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GIT Physiology > Gastric Secretions > Flashcards

Gastric Secretions Flashcards

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1
Q

What are gastric secretions?

A
  • fluid secreted in the stomach
  • Mixture of secretions of the surface epithelial cells and gastric glands
  • gastric secretions increase after a meal
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2
Q

What are gastric secretions made up of?

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

What is the pH of the gastric contents?

A

ph = 2

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4
Q

Name the cells in the mucosa?

A
  1. chief
  2. parietal
  3. mucous neck
  4. G cells
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5
Q

Chief cells secrete?

A
  1. pepsinogen - pepsin (activated form) breaks down proteins into peptides
  2. gastric lipase - split tryglycerides into fatty acids and monoglycerides
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6
Q

Parietal cells secrete?

A
  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
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7
Q

Surface mucous cells + mucous neck cells secrete?

A

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

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8
Q

Surface mucous cells + mucous neck cells absorb?

A

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

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9
Q

How is HCl produced?

A
  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-
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10
Q

What catalyses the reaction of HCl production?

A

carbonic anhydrase

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11
Q

What are the functions of HCl?

A
  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
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12
Q

What are the functions of intrinsic factor?

A

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)

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13
Q

What is the function of mucous and bicarbonate?

A

protect the stomach from mechanical and chemical damage

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14
Q

What are the 3 phases of gastric secretions?

A
  1. cephalic phase 2. gastric phase 3. intestinal phase
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15
Q

What is the cephalic phase?

A

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

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16
Q

The cephalic phase is controlled by?

A

CNS - prepares the stomach to receive food

17
Q

How does the CNS control the cephalic phase?

A
  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
18
Q

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

A

500 ml/hr

19
Q

What could potentially slow digestion in the stomach?

A
  1. anger 2. fear 3. anxiety
20
Q

How long does the cephalic phase last for?

A

lasts for a few moments before gastric stage starts

21
Q

What stimuli initiates the gastric phase?

A
  1. distension of stomach
  2. an increase in ph of gastric contents
  3. presence of undigested materials in the stomach especially proteins and peptides
22
Q

Describe the functions of the gastric phase?

A
  1. enhance secretion started in cephalic phase
  2. homogenize and acidify chyme
  3. initiate digestion of proteins by pepsin
23
Q

How long does the gastric phase last for?

A

long : 3-4 hours

24
Q

What mechanisms does the gastric response consist of?

A
  1. neuronal 2. hormonal 3. local
25
Q

Describe the neural mechanism of the gastric phase?

A

short reflexes triggered by :

  1. stimulation of stretch receptors as stomach fills
  2. stimulation of chemoreceptors as pH increases
26
Q

Describe the hormonal mechanism of the gastric phase?

A

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

27
Q

Describe the local mechanism?

A

release of histamine by mast cells as stomach fills

28
Q

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

A
  1. increased acid and pepsinogen production

2. increased motility and initiation of mixing waves

29
Q

When does the intestinal phase start?

A

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

30
Q

What is the function of the intestinal phase?

A
  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
31
Q

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

A

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

32
Q

How long does the intestinal phase last for?

A

long - hours

33
Q

Describe the neural mechansim?

A

short reflexes (enterogastric reflex) triggered by distension of duodenum

34
Q

Describe the hormonal mechansims?

A
  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
35
Q

What is the action of the intestinal phase?

A
  1. feedback inhibition of gastric acid and pepsinogen production
  2. reduction of gastric motility
36
Q

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

A
  1. CNS
  2. short reflexes coordinated in the stomach walls
  3. digestive tract hormones hormones - local factors
37
Q

Describe the neural response?

A
  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
38
Q

Describe the hormonal response?

A
  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
39
Q

Describe the local response?

A
  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

GIT Physiology (5 decks)

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