Upper GI tract

Question 1

What is the function of the digestive system?

Answer 1

Breaks food down into macromolecules to allow for absorption via the gastrointestinal tract into the bloodstream - follows the processed of ingestion, digestion, absorption and excretion

Question 2

What are the components of the gastrointestinal (GI) system?

Answer 2

Salivary glands include the parotid, sublingual and submandibular glands

Duodenum - first part of the small intestine, connects to the stomach

The colon (large intestine) is divided into 5: ascending, transverse, descending, sigmoid and rectum

Liver, gallbladder and pancreas are connected to the upper GI tract

Question 3

What are the foregut structures (embryologically)?

What are the midgut structures?

What are the hind gut structures?

Answer 3

Oesophagus, stomach, liver, pancreas and the biliary system

Small intestine and a part of the large intestine

Remainder of the colon and rectum

Question 4

The gut wall carries out which 2 functions?

How is it adapted to carry out these functions? What is the basic plan of the gut wall (cells and tissues)?

Answer 4

Digestion and absorption

1. Mucosa (epithelium) - many functions, but mainly secretory as well as absorptive
2. Submucosa (connective tissue) - through which the blood vessels and capillaries are found, also contains the nerve plexus
3. Muscularis (smooth muscle) - it has 2 functions: 1. inner layer called the circular muscle layer that constricts the bowel to divide it into segments. Food is churned in these individual segments to prolong time for absorption. 2. Longitudinal outer muscle layer carries out peristalsis, involuntary contraction of circular muscle to propagate the food / bolus from the oesophagus to the rectum
4. Serosa (covering / lining) - has protective functions

Question 5

What muscle divides the oesophagus into segments and what is its function?

What is the function of the longitudinal muscles?

Answer 5

Inner circular muscle - pushes the food

Longitudinal muscles - carry out peristalsis to propagate the food

Question 6

How many teeth are found in the oral cavity and what are the different types?

What are the common muscles found in the oral cavity?

How is food broken down in the oral cavity?

Answer 6

32 teeth in total - 8 incisors, 4 canines, 8 pre-molars, 12 molars

Masseter - largest jaw muscle, responsible for biting

Mastication - chewing movement facilitated by the muscles, teeth that break down the food. enzymes to begin digestion

Question 7

What are the 3 salivary glands found in the oral cavity? What does saliva contain and what are its functions?

What are the functions of the saliva?

What are the tongue muscles and what is the function of the tongue?

Answer 7

Parotid, sublingual and submandibular - together produce saliva, which contains aqueous secretion and digestive enzymes (lingual lipase and salivary amylase) for lipid and carbohydrate digestion

Saliva lubricates the food and begin the process of digestion

Tongue = propagates the food into the pharynx - intrinsic tongue muscles = fine motor control and moving food; extrinsic tongue muscles = overall movement of tongue (up, down, in, out) and assists mechanical digestion

Question 8

Where does the pharynx lead into?

What is the oesophagus?

What are the structures of the oesophagus and how do the different structures link to its function?

Answer 8

The oesophagus

A muscular tube that propagates food, drinks and swallowed secretions from the pharynx to the stomach - traversing through 3 different body cavities (neck, thorax, and abdomen)

Lined by squamous epithelium that is non-keratinising - protective, ‘wear and tear’ lining that protects from extreme temperatures (too hot or cold) and textures

Contains mucus secreting glands - lubricate the food as it passes down

Strong smooth muscles - peristalsis

Upper and lower oesophageal sphincters - control how much food first enters the oesophagus and how much food then leaves the oesophagus, so to control the speed at which food moves through the oesophagus into the stomach

Question 9

What is the name given to the junction where the oesophagus and stomach meet? What occurs here?

How are the oesophagus and stomach mucosa different?

Answer 9

The gastroesophageal junction - pale pink mucosa of the oesophagus meets the red mucosa of the stomach

Oesophagus = squamous lined so its function is mainly protective, whereas stomach = secretory (becoming columnar lined mucosa - secretes many disgestive juices)

Question 10

What is a common condition associated with the lower sphincter and the gastroesophageal junction?

What occurs if this condition persists for many years?

Answer 10

Acid reflux - acid produced in the stomach refluxes into the oesophagus, produces symptoms of heartburn

Although the oesophagus is made of squamous mucosa that is protective and can protect the oesophagus from the acid, there is still inflammation and irritation

After many years, metaplasia occurs - the oesophagus lining changes from squamous to columnar (like stomach lining) and also has secretory roles to further protect itself from the acid. The condition is called barrett’s oesophagus, and can sometimes progress to cancer

Question 11

How does metaplasia progress to dysplasia, and eventually to cancer?

Answer 11

Metaplasia = abnormal change in the type of tissue, dysplasia = presence of abnormal cells in a certain type of tissue, cancer = abnormal growth (uncontrollable cell division)

Question 12

The gastroesophageal junction goes through what important structure? Why is this important?

Answer 12

Diaphragmatic / oesophageal hiatus - the diaphram being a muscle pinches the lower end of the oesophagus to help prevent acid from refluxing back up

Question 13

Why is acid reflux on a daily basis not a normal occurence?

Answer 13

Angle between the oesophagus and the stomach is slightly acute

Mucosal folds in the gastroesophageal junction to prevent the acid from going up

Pressure in the thorax is negative (as the lungs must stay inflated), so relatively the pressure in the abdomen is positive - therefore the difference keeps the lower end of the oesophagus in the abdominal cavity in a collapsed state to prevent the acid from going up

The phrenoesophageal ligaments suspend the gastroesophageal junction at an angle at which acid cannot easily go back up

Question 14

What is a hiatus hernia? What are its implications?

Answer 14

When the hiatus of the diaphram becomes too large so can no longer constrict the oesophagus end, and so a part of the stomach begins migrating to the chest cavity (thorax)

It keeps the gastroesophageal junction open allowing for the acid in the stomach to continuously travel up the oesophagus

Question 15

What are the 4 phases of swallowing?

Answer 15

1. Stage 0 (oral phase) = chewing allows for food to be broken down by the teeth, saliva mixes with it to prepare the bolus. Both oesophageal sphincters are constricted
2. Stage 1 (pharyngeal phase) = food bolus is guided from the oral cavity into the pharynx. Both oesophageal sphincters open
3. Stage 2 (upper oesophageal phase) = upper oesophageal sphincter closes and the circular muscular rings contract to move the bolus down the oesophagus. Sequential contractions of the longitudinal muscles as well
4. Stage 3 (lower oesophageal phase) = lower sphincter closes as food passes through to the stomach

Question 16

Can you survive without the stomach?

Can you survive without the oesophagus?

What are the different sections of the stomach? Fill in the covered labels on the diagram below:

Answer 16

No, but you can survive without most of it

Yes, it has been done before with oesophageal cancer - the oesophagus is removed, and the stomach is stretched into a long tube to take its place. Vagus nerve supply to the stomahc is also usually cut

Question 17

What are the different functions of the stomach?

What do the different parts of the stomach secrete?

Answer 17

Breaks food into smaller molecules; holds food, releases it in a controlled steady rate to the duodenum; kills parasites and certain bacteria

Cardiac and pyloric region = mucus only

Body and fundus = mucus, HCl and pepsinogen

Antrum = gastrin

Question 18

Why is acid secretion in the stomach important?

Why is mucus secretion in the stomach important?

How does the pH of the columnar epithelial cells of the stomach differ compared to the lumen?

Which enzyme is secreted in the stomach predominantly?

Answer 18

Helps in breaking down the food (and so starting the process of digestion) - 2L of acid produced per day

The mucus contains bicarbonate that increases the local pH of the columnar eptihelial (for protection - so they do not get damaged)

Epithelial lining = pH 6-7; lumen = pH 1-2

Pepsin = breaks down proteins

Question 19

What is the structure of the lining of the stomach?

What are the different types of cells lining the folds and pits?

Answer 19

It is composed of folds (gastric folds) and pits, many different cells line the folds and pits

Chief cells, parietal cells

Question 20

What is the function of the gastric chief cell?

How is it adapted to perform its function?

Answer 20

Secretes pepsinogen (which is converted to pepsin under the acidic conditions of the stomach)

Abundant in RER - as the ribosomes carry out proteinsynthesis

Golgi apparatus then packages the proteins, and the apex of the cell contains the secretory granules, from which the pepsinogen is secreted

Question 21

What is the function of the parietal cell?

How is it adapted to perform its function?

Answer 21

Helps to create HCl

Does not have a protein systhesis function, instead contains an active pump called the H⁺/K⁺ ATPase pump, and it requires a lot of energy

Many mitochondria present in the cell to provide ATP for the pump

It also contains a system of tubules and canaliculi whih release the HCl into the lumen

Question 22

How do parietal cells produce HCl?

Answer 22

CO₂ enters the parietal cell from the capillaries. CO₂ mixes with H₂O using the enzyme carbonic anhydrase to catalyse the process forming H⁺ and HCO₃^-. K⁺ and Na⁺ are exchanged, whilst Na⁺ leaves the parietal cell to go into the capillaries, K⁺ enters the cell from the capillaries and is secreted into the gastric lumen. Then the K+ is exchanged for the H⁺ via the H⁺/K⁺ ATPase pump. The HCO₃^- is exchanged for a Cl^-, HCO₃^- exits the cell to go into the capillary, whilst the Cl^- enters the cell and is secreted into the gastric lumen. This allows for the H⁺ and Cl^- to combine to form HCl

Question 23

Why is the knowledge of how HCl is produced useful?

Answer 23

Previously, surgeries were used to treat peptic ulcer disease - now medications can be used instead

So HCl secretion can be reduced by using carbonic anhydrase inhibitors, or stopping other parts of sequence of events

Question 24

Why is HCl important?

Answer 24

Functioning of pepsinogen / pepsin - converts pepsinogen to pepsin so it can break down proteins

Acidic conditions kill parasites and some bacteria

Question 25

What is gastrin? Where is it secreted from?

What is the function of gastrin?

Why is gastrin important clinically?

Answer 25

Gastrin is a hormone secreted by the G-cells present in the antrum of the stomach

Gastrin stimulates the parietal cells to secrete HCl

Some patients present with persistent peptic ulcers - perhaps be indicative of a gastrinoma (benign tumour of the gastric producing cells)

Question 26

Parietal cells require various stimulants and suppressants for HCl secretion. What are the 3 types?

Answer 26

1. Neuro-stimulation - thought, sight, smell of food, travels via the vagus nerves
2. Hormonal stimulation - e.g. gastrin stimulates parietal cells to secrete HCl
3. Local reflexes - when the stomach distends with food, nerves causes a local neural response via the Meissner and myenteric neural plexi. Local response = greater secretion of HCl and enzymes

Question 27

What are the issues associated with the surgical removal of the oesophagus? It involves stretching the stomach to a tube, and cutting off the vagus nerve supply:

Answer 27

Stomach has been stretched and has lost its nerve supply, stomach does not move as well, so stomach cannot be emptied as effectively - symptoms include reduced appetite / portions, and nausea

No peristalsis as vagus nerve supply has been cut

Pyloris sphincter cannot relax as there is no peristaltic wave to trigger its relaxation - sphincter remains in a constant state of contraction / spasm - symptoms often include vomiting, nausea etc.

Cut off vagus nerve supply also = lost stimulation for muscle movement and HCl production (but gastrin can still stimulate HCl production)

Question 28

What are the 2 types of muscles found in the stomach and what are their roles?

Which branches of the nervous system innervate these 2 types of muscles?

Answer 28

Circular muscle = 80% of stomach’s contractions, responsible for segmentation function. Churns the chyme so it remains in the stomach for some time to allow for it to be broken down by the acid and enzymes. Stretching of the stomach activates the enteric NS (a branch of the ANS)

Longitudinal muscle = 20% of stomach’s contractions, responsible for peristalsis function. Propels chyme towards the colon through the pyloric sphincter. ANS function.

Question 29

Can the food leave the stomach and enter the small intestine if the pyloric sphincter spasms?

Where might this be seen?

Answer 29

No - sometimes seen in neonates who are born with hypertrophic pyloric stenosis. Symptoms include repeated, non-bililous vomiting as the milk ingested cannot exit the stomach and enter the duodenum

Question 30

As the food exits the stomach and enters the small intestine, what is used to signal between these 2 organs?

What is this reflex called?

Answer 30

Enterohormones, such as gastri inhibitory peptides, cholecystokinin, and secretin. These feedback to the stomach to stimulate / inhibit secretion of HCl and pepsin. There are also neural pathways from the intestines that send signals back to the brain, which then send signals via the vagus nerve to the stomach

This is known as the gastrointestinal reflex

Question 31

What are the 3 phases of gastric secretion?

Answer 31

1. Cephalic - neural stimulation, sight, smell and taste of the food
2. Gastric - distension through the neural plexi increase stomach activity
3. Intestinal - pH and hormonal activity

Question 32

The stomach has a thick layer of mucosa to protect itself from the HCl (preventing self-digestion). Why might this layer be broken down?

Answer 32

e.g. H-pylori bacteria converts urea into ammonia. Ammonia creates an alkaline medium that then neutralises the acid. It also breaks down binding channels that hold the cells of the stomach lining together, breaking the mucosal barrier. The HCl can then destroy further cells in the stomach, giving rise to peptic ulcers

Question 33

What different factors can stimulate HCl production?

Using that information, what drugs may be used to reduce HCl production?

Answer 33

Innervation from the vagus nerve - acetylcholine (ACh) binds to the ACh receptors

Other cells in the stomach wall secrete histamine

G-cells secrete gastrin

Anti-histamines / histamine receptor blockers

Blocking gastrin by giving the hormone somatostatin, which suppresses the g-cell receptors to reduce HCl secretion

Blocking the H⁺/K⁺ pump

Question 34

Which of the following stimuli is most likely to decrease aid secretion int he stomach?

A. Chyme fatty acid content

B. Increased ACh secretion

C. Increased gastrin secretion

D. Protein content of the meal

E. Stomach distension

Answer 34

A - enterogastric reflex will tell the stomach to stop secreting acid