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MRCEM Primary > GI tract physiology > Flashcards

GI tract physiology Flashcards

1
Q

What are the 4 layers of the gut wall from external to internal?

A

Serosa
Muscular layer (muscularis externa)
Submucosa
Mucosa

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2
Q

What does the serosa consist of?

A

Epithelial layer
Thin layer of connective tissue

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3
Q

What is the function of the epithelial layer of the serosa?

A

Secretes serous fluid (also known as mesothelium)

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4
Q

What is the function of the thin layer of connective tissue in the serosa?

A

Reduces friction during digestion
Supplies food vessels and nerves to epithelial layer
Binding layer to allow serous membrane to adhere to organs + structures

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5
Q

In the retroperitoneal areas of the GI tract, what is the serial layer replaced by?

A

Adventitia (connective tissue layer)

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6
Q

What is the function of the muscularis external of the gut wall?

A

Produces segmental contractions and peristaltic movements that churns food + digestive enzymes together.

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7
Q

How many muscle layers are there in the muscularis externa?

A

2

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8
Q

What are the 2 muscle layers in the muscularis externa?

A

Inner layer
Outer layer

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9
Q

Describe the muscle in the inner layer of the muscularis externa.

A

Circular ring arrangement around the tract

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10
Q

Describe the muscle in the outer layer of the muscularis externa.

A

Longitudinal arrangement.

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11
Q

What does the circular ring, inner muscle layer of the gut wall prevent?

A

Prevents food from travelling backwards

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12
Q

In which part of the GI tract is the muscularis externa thicker?

A

Colon

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13
Q

Which extra muscle layer does the stomach have?

A

Inner oblique muscular layer

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14
Q

What does the inner oblique muscular layer of the stomach do?

A

Helps to churn chyme in the stomach

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15
Q

Which muscular layer forms the pyloric sphincter?

A

Inner circular layer

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16
Q

What nerve plexus is responsible for the control of peristalsis in the GI tract?

A

Myenteric plexus (Auerbach’s plexus)

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17
Q

What does stimulation of the myenteric plexus (Auerbach’s plexus) cause?

A

Increase in gut wall tone and increase in intensity of rhythmical contractions.

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18
Q

What is the submucosa?

A

A dense, irregular layer of connective tissue with large blood vessels, lymphatics, glands and nerves.

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19
Q

What does the submucosa do?

A

Supports the mucosa beneath it.

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20
Q

Which nerve plexus is contained within the submucosa?

A

Submucosal plexus (Meissner’s plexus)

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21
Q

What is the submucosal plexus the main control for?

A

GI secretion and local blood flow

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22
Q

What cells is the mucosa of the gut wall formed by?

A

Simple epithelium cells

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23
Q

What does the mucosa come into direct contact with?

A

Chyme in the stomach

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24
Q

What is the absorptive and secretory layer of the GI tract?

A

Mucosa

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25
Q

What are the 3 layers of the mucosa?

A

Epithelium
Lamina propria
Muscularis mucosa

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26
Q

What is the epithelial layer in the mucosa responsible for?

A

Digestive, absorptive and secretory processes.

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27
Q

What is the lamina propria in the mucosa?

A

A thin layer of loose connective tissue

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28
Q

What is the muscularis mucosa?

A

Thin layer of smooth muscle

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29
Q

What type of epithelium is present in the oesophagus?

A

Stratified squamous (non-keratinising)

**For protective purposes

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30
Q

What type of epithelium is present in the stomach?

A

Simple columnar organised into pits and glands

**To deal with secretion

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31
Q

What type of epithelium is present in the small intestine?

A

Villi + microvilli to increase surface area

**Specialised for absorption

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32
Q

What are 3 functions of saliva?

A

Maintains pH of mouth

Contributes to digestion of food

Maintains oral hygiene

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33
Q

Which receptors give the sensation of taste?

A

Gustatory receptors

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34
Q

Saliva begins early digestion of what?

A

Polysaccharides

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35
Q

What are the 6 main constituents of saliva?

A

Water
Electrolytes
Bicarbonate ions
Antimicrobial agents
Mucous
Enzymes

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36
Q

What percentage of saliva is made up of water?

A

99%

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37
Q

Which electrolytes are present in saliva?

A

Na + Cl (lower concentration than plasma)

K + I + Ca (higher concentrations than plasma)

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38
Q

What constituent of saliva helps to maintain an alkaline environment?

A

HCO3- ions

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39
Q

Which immunoglobulin is present in saliva?

A

IgA

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40
Q

What are the antimicrobial constituents of saliva?

A

Lysozymes
Lactoferrin
IgA
Salivary lactoperoxidase

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41
Q

What are the 3 main enzymes that are present in saliva?

A

Amylase
Lingual lipase
Kallikrein

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42
Q

Which cells secrete amylase?

A

Acinar cells of the parotid and submandibular glands

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43
Q

What does amylase do in the mouth?

A

Starts the digestion of starch

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44
Q

What does lingual lipase do?

A

Catalyses the first reaction in the digestion of dietary lipids.

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45
Q

What is kvllikrein?

A

Serine protease that converts kininogens to bradykinin

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46
Q

What is bradykinin?

A

A vasodilator involved in mediation of the immune response.

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47
Q

Which 3 pairs of salivary gland secrete saliva during mastication?

A

Parotid glands
Submandibular glands
Sublingual glands

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48
Q

What percentage of total saliva do the parotid glands produce?

A

25%

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49
Q

Saliva from which glands contains IgA?

A

Parotid glands

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50
Q

What percentage of saliva is produced by the submandibular glands?

A

70%

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51
Q

What percentage of saliva is produced by the sublingual glands?

A

5%

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52
Q

The secretion of saliva is controlled by what?

A

Autonomic nervous system.

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53
Q

The salivary secretion reflex is stimulated by what?

A

Salivary nuclei in the medulla

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54
Q

Which 3 ways can secretion of saliva be stimulated?

A

Mechanoreceptors in the mouth
Chemoreceptors in the mouth
Higher CNS centres (e.g. via smell of food)

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55
Q

What initiates swallowing?

A

Stimulation of tactile receptors on the faces, tonsils, soft palate, base of tongue and posterior pharyngeal wall.

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56
Q

Which cranial nerves transmit sensory impulses to the swallowing centre in the brainstem?

A

VII, IX, X

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57
Q

Where are the swallowing centres?

A

Brainstem

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58
Q

Which cranial nerves transmit efferent (motor( fibres?

A

IX, X, XII

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59
Q

Which phase of swallowing is voluntary?

A

Oral phase

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60
Q

What are the adaptations of the mucosa and submucosa of the small intestine?

A

Circular folds
Villi
Microvilli

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61
Q

In which part of the small intestine does the majority of absorption occur?

A

Proximal two thirds

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62
Q

What are the circular folds in the wall of the small intestine called?

A

Plica circulare

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63
Q

What are the villi in the small intestine covered by?

A

Mucosa (absorptive surface)

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64
Q

What structures form the brush border in the small intestine?

A

The microvilli

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65
Q

What structures are found between each villus in the small intestine?

A

A deep crevice leading to a crypt of lieberkühn (gland)

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66
Q

What is the function of the crypts of lieberkühn?

A

Secrete alkaline intestinal juice (pH 7.4-7.8)

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67
Q

What causes secretion of the alkaline intestinal juice from the crypts of lieberkühn in the small intestine?

A

Distension of small intestine
Irritating effects of chyme on intestinal mucosa

68
Q

What do Brunner’s glands produce?

A

A bicarbonate rich alkaline mucous (neutralises acidic chyme).

69
Q

Where are Brunner’s glands found?

A

Duodenum

70
Q

What are Peyer’s patches?

A

Small masses of lymphatic tissue

71
Q

Where are Peyer’s patches found?

A

Ileum

72
Q

What do Peyer’s patches function as?

A

Immune surveillance system of intestinal lumen - facilitate generation of immune response within mucosa.

73
Q

What do I cells secrete?

A

Cholecystokinin (CCK)

74
Q

Which cells secrete cholecystokinin?

A

I cells

75
Q

What do S cells secrete?

A

Secretin

76
Q

Which cells secrete secretin?

A

S cells

77
Q

What do K cells secrete?

A

Gastric inhibitory peptide (GIP)

78
Q

Which cells secrete gastric inhibitory peptide (GIP)?

A

K cells

79
Q

What do enterochromaffin cells secrete?

A

Serotonin

80
Q

Which ells secrete serotonin?

A

Enterochromaffin cells

81
Q

What is cholecystokinin secreted in response to?

A

Presence of fat in small intestine

82
Q

What causes secretin to be secreted?

A

Low pH of the chyme

83
Q

What causes gastric inhibitory peptide to be secreted?

A

Chyme entering small intestine

84
Q

What causes serotonin to be realised in the small intestine?

A

Mechanical stimulation

85
Q

Where are CCK cells concentrated?

A

Proximal small intestine

86
Q

What is the most potent stimulant for production of CCK?

A

Presence of partially digested fats and proteins in duodenum

87
Q

What are the 4 functions of CCK?

A

Stimulates delivery of digestive enzymes from pancreas

Stimulates contraction of gallbladder and relaxation of sphincter of oddi = bile into duodenum

Inhbits gastric emptying

Decreases gastric acid secretion

Induces satiety

88
Q

What does CCK do to gastric emptying?

A

Inhibits it

89
Q

What causes secretin to be released?

A

Acidity in duodenum
Presence of fatty acids

90
Q

What is the main function of secretin?

A

Neutralisation of acidic chyme

91
Q

Name 4 functions of secretin.

A

Enhances effects of CCK

Stimulates insulin release from pancreas

Stimulates pepsinogen release from pancreas

Stimulates glucagon release

Stimulates pepsin release

Stimulates pancreatic polypeptide release

Stimulates somatostatin release

92
Q

Digestive enzymes from the pancreas enter the small intestine in response to what?

A

Cholecycstokinin

93
Q

How is sodium absorbed by enterocytes in the small intestine?

A

Co-transport with organic nutrients

AND

By exchange with protons

94
Q

How is a low intracellular volume of sodium maintained in enterocytes?

A

Large number of Na+/K+ ATPases in basolateral membrane

95
Q

How many of each ion do Na+/K+ ATPases transport?

A

3 Na
2 K

96
Q

Which enzymes cleave polypeptides into oligopeptides?

A

Trypsin
Chymotrypsin

97
Q

What breaks oligopeptides down into amino acids?

A

Carboxypeptidase (from brush border)

98
Q

How do amino acids enter cells in the small intestine?

A

Enter epithelial cells by secondary active transport coupled to sodium ions

AND

through different co-transporter mechanisms

99
Q

What is the main enzyme involved in lipid digestion?

A

Pancreatic lipase

100
Q

What does pancreatic lipase do?

A

Breaks down triglycerides into free fatty acids and monoglycerides

101
Q

What is formed when free fatty acids or monoglycerides join with bile salts?

A

Micelles

102
Q

Why are micelles formed?

A

Because the hydrophilic outer layer allows micelles to enter aqueous layers surrounding the microvilli.

They can then diffuse passively into the small intestinal cells.

103
Q

Which are the fat soluble vitamins?

A

A, D, E, K

104
Q

How are fat soluble vitamins absorbed?

A

Along with free fatty acids and monoglycerides.

105
Q

How are water soluble vitamins absorbed?

A

Diffusion or mediated transport (except B12 - intrinsic factor)

106
Q

How are monosaccharides transported across the epithelium in the small intestine?

A

Co-transporter molecules that link monosaccharides with sodium ions

107
Q

How is calcium absorbed in the small intestine?

A

Via a concentration gradient or via a carrier mechanism

108
Q

How is calcium transported from the enterocytes into the bloodstream?

A

Ca2+ ATPase

OR

Na+/Ca2+ antiporter

109
Q

Why can’t the majority of dietary iron be absorbed?

A

Because it is in the ferris (Fe3+) form

110
Q

Which form of dietary iron can be absorbed?

A

Fe2+ (ferrous form)

111
Q

How is Fe2+ (ferrous iron) absorbed in the small intestine?

A

As an Fe2+-ascorbate complex with a carrier protein

112
Q

Where in the small intestine is iron absorbed?

A

Duodenum

113
Q

Where is B12 absorbed?

A

Terminal ileum

114
Q

What does B12 need to bind to in order to be absorbed?

A

Intrinsic factor

115
Q

Where is intrinsic factor secreted from?

A

Parietal cells in stomach wall

116
Q

What volume of chyme enters the large intestine per day?

A

1.5L

117
Q

What are the 3 primary functions of the large intestine?

A

Absorbing water and electrolytes
Producing and absorbing vitamins
Forming and propelling faeces towards the rectum

118
Q

What is the primary function of the ileocaecal valve?

A

To limit the reflux of contents into the ileum

119
Q

What results in the opening of the ileocaecal valve?

A

Distension of the terminal ileum

120
Q

What causes the ileocaecal valve to close?

A

Distension of the caecum

121
Q

What are the teniae coli?

A

Longitudinal ribbons of muscle

122
Q

Where are the teniae coli found?

A

Just below the serosa

123
Q

What happens when the teniae coli contract length wise?

A

They produce the haustra

124
Q

What is the parasympathetic innervation of the caecum and ascending colon?

A

Vagus nerve

125
Q

What is the parasympathetic innervation of the descending and sigmoid colon, rectum and anal canal?

A

Branches of pelvic nerves from the sacral spinal cord.

126
Q

What is the sympathetic innervation of the proximal and vital parts of the large intestine?

A

Superior mesenteric plexus
Inferior mesenteric plexus
Superior hypogastric plexus

127
Q

What is the sympathetic innervation of the rectum and anal canal?

A

Inferior hypogastric plexus

128
Q

Parasympathetic innervation of the colon causes what?

A

Segmental contraction

129
Q

Sympathetic innervation of the colon causes what?

A

Stoppage of colonic activity

130
Q

Where are the majority of bacteria in the GI tract found?

A

Large intestine

131
Q

What are the roles of the bacteria in the large intestine?

A

Synthesis of B6 and B12
Synthesis of thiamine and riboflavin
Breakdown fo primary bile acids to secondary bile acids
Conversion of bilirubin to non-pigmented metabolites
Break down of cholesterol
Break down of some food additives and drugs

132
Q

What is the internal anal sphincter innervated by?

A

Parasympathetic neutrons of the. myenteric plexus (Auerbach’s plexus)

133
Q

What is the external anal sphincter innervated by?

A

Somatic efferent neurons

134
Q

What are the 4 main functions of the stomach?

A

Food storage
Food digestion
Chyme regulation
Intrinsic factor secretion

135
Q

What 2 substances do parietal cells secrete?

A

Intrinsic factor
Hydrochloric acid

136
Q

What are 3 stimuli that cause parietal cells to secrete hydrochloric acid?

A

Histamine on H2 receptors (biggest stimulus)
ACh on M3 receptors
Gastrin in CCK2 receptors

137
Q

What do gastric chief cells secrete?

A

Pepsinogen
Gastric lipase

138
Q

What converts pepsinogen to pepsin?

A

Gastric (hydrochloric) acid

139
Q

What 3 factors stimulate chief cells to secrete pepsinogen/ gastric lipase?

A

ACh (parasympathetic activity)
Decrease in pH
Secretin

140
Q

What does pepsin do?

A

Breaks down proteins into smaller peptides

141
Q

What is the function of gastric lipase?

A

Digestion of fats in the stomach

142
Q

Where are gastric G cells located?

A

Antrum of stomach
Duodenum
Pancreas

143
Q

What do G cells secrete?

A

Gastrin

144
Q

Which 4 factors stimulate gastrin release from G cells?

A

Vagal stimulation
Hypercalcaemia
Stomach distension
Presence of partial digested proteins

145
Q

Which factors inhibit gastrin release?

A

Somatostatin
Presence of acid (low pH)

146
Q

What do enterochromaffin cells release?

A

Histamine

147
Q

What do gastric mucous-neck cells secrete?

A

Mucous and bicarbonate

148
Q

What do D cells produce?

A

Somatostatin

149
Q

Which cells produce somatostatin?

A

D cells

150
Q

Where are gastric D cells found?

A

Gastric antrum
Duodenum
Pancreatic islets

151
Q

What does somatostatin do to parietal cells?

A

Inhibits gastric acid secretion

152
Q

Secretion of which hormones are inhibited by somatostatin?

A

Growth hormone
TSH
Gastrin
CCK
Secretin
Motilin
VIP
GIP
Insulin
Glucagon

153
Q

What does somatostatin do to gastric emptying?

A

Reduces rate of gastric emptying

154
Q

What volume of gastric acid is secreted by the stomach per day?

A

2-3L per day

155
Q

What is the pH of gastric acid?

A

1.5 - 3.5L

156
Q

What are the components of gastric acid?

A

Water
HCl
Pepsinogen
Mucous
Intrinsic factor

157
Q

How are hydrogen ions generated within parietal cells?

A

Dissociation of water

158
Q

Hydroxyl ions formed by the dissociation of water combine with what to form bicarbonate ions?

A

CO2

159
Q

What catalyses the reaction that joins a hydroxyl group with CO2 to produce a bicarbonate ion?

A

Carbonic annhydrase

160
Q

Bicarbonate is transported out of the base lateral membrane of parietal cells in exchange for what?

A

Chloride ions

161
Q

What are the 3 phases of gastric secretions?

A

Cephalic phase
Gastric phase
Intestinal phase

162
Q

What is the cephalic phase of gastric secretions triggered by?

A

Sight, smell and taste of food

163
Q

What is the gastric phase of secretion initiated by?

A

Entry of food into the stomach

164
Q

What are the main stimulants of the gastric phase of secretion?

A

Stomach distension
Chemical composition of food

165
Q

When does the intestinal phase of secretion begin?

A

When partially digested food enters the duodenum

166
Q

Which 5 factors promote gastric emptying?

A

Increased food volume in stomach
Gastric
Motilin
Parasympathetic innervation (vagus nerve)
Prokinetics (metoclopramide/ erythromycin)

167
Q

Name the factors which inhibit gastric emptying.

A

Duodenal distension
CCK
Secretin
Somatostatin
VIP
GIP
Sympathetic innervation (coeliac plexus)
Pregnancy (increased progesterone levels)

MRCEM Primary (39 decks)

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