Lecture 10: Stomach

Question 1

What is the stomach?

Answer 1

Dilated part of the digestive tract, right aftr the oesophagus and leading onto the duodenum

Question 2

What is the role of the stomach?

Answer 2

• store food
• start digestion of protein
• some carbohydrate and fat digestion (salivary amylase and lingual lipase)
• disinfect (acidic)

Question 3

What are the landmarks of the stomach?

Answer 3

Cardia: first part of the stomach straight after the oesophagus
Fundus: top bulge of stomach
Body: main bulk of the stomach
Antrum: narrower area at the bottom which leads to the pyloric sphincter which controls the release of chyme into the duodenum

Question 4

What is the change in epithelia between the oesophagus and the stomach?

Answer 4

Stratified squamous to simple columnar

Question 5

What is the junction between the oesophagus and the stomach?

Answer 5

Lower oesophageal sphincter: composed of smooth muscle from stomach (intrinsic part) and right crus of the diaphragm (crural part of diaphragm) wraps around the sphincter and acts to close it when intraabdominal pressure increases
-stops contents of stomach refluxing

Question 6

What stops reflux into the oesophagus from the stomach?

Answer 6

• angle of entry into the stomach
• lower oesophageal sphincter
• receptive relaxation

Question 7

What is receptive relaxation?

Answer 7

• we swallow and peristalsis in oesophagus alerts the stomach that something is about to enter
• causes a reflex relaxation of proximal stomach (fundus distends: stomach has rugae-folds-)
• this means the pressure in the stomach doesn’t go up that greatly

Question 8

What are the different muscles of the stomach?

Answer 8

-oblique
-circular
-longitudinal
Proximal wall of stomach is much thinner than the distal part which is more muscular, giving rise to the stomach distinct shape. Therefore the food starts to move faster as it travels distally, which helps to accelerate the smaller parts to the pyloric sphincter and the larger particles to remain in the stomach.

Question 9

What is the lining of the stomach made from?

Answer 9

Surface mucous cells
-produce mucus to form protection between lumen of stomach and epithelial layer
Between the epithelia are gastric pits which contain gastric glands

Question 10

What is the function of the gastric pits?

Answer 10

-parietal cells
-chief cells
-enteroendocrine cells
All reside in the gastric glands

Question 11

What cells are found in the gastric glands in the BODY of the stomach?

Answer 11

Parietal cells: produce HCL
Chief cells: produce pepsinogen (inactive form of pepsin, and the conversion to pepsin is facilitated by the acidic environment of the stomach)

Question 12

What cells are found in the gastric glands in the ANTRUM of the stomach?

Answer 12

Enteroendocrine cells: G-cells

-produce gastrin

Question 13

What protects the stomach from HCL/Pepsin?

Answer 13

• mucus
• HCO3- ions: forms a pH neutral layer
• rich blood supply to the gastric mucosa so we can remove any acid quickly from epithelia
• prostaglandins released: increase blood flow/support mucus layer
• high turnover of epithelial cells to keep barrier in tact if some epithelia are damaged

Question 14

How does the parietal cell produce acid?

Answer 14

Exchanges intraluminal potassium for hydrogen ions using the proton pump
We don’t want to be producing acid all the time so:
Resting phase
-tubulovesicles are found in the cell and these contain the proton pumps and lack potassium permeability so the apical membrane contains the potassium channels and lacks hydrogen pumps
Stimulated phase
-tubulovesicles and apical membrane come together

Question 15

How is the apical membrane of the parietal cell adapted?

Answer 15

• canaliculi (involutions)

- microvilli

Question 16

What stimulates the parietal cells to go from resting to stimulated phase?

Answer 16

• sensory triggers (sight/taste/smell)
• stretch of stomach
• presence of amino acids and small peptides
• chyme entering the duodenum

Question 17

What are the phases of digestion and how much does each stage contribute to HCL production?

Answer 17

Cephalic phase: sensory triggers (30% of HCL)
Gastric phase: 60% of HCL
Intestinal phase: chyme entering duodenum (10% of HCL)

Question 18

How are the pariteal cells ‘switched on’?

Answer 18

Gastrin receptors
-G cells are stimulated from peptides in stomach, they produce gastrin which is a hormone that enters the bloodstream, the gastrin then attaches to the CCK receptor on the parietal cell

Histamine receptors
-entero-chromaffin like cell (ECL) produces histamine which binds to the parietal cell to stimulate

Muscarinic receptors (vagus nerve)

Question 19

Why does gastrin bind to the CCK receptor?

Answer 19

Cholecystokinin and gastrin come from the same family of hormones and are similar in structure

Question 20

What can stimulate G cells other than peptide in the stomach?

Answer 20

Vagus nerve: parasympathetic triggered by stretching or sensory triggers

• acetylcholine receptor on the G cell (muscarinic receptor)
• GRP (gastrin releasing peptide)

Question 21

What stimulates the ECL cells to produce histamine?

Answer 21

ECL cell has a muscarinic receptor stimulated by acetylcholine, via vagal stimulation. It also has a CCK receptor so can be stimulated by gastrin

Question 22

How do you switch off acid production?

Answer 22

• stomach starts to empty and the stretch receptors in stomach aren’t stimulated, so vagal stimulation stops
• as acid in stomach builds up, pH drops, the D cells then detect this and are stimulated
• the D cells now produce somatostatin which is an inhibitory hormone which inhibits G cells from producing gastrin by binding to a somatostatin receptor

Question 23

How is HCL formed?

Answer 23

-water and carbon dioxide bind within the parietal cells to form carbonic acid (H2CO3)
-the carbonic acid dissociates into hydrogen ions and bicarbonate ions
This is all catalysed by carbonic anhydrase in cytosol
-bicarbonate ion exits basolateral surface of parietal cell to venous blood causing a raise in pH ofvenous blood leaving the stomach: ALKALINE TIDE, the bicarbonate ions are exchange with Cl- into the cell
-hydrogen ions are exiting the apical surface via H/K ATPase , but there is also a K+ channel so K+ can leave cell again
-there is a Cl- channel in apical membrane which allows the Cl- out of the parietal cell
-H+ and Cl- then react to form HCL