Physiology- Salivary and gastric secretions

Question 1

Outline the three functions of saliva

Answer 1

1. Lubrication- moistens mouth for swallowing allows movement of mouth and tongue for speech and dissolve chemicals in food to present to receptors.
2. Protection- alkalinity neutralises acids released by mouth bacteria, washes oral bacteria from teeth and reduce bacteria growth.
3. Digestion- breakdown of carbohydrates and fats via the enzymes a-amylase and lingual lipase/

Question 2

What happens in Sjogrens syndrome?

Answer 2

Autoimmune destruction of exocrine glands ->dry mouth/eyes

Question 3

How much saliva is produced per day?

What is its composition?

What is its source?

Answer 3

1.5 L

Serous - protein reach secretions (ptyalin hydrolsyses starch)
Mucous- mucin reach lubricant
Composition changes with flow rate. At high flow rate there is less time for ducts to absorb salt so saliva more closely resembles primary isotonic solution.

Submandibular glands -70%
Parotid (for serous secretion)- 25%
Sublingular glands (for mucus secretion) - 5%

Question 4

Describe the role of acinar cells in salivary secretion

Answer 4

Acinus produce primary isotonic solution (resembles interstitual fluid) and connect with ducts draining into major ducts. (Differ between mucus and serous producing)

Apical side pumps HCO3- and K+
Cl- release via Ca2+ activated Cl- channel
H20 release via aquaporins
Na+ release through leaky tight junctions

Acinar cells are also surrounded by myoepithelial cells

Question 5

Describe the role of the striated duct in salivary secretion

Answer 5

Makes secretion hypotonic as salt is absorbed without water

CL- uptake basolaterally via cotransporter
Na+ uptake via basolateral ATPase

Question 6

Describe central control of the stimulation of saliva production

Answer 6

Aldosterone increases ductal Na+ absorption and K+ secretion

Sight, smell, taste and thought provoke salivary nuclei in PONS

Efferent nerves (vagus) reach salivary glands via glossopharyngeal and facial nerves:

1. Ach on muscarinic receptor-> acinar secretion
2. Parasymp stim increases blood flow and promotes watery secretion of myoepithelial cells surrounding acinar and ducts contract and eject preformed saliva
3. Symp stim increases amylase output and decreases blood flow

Question 7

Describe the composition of gastric juice

Answer 7

1. Water and electrolytes dissolve and dilute food
2. HCL hydrolyse fat and starch, act as antiseptic, conversion of pepsinogen-> pepsin
3. Pepsins (endopeptidase) secreted as inactive form
4. Mucus- protects surface ep.cells from acid/pepsin erosion
5. Intrinsic factor- a glycoprotein that bind B12 necessary absorption in ileum

Question 8

Which areas of the stomach have an exocrine/ endocrine function?

Answer 8

Fondus and corpus = exocrine (pepsin, IF, HCL)

Antrum= endocrine (gastrin)

Question 9

Describe which cells occupy the oxyntic gland

Answer 9

Surface mucous cell- secrete protective barrier
Endocrine D cells- secrete somatostatin which regulates gastrin release
Endocrine G cells- produce gastrin
Chief cells- secrete pepsinogen (converted by acid to pepsin)
Exocrine parietal cell- acid secreting IF
Histamine cells

Question 10

Why does the gut need to be protected and how does it protect against self digestion and mechanical damage?

Answer 10

pH<2 + active pepsin

1. Mucus layer (contains mucoplysaccharides and mucuproteins which protect from mechanical damage)
   Basic side chains increase local pH to approx 7 and HCO3- secretion from epithelium neutralises H+
2. Tight junctions stop acid entry into underlying tissue
3. Continual replacement by stem cells

Question 11

What is Gastritis?

What is restitution?

Answer 11

Inflammation of gastric mucosa, commonly caused by infection by Helicobacter pylon. Also smoking, alcohol, NSAIDs and chronic stress

Restitution is the rapid regeneration of damaged surface epithelium from rapid division of stem cells in crypts.

Question 12

Describe acid secretion oxyntic cells

Answer 12

Resting structure contains tubulovesicles that have membranes containing H+/K+ pump

Cytoskeletal rearrangement causes these to fuse to lumen and increase SA via microvilli

Canaliculus formed have H+/K+ Atpase inserted into membrane and dense mitochondria to maintain energy needed for high metabolic activity.

Cl-/HCO3- exchanger on basolateral membrane of oxyntic cell pumps CL- into cell to maintain conc grad for apical CL- channel pumping Cl- into gastric lumen for HCL. HCO3- makes gastric venous blood alkalotic= POSTPRANDIAL ALKALINE TIDE

In oxyntic cell carbonic anhydrase breaks down H2CO3. H+ pumped into lumen via H+/K+ pump against steep conc grad (energy expensive)

K+ recycled out of apical side into lumen via K+ channel to maintain conc grad for H+/K+ ATPase

Question 13

State three stimulators of acid secretion

Answer 13

1. Gastrin- increases intracellular Ca2+ conc
2. Histamine- increases cAMP
3. ACh-> increased Ca2+ from post synaptic vagal fibres innervating gastric mucosa

Question 14

State two paracrine inhibitors of acid secretion

Answer 14

1. Somatostatin inhibits gastrin release from G cells. Released from D cells
2. Prostaglandins from mucosal cells anatagonise histamine (NSAIDs inhibit this)

Question 15

What is atrophic gastritis?

Answer 15

Dysfunction of parietal cells
Antibody-mediated destruction of parietal cells -> hypochlorhydria and decreased IF (malabsoprtion of B12 -> pernicous anaemia)

Question 16

How is histamine release?

Answer 16

parietal cell send paracrine signal stimulating ECL cell to release histamine

This also leads to local vasodilation of arterioles.

Question 17

Consider regulation of gastric secretions. What is the affect of protein digestion products in duodenum

Answer 17

Secretion of secretin, CCK, GIP which inhibits paretial cells to release gastric secretions

Stimulation of G cells in duodenum which provoked parietal cells to release gastic secretions.