Upper GI tract

Question 1

Define digestion and absorption

Answer 1

Digestion is the process of breaking down macromolecules to allow absorption.
Absorption is the process of moving nutrients & water across a membrane. Absorption can occur through various means: directly into bloodstream or through lymphatic system.

Question 2

What are the components of the GI system?

Answer 2

Upper GI tract: Oesophagus + Stomach – arise from foregut. Other foregut structures are liver, pancreas, gall bladder and a portion of the duodenum.
Mid GI tract: Small Intestine, portion of large intestine (transverse colon) – arise from midgut
Lower GI tract: Entire colon, rectum + anus – arise from hindgut

Question 3

Describe wall structure of oesophagus

Answer 3

First layer is the mucosa, contains epithelium, lamina propria (loose connective tissue) and muscularis mucosae. Submucosa contains connective tissue (containing nerve plexus). Nerve plexus of submucosa important for peristalsis and other functions. Muscularis contains smooth muscle with nerve plexus. inner is circular while outer is longitudinal – both serve different functions. Serosa/Adventitia contains connective tissue +/- epithelium.

Question 4

Describe organisation of teeth and muscles in oral cavity

Answer 4

Adult mouth has 32 teeth. 8 incisors, 4 canines, 8 premolars, 12 molars. Masseter muscle is the largest jaw muscle and responsible for biting. Several muscles control position of mandible.

Question 5

What is the function of the salivary glands?

Answer 5

Food mixed with saliva (aqueous secretion & digestive enzymes). Contains lingual lipase (fat digestion) and salivary amylase (carbohydrate digestion).

Question 6

What is the function of the tongue?

Answer 6

Contains intrinsic and extrinsic muscles. Intrinsic muscles responsible for fine motor control & moving food. Extrinsic muscles responsible for gross movement of tongue (in, out, up & down) and assists mechanical digestion.

Question 7

How is the oesophagus divided up?

Answer 7

One way of dividing the oesophagus is into cervical oesophagus, upper thoracic, middle thoracic and lower thoracic oesophagus but middle two stratifications contain most of oesophagus as cervical only consists of few centimetres in neck while lower thoracic only consists of few centimetres in abdomen. Second division can be done on the basis of distance from incisors into proximal, middle and distal portions.

Question 8

What is the function of the oesophagus?

Answer 8

Conduit for food, drink & swallowed secretions from pharynx to stomach. Epithelium is non-keratinising which acts as a ‘wear & tear’ lining since can take extremes of temp. & texture. Another function is lubrication - contains mucus glands and saliva.

Question 9

Describe muscular control of oesophagus

Answer 9

Two sphincters: the upper and lower oesophageal sphincters. The upper is skeletal muscle and hence under conscious control - existence of lower is controversial. These are tonically active and swallowing centres.

Question 10

Describe the gastroesophageal junction and a disorder of it

Answer 10

A Z-line marks this junction where the pink mucosa of squamous epithelium of the oesophagus meets the red mucosa of gastric epithelium. Barrett’s oesophagus is a condition where lower oesophagus epithelium undergoes a change (metaplasia) and gastric mucosa extends into the oesophagus.

Question 11

What is the role of the muscles of the oesophagus?

Answer 11

Circular muscle layer has a more segmental role as allows food to remain in certain segs of oesophagus while longitudinal muscle contracts and propels the food caudally allowing bolus to move downwards

Question 12

Describe significant anatomical features of the gastroesophageal junction

Answer 12

Diaphragm plays a role in allowing movement from oesophagus into stomach and vice versa so prevents reflux. Also the area of epithelial transition stratified squamous to simple columnar. Gastric folds called rugae present.

Question 13

Why is there an increased number of cancers of gastroesophageal region?

Answer 13

Seeing more cancers of the gastro-oesophageal region. In the past, mainly cancers of the squamous epithelium of oesophagus but now increasingly seeing adenocarcinomas of metaplastic epithelia in gastro-oesophageal region. Theory is that prolonged acid exposure causes squamous epithelium to change and become columnar which is protective against that acid damage but constant exposure makes these cells dysplastic and eventually cancerous.

Question 14

How is acid reflux prevented?

Answer 14

1. Diaphragm pinches the oesophagus preventing acid from travelling up and causing reflux – hence when you get a hiatus hernia where opening of oesophagus surrounded by diaphragm becomes larger, a portion of stomach slips into chest causing prolonged exposure of lower oesophagus to acid.
2. Gastro-oesophageal junction lies at an angle to the stomach + phreno-oesophageal ligaments prevent acid reflux.

Question 15

What is the purpose of rugae?

Answer 15

The gastric folds (or gastric rugae) are coiled sections of tissue that exist in the mucosal and submucosal layers of the stomach. They provide elasticity by allowing the stomach to expand when a bolus enters it.

Question 16

What are the stages of swallowing?

Answer 16

Stage 0: Oral phase - Chewing & 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 closes. Superior circular muscle rings contract & inferior rings dilate. Sequential contractions of longitudinal muscle.
Stage 3: Lower oesophageal phase - Lower sphincter closes as food passes through

Question 17

What are the functions of the stomach?

Answer 17

1. Breaks food into smaller particles (acid & pepsin)
2. Holds food, releasing it in controlled steady rate into duodenum
3. Kills parasites & certain bacteria

Question 18

What are the sections of the stomach?

Answer 18

Split into three sections: Cardiac, gastric and pyloric. Cardia found at bottom of oesophagus and fundus is the superior lobe formation of stomach. Gastric section contains the body of the stomach - main section. Pyloric antrum and canal lead to pylorus in final section of stomach.

Question 19

What chemicals are found in the different sections of the stomach?

Answer 19

Cardia & Pyloric Region: Mucus only
Body & Fundus: Mucus, HCl, pepsinogen
Antrum: Gastrin

Question 20

Describe chemical environment of stomach

Answer 20

2L/day acid produced. 3 million times more H+ present here than in blood. HCO3- trapped in mucus gel coating epithelia. pH at epithelial surface is 6-7 while it is 1-2 in lumen.

Question 21

What is the role of peristalsis and segmentation in movement of bolus?

Answer 21

Peristalsis comprises 20% of stomach contractions. Propels chyme towards colon and more powerful as moves from LOS (lower oesophageal sphincter) to pyloric sphincter. Relies on autonomic nervous system. Segmentation comprises 80% of stomach contractions and is weaker. Fluid chyme is moved towards pyloric sphincter while solid chyme pushed back to body. Stretching activates the enteric nervous system.

Question 22

What is the role of the gastric chief cell?

Answer 22

Is a protein-secreting epithelial cell with extensive rough endoplasmic reticulum and Golgi apparatus for packaging and modifying for export. Contains masses of apical secretion granules and secretes pepsinogen.

Question 23

What is the role of the parietal cell?

Answer 23

Secretory cell. Contains many mitochondria (requires lots of ATP) and cytoplasmic tubulovesicles (contain H+/K+ ATPase). Has many internal canaliculi (extend to apical surface). When secreting, tubulovesicles fuse with membrane and microvilli project into canaliculi. Responsible for producing HCl.

Question 24

Describe process of HCl production

Answer 24

1. Water and carbon dioxide combined in parietal cell cytoplasm to form H2CO3 catalysed by carbonic anhydrase.
2. Carbonic acid spontaneously dissociates into hydrogen ion and bicarbonate ion.
3. The hydrogen ion that is formed is transported into the stomach lumen via the H+– K+ ATPase ion pump. This pump uses ATP as an energy source to exchange potassium ions with H+ ions. H+ passes into lumen while K+ transported into parietal cell.
4. The bicarbonate ion is transported out of the cell into the blood via a transporter protein called anion exchanger which transports the bicarbonate ion out the cell in exchange for a chloride ion (Cl–). This chloride ion is then transported into the stomach lumen via a chloride channel.

Question 25

How is acid production controlled?

Answer 25

Very few H+/K+ ATPases present at rest as most enclosed within tubulovesicles. These fuse with canaliculi membrane when there is a stimulus, hence allowing for the increased movement of hydrogen ions into the stomach thus increasing acid production. Three methods of control: ACh release via vagus nerve, gastrin binding to CCK receptors and histamine secretion by enterochromaffin like cells.

Question 26

Describe cephalic phase of gastric secretion

Answer 26

During the cephalic phase, gastric acid and pepsinogen secretion is activated by the thought, sight or smell of food, and by food in the mouth. Sensation carried via vagus nerve and vagus nerve then stimulates parietal cell through acetylcholine release which binds to M3 muscarinic receptor. This causes calcium ion influx in parietal cell and tubulovesicles fuse. Gatrin release also stimulated.

Question 27

Describe gastric phase of gastric secretion

Answer 27

Stimulus is stretch and chemoreceptors in the stomach triggered by distension caused by food arriving in stomach. Local reflexes of HCl secretion as a response to the distension. Signal coordinated by vagus nerve. Histamine, gastrin and acetylcholine control this phase.

Question 28

How does contents of food alter HCl secretion?

Answer 28

As dietary protein is digested, it breaks down into smaller peptides and amino acids that directly stimulate the G cells to secrete even more gastrin: this is a positive feedback loop that accelerates protein digestion. Small peptides also buffer the stomach acid so the pH does not fall excessively low. As digestion continues and these peptides empty from the stomach, the pH drops lower and lower. Below pH of 2, stomach acid inhibits the parietal cells and G cells: this is a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.

Question 29

Describe intestinal phase of gastric secretion

Answer 29

Excitatory intestinal phase occurs when protein concentration in duodenum stimulates gastrin secretion. Enterogastric reflex also occurs in this phase where increased stomach distension results in decreased motility and hence slower emptying of stomach. If arriving chyme in duodenum has a pH below 2 or high lipid content, 3 hormones inhibit acid release in stomach namely secretin, gastric inhibitory peptide and cholecystokinin.

Question 30

How does omeprazole work?

Answer 30

Is a proton pump inhibitory which bind to the H+– K+ ATPase (proton pump), hence preventing the transportation of hydrogen ions into the stomach lumen. PPIs completely prevent stomach acid formation due to hydrogen ions not being able to react with chloride ions in the stomach.

Question 31

How does ranitidine work?

Answer 31

Is a H2 antagonist and so binds to the H2 receptors preventing the binding of histamine and thus reduce acid secretion.