief cells secrete?

Answer 23

Pepsinogen

Located in the body of the stomach

Question 24

What do G cells secrete?

Answer 24

Gastrin into the circulation

Located in the antrum of the stomach

Question 25

What do mucous cells secrete?

Answer 25

Mucous, HCO3 and pepsinogen | Located in antrum

Question 26

The number of parietal cells determines what?

Answer 26

The maximal secretory rate

Question 27

What is the function of the low gastric pH (1-2)?

Answer 27

To convert pepsinogen to pepsin

Question 28

Where is HCl formed?

Answer 28

At the villus like membranes of the canaliculi of the parietal cell

Question 29

Gastric juice can be seen as a mixture of what two separate secretions?

Answer 29

Parietal and non-parietal secretions

Question 30

What are non-parietal secretions?

Answer 30

Basal alkaline secretion of constant and low volume It’s primary constituents are Na and Cl; K is present at the same concentration as plasma HCO3 is secreted at a concentration of ~30mEq/L

Question 31

What are parietal secretions?

Answer 31

Slightly hyperosmotic Contains 150-160 mEq/L of h and 10-20 mEq/L of K Cl is the only anion present As the secretion rate increases the concentrations of electrolytes begin to approach those of pure parietal cell secretion

Question 32

Pepsinogen is only secreted during what conditions?

Answer 32

When the gastric pH is acidic enough to convert it to pepsin

Question 33

What is the most important stimulus for pepsinogen secretion?

Answer 33

Vagus nerve stimulation

Question 34

What triggers local cholinergic reflexes that stimulate chief cells to secrete pepsinogen?

Answer 34

H+ ions

Question 35

Pepsin can convert what?

Answer 35

``` More pepsinogen into pepsin Proteolytic enzyme (splits interior peptide linkages) ```

Question 36

What is the optimal pH for pepsin?

Answer 36

Between 1.8-3.5 Reversibly inactivated at > pH 3.5-5 Irreversibly inactivated >pH 7-8

Question 37

What is intrinsic factor?

Answer 37

Mucoprotein secreted by parietal cells that is required for the absorption of vitamin B12 in the ileum Binds to vitamin B12 Only secretion by the stomach that is required (essential) Failure to secrete IF is associated with achlorhydria (no acid production) and with absence of parietal cells

Question 38

Describe growth of the gastric mucosa

Answer 38

Gastric epithelium secretes HCO3 and mucus to form the gel like mucosal barrier Mucous cells secrete mucous and surface epithelial cells secrete HCO3 The mucosal barrier protects the gastric mucosal epithelium against the HCl and pepsin

Question 39

What factors protect the gastric mucosa?

Answer 39

HCO3, mucus, prostaglandins (e.g. Misoprostol), mucosal blood flow, gastrin, and growth factors

Question 40

Which factors damage gastric mucosa?

Answer 40

Acid (H+), pepsin, NSAIDs (e.g. aspirin), H pylori, alcohol, bile, stress and smoking

Question 41

What is pernicious anemia?

Answer 41

Results from the absence of intrinsic factor which leads to decreased absorption of vitamin B12

Question 42

What are the common causes of pernicious anemia?

Answer 42

``` Atrophic gastritis (chronic inflammation of the stomach mucosa that leads to loss of parietal cells) Autoimmune metaplastic atrophic gastritis (immune system attaches IF protein or gastric parietal cells) ```

Question 43

What can disrupt the absorption of vitamin B12?

Answer 43

``` Gastrectomy (loss of parietal cells which is the source of IF) Gastric bypass (exclusion of the stomach, duodenum, and proximal jejunum alters absorption of vitamin B12 ```

Question 44

What does pancreatic juice contain?

Answer 44

HCO3- for the neutralization of H from the stomach and enzyme secretions to digest carbs, proteins and lipids into absorbable molecules

Question 45

What is the sympathetic innervation of the exocrine pancreas?

Answer 45

Postganglionic nerves from the celiac and superior mesenteric plexus Inhibits pancreatic secretion

Question 46

Describe the parasympathetic innervation of the exocrine pancreas

Answer 46

Vagus nerve Preganglionic fibers synapse in the ENS Postganglionic fibers synapse on the exocrine pancreas Stimulates pancreatic secretion

Question 47

Describe the organization of the exocrine pancreas

Answer 47

Organized like salivary glands Acinus contains acinar cells that synthesize and secrete major enzymes for digestion Ducts are lined by ductal epithelial cells; extends to the region in the acinus containing centroacinar cells