Upper GI Tract

Question 1

What is the difference between the gastrointestinal system, the alimentary system and the digestive system?

Answer 1

There isn’t one they’re the same

Question 2

What are the components of the GI system?

Answer 2

Upper: oesophagus and stomach

Mid: small intestine and a bit of the large

Lower: colon, rectum and anus

Question 3

What is the structure of the gut wall?

Answer 3

Mucosa:

Epithelium, lamina propria ( loose connective tissue), muscularis mucosae

Submucosa:

Connective tissue (containing nerve plexus)

Muscularis: smooth muscle (containing nerve plexus)

Divided into two: circular (inner), longitudinal (outer)

Serous/adventitia: connective tissue +/- epithelium

Question 4

What is the anatomy of the oral cavity?

Answer 4

Teeth: 32 in total (8 incisors, 4canines, 8 premolars, 12 molars)

Muscles:

Masseter - largest, responsible for biting

Temporalis

And several muscles control position of biting

Salivary glands:

Lingual lipase, salivary amylase

Parotid, submaxillary, sublingual

Tongue:

Intrinsic muscles - fine motor control and moving food

Extrinsic muscles - gross movement of tongue (in/out/up/down)

Assists mechanical digestion

Question 5

What is the oesophagus?

Answer 5

From mouth to stomach

Can be divided into parts: upper 1/3 (24cm), middle 1/3 (24-32cm), lower 1/3 (32-40cm)

It starts level with C5 and ends level with T10

Function: acts as a conduit for the food bolus from pharynx to stomach

Epithelium:

Non keratinising, squamous

Wear and tear lining (can handle extremes)

Lubrication- mucous secreting glands and saliva

Muscle:

Tonically active, swallowing centre, peristalsis

Has two sphincters, upper oesophageal syphincter (true) and lower (debated)

Z line - between pink mucosa of oesophagus and red mucosa of stomach

Question 6

What is the gastro-oesophageal junction?

Answer 6

Z line between them

Reflux - prevented by the diaphragm

Epithelial transition - stratified squamous to simple columnar

Gastric folds - rugae , increase surface area for more digestion

Question 7

What is the mechanism of swallowing?

Answer 7

Stage 0: oral phase

Chewing and saliva prepare bolus

Both oesophageal sphincters constricted

Stage 1: pharyngeal phase

Pharyngeal musculature guides food bolus towards oesophagus

Both oesophageal sphincters open

Stage 2: upper oesophageal phase

Upper sphincter closed

Superior circular muscle rings contract and inferior rings dilate

Sequential contractions of longitudinal muscle

Stage 3: lower oesophageal phase

Lower sphincter closed as food passes through

Question 8

What are some features of the stomach?

Answer 8

Breaks food into smaller particles due to secretion of acid and pepsin

Holds food, releasing it in controlled steady rate into duodenum

Kills parasites and certain bacteria

Structure:

Cardia and pyloric region- mucus only

Body and fundus- mucus, HCL, Pepsinogen

Antrum: gastrin

Question 9

What are the contents of the stomach?

Answer 9

Acid:

2L/day

150 mM H+ (3 million times that in the blood)

Mucin:

HCO3- (bicarbonate) trapped in mucus gel

pH:

At epithelial surface: 6-7

In lumen: 1-2

Question 10

What are the muscular movements of the stomach?

Answer 10

Peristalsis:

20% of stomach contraction

Longitudinal muscle

Propels chyme towards colon

More powerful as moves from lower oesophageal sphincter to pyloric sphincter

ANS essential

Segmentation:

80%

Circular muscle

Weaker

Fluid chyme towards pyloric sphincter

Solid chyme pushed back to body of stomach

Stretching activated enteric nervous system

Question 11

What are chief cells?

Answer 11

Protein secreting epithelial cell

Abundant RER

Golgi picking and modifying for export

Masses of apical secretion granules

Secretes pepsinogen (broken down into pepsin by HCL)

Question 12

What are parietal cells?

Answer 12

Many mitochondria, requires lots of ATP

Cytoplasmic tubulovesicles (contain H+/K+ ATPase)

Internal cannuliculi, extend to apical surface

Secrete HCL

When actively secreting, tubulovesicles fuse with membrane

Microvilli project into cannuliculi

Carbonic anhydrase: breaks down H2CO3 into hydrogen ions and bicarbonate

These hydrogen ions leave the cell into the lumen via the hydrogen potassium ATPase

Chloride enters the cell using n antiporter as bicarbonate leaves the cell

Chloride ions are also secreted into the lumen along with potassium

The trigger for all of this is histamine binding to parietal cells

So the stomach contents have high concentrations of HCL and potassium

Question 13

What breaks pepsinogen into pepsin?

Answer 13

HCL

Question 14

What is Gastrin?

Answer 14

Secreted in pyloric antrum

Local peptide hormones

Stimulates histamine release from chromaffin cells (lamina propria)

Secreted by G cells

Question 15

What is the cephalic phase of gastric secretion?

Answer 15

Thiught, sight, smell and taste of food causes gastric secretion

Via vagus nerves

Vagus nerve stimulates parietal cell (uses ACh)

Question 16

What is the gastric phase of gastric secretion?

Answer 16

Food in stomach

Stretch and chemo receptors

Goes to and back from the brain via vagus nerves

Stimulates parietal cells to secrete HCL and chief cells to secrete pepsinogen

Question 17

What is the intestinal phase of gastric secretion?

Answer 17

Happens when the food has reached the duodenum

There is both and inhibition and stimulation of gastric acid

But main effect is: when chyme of pH<2 or lipids reach the duodenum, this triggers a signal to the brain via the vagus nerves. This causes inhibition of secretion of HCL and pepsin.

This is done using the hormones gastric inhibitory peptide, cholecystokinin and secretin

Question 18

What are some drug types to stop HCL secretion?

Answer 18

H2 receptor blockers (stop histamine binding)

Proton pump inhibitors