Gastric Secretions

Question 1

What are gastric secretions?

Answer 1

• fluid secreted in the stomach
• Mixture of secretions of the surface epithelial cells and gastric glands
• gastric secretions increase after a meal

Question 2

What are gastric secretions made up of?

Answer 2

1. HCl
2. pepsins
3. intrinsic factor
4. mucus
5. bicarbonate

Question 3

What is the pH of the gastric contents?

Answer 3

ph = 2

Question 4

Name the cells in the mucosa?

Answer 4

1. chief
2. parietal
3. mucous neck
4. G cells

Question 5

Chief cells secrete?

Answer 5

1. pepsinogen - pepsin (activated form) breaks down proteins into peptides
2. gastric lipase - split tryglycerides into fatty acids and monoglycerides

Question 6

Parietal cells secrete?

Answer 6

1. hydrochloric acid
   - kills microbes in food
   - denature proteins
   - convert pepsinogen into pepsin
2. intrinsic factor
   - needed for absorption of vitamin B12 which is used in red blood cell formation

Question 7

Surface mucous cells + mucous neck cells secrete?

Answer 7

mucous - form a protective barrier that prevents digestion of stomach wall

Question 8

Surface mucous cells + mucous neck cells absorb?

Answer 8

small quantity of water, ions, short chain fatty acids and some drugs enter the bloodstream

Question 9

How is HCl produced?

Answer 9

1. H+ and Cl– secreted into gastric lumen separately by the parietal cells
2. Proton pumps powered by H+/K+ATPase actively transport H+ into lumen in exchange for K+
   - The H+/K+ATPase pumps mediate primary active transport
3. Cl-/HCO3- antiporters move out HCO3- (bicarbonate) and take in Cl-

Question 10

What catalyses the reaction of HCl production?

Answer 10

carbonic anhydrase

Question 11

What are the functions of HCl?

Answer 11

1. HCl catalyses the cleavage of inactive pepsinogens to active pepsins
2. kills bacteria
3. provides a low pH environment which is required for the actions of pepsins in digesting proteins and peptides

Question 12

What are the functions of intrinsic factor?

Answer 12

Intrinsic factor binds vitamin B12 and allows it to be absorbed in the ileum (vitamin B12 deficiency causes anaemia and can lead to permanent nervous tissue damage)

Question 13

What is the function of mucous and bicarbonate?

Answer 13

protect the stomach from mechanical and chemical damage

Question 14

What are the 3 phases of gastric secretions?

Answer 14

1. cephalic phase 2. gastric phase 3. intestinal phase

Question 15

What is the cephalic phase?

Answer 15

occurs before food has entered the stomach, especially while it is being eaten - it results from sight, smell, thought of and taste of food : the greater the appetite the more intense the stimulation

Question 16

The cephalic phase is controlled by?

Answer 16

CNS - prepares the stomach to receive food

Question 17

How does the CNS control the cephalic phase?

Answer 17

1. the cerebral cortex and feeding centres in the hypothalamus send nerve impulses to the medulla oblongata
2. From the medulla, impulses are transmitted to the stomach through the parasympathetic division of ANS via the vagus nerves, synapsing on the submucosal plexus
3. Postganglionic fibres innervate mucous cells, parietal cells, chief cells and G cells of the stomach
4. These impulses stimulate gastric glands to secrete pepsinogen, hydrochloric acid and mucus into the stomach chyme, and gastrin into blood

Question 18

What is the rate of production of gastric juice in response to this CNS stimulation?

Answer 18

500 ml/hr

Question 19

What could potentially slow digestion in the stomach?

Answer 19

1. anger 2. fear 3. anxiety

Question 20

How long does the cephalic phase last for?

Answer 20

lasts for a few moments before gastric stage starts

Question 21

What stimuli initiates the gastric phase?

Answer 21

1. distension of stomach
2. an increase in ph of gastric contents
3. presence of undigested materials in the stomach especially proteins and peptides

Question 22

Describe the functions of the gastric phase?

Answer 22

1. enhance secretion started in cephalic phase
2. homogenize and acidify chyme
3. initiate digestion of proteins by pepsin

Question 23

How long does the gastric phase last for?

Answer 23

long : 3-4 hours

Question 24

What mechanisms does the gastric response consist of?

Answer 24

1. neuronal 2. hormonal 3. local

Question 25

Describe the neural mechanism of the gastric phase?

Answer 25

short reflexes triggered by : 1. stimulation of stretch receptors as stomach fills 2. stimulation of chemoreceptors as pH increases

Question 26

Describe the hormonal mechanism of the gastric phase?

Answer 26

stimulation of gastrin release by G cells by parasympathetic activity and presence of peptides and amino acids in chyme

Question 27

Describe the local mechanism?

Answer 27

release of histamine by mast cells as stomach fills

Question 28

Describe the actions seen as a consequence of the gastric phase?

Answer 28

1. increased acid and pepsinogen production | 2. increased motility and initiation of mixing waves

Question 29

When does the intestinal phase start?

Answer 29

when chyme starts to enter the small intestine due to activation of receptors in the small intestine

Question 30

What is the function of the intestinal phase?

Answer 30

1. to control the rate of gastric emptying and ensure that secretory, digestive and absorptive functions of the small intestine can proceed efficiently 2. inhibits secretory and motility activities of the stomach 3. Allows slow exit of chyme from the stomach, and prevent overloading of the duodenum 4. Promotes continued digestion of foods that have reached the small intestine

Question 31

What happens when chyme containing fatty acids and glucose enter the SI?

Answer 31

stimulates enteroendocrine cells in the SI mucosa to release into blood two hormones that affect the stomach - secretin and cholecystokinin (CCK)

Question 32

How long does the intestinal phase last for?

Answer 32

long - hours

Question 33

Describe the neural mechansim?

Answer 33

short reflexes (enterogastric reflex) triggered by distension of duodenum

Question 34

Describe the hormonal mechansims?

Answer 34

1. primary - stimulation of CCK, gastric inhibitory peptide and secretin release by presence of acid, carbohydrates and lipids 2. secondary - release of gastrin stimulated by presence of undigested peptides

Question 35

What is the action of the intestinal phase?

Answer 35

1. feedback inhibition of gastric acid and pepsinogen production 2. reduction of gastric motility

Question 36

Production of acid and enzymes by gastric mucosa can be controlled by?

Answer 36

1. CNS 2. short reflexes coordinated in the stomach walls 3. digestive tract hormones hormones - local factors

Question 37

Describe the neural response?

Answer 37

1. Stimulation of stretch receptors in the stomach wall and chemoreceptors in the mucosa triggers short reflexes coordinated in the submucosal and myenteric plexuses 2. Postganglionic fibres leaving the submucosal plexuses innervate parietal cells and chief cells and the release of acetylcholine stimulates their secretion

Question 38

Describe the hormonal response?

Answer 38

1. Neural stimulation and presence of peptides and amino acids in the chyme stimulates secretion of gastrin by the G cells of the pyloric antrum 2. Parietal and Chief cells respond to gastrin by accelerating their secretion rates 3. Gastrin also stimulates gastric motility

Question 39

Describe the local response?

Answer 39

1. Distortion of the gastric wall also stimulates release of histamine in the lamina propria 2. Histamine binds to receptors on the parietal cells and stimulates acid secretion 3. Acetylcholine and gastrin work better in the presence of histamine