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
2
Q
What does the serosa consist of?
A
Epithelial layer
Thin layer of connective tissue
3
Q
What is the function of the epithelial layer of the serosa?
A
Secretes serous fluid (also known as mesothelium)
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
5
Q
In the retroperitoneal areas of the GI tract, what is the serial layer replaced by?
A
Adventitia (connective tissue layer)
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.
7
Q
How many muscle layers are there in the muscularis externa?
A
2
8
Q
What are the 2 muscle layers in the muscularis externa?
A
Inner layer
Outer layer
9
Q
Describe the muscle in the inner layer of the muscularis externa.
A
Circular ring arrangement around the tract
10
Q
Describe the muscle in the outer layer of the muscularis externa.
A
Longitudinal arrangement.
11
Q
What does the circular ring, inner muscle layer of the gut wall prevent?
A
Prevents food from travelling backwards
12
Q
In which part of the GI tract is the muscularis externa thicker?
A
Colon
13
Q
Which extra muscle layer does the stomach have?
A
Inner oblique muscular layer
14
Q
What does the inner oblique muscular layer of the stomach do?
A
Helps to churn chyme in the stomach
15
Q
Which muscular layer forms the pyloric sphincter?
A
Inner circular layer
16
Q
What nerve plexus is responsible for the control of peristalsis in the GI tract?
A
Myenteric plexus (Auerbach’s plexus)
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.
18
Q
What is the submucosa?
A
A dense, irregular layer of connective tissue with large blood vessels, lymphatics, glands and nerves.
19
Q
What does the submucosa do?
A
Supports the mucosa beneath it.
20
Q
Which nerve plexus is contained within the submucosa?
A
Submucosal plexus (Meissner’s plexus)
21
Q
What is the submucosal plexus the main control for?
A
GI secretion and local blood flow
22
Q
What cells is the mucosa of the gut wall formed by?
A
Simple epithelium cells
23
Q
What does the mucosa come into direct contact with?
A
Chyme in the stomach
24
Q
What is the absorptive and secretory layer of the GI tract?
A
Mucosa
25
What are the 3 layers of the mucosa?
Epithelium
Lamina propria
Muscularis mucosa
26
What is the epithelial layer in the mucosa responsible for?
Digestive, absorptive and secretory processes.
27
What is the lamina propria in the mucosa?
A thin layer of loose connective tissue
28
What is the muscularis mucosa?
Thin layer of smooth muscle
29
What type of epithelium is present in the oesophagus?
Stratified squamous (non-keratinising)
**For protective purposes
30
What type of epithelium is present in the stomach?
Simple columnar organised into pits and glands
**To deal with secretion
31
What type of epithelium is present in the small intestine?
Villi + microvilli to increase surface area
**Specialised for absorption
32
What are 3 functions of saliva?
Maintains pH of mouth
Contributes to digestion of food
Maintains oral hygiene
33
Which receptors give the sensation of taste?
Gustatory receptors
34
Saliva begins early digestion of what?
Polysaccharides
35
What are the 6 main constituents of saliva?
Water
Electrolytes
Bicarbonate ions
Antimicrobial agents
Mucous
Enzymes
36
What percentage of saliva is made up of water?
99%
37
Which electrolytes are present in saliva?
Na + Cl (lower concentration than plasma)
K + I + Ca (higher concentrations than plasma)
38
What constituent of saliva helps to maintain an alkaline environment?
HCO3- ions
39
Which immunoglobulin is present in saliva?
IgA
40
What are the antimicrobial constituents of saliva?
Lysozymes
Lactoferrin
IgA
Salivary lactoperoxidase
41
What are the 3 main enzymes that are present in saliva?
Amylase
Lingual lipase
Kallikrein
42
Which cells secrete amylase?
Acinar cells of the parotid and submandibular glands
43
What does amylase do in the mouth?
Starts the digestion of starch
44
What does lingual lipase do?
Catalyses the first reaction in the digestion of dietary lipids.
45
What is kvllikrein?
Serine protease that converts kininogens to bradykinin
46
What is bradykinin?
A vasodilator involved in mediation of the immune response.
47
Which 3 pairs of salivary gland secrete saliva during mastication?
Parotid glands
Submandibular glands
Sublingual glands
48
What percentage of total saliva do the parotid glands produce?
25%
49
Saliva from which glands contains IgA?
Parotid glands
50
What percentage of saliva is produced by the submandibular glands?
70%
51
What percentage of saliva is produced by the sublingual glands?
5%
52
The secretion of saliva is controlled by what?
Autonomic nervous system.
53
The salivary secretion reflex is stimulated by what?
Salivary nuclei in the medulla
54
Which 3 ways can secretion of saliva be stimulated?
Mechanoreceptors in the mouth
Chemoreceptors in the mouth
Higher CNS centres (e.g. via smell of food)
55
What initiates swallowing?
Stimulation of tactile receptors on the faces, tonsils, soft palate, base of tongue and posterior pharyngeal wall.
56
Which cranial nerves transmit sensory impulses to the swallowing centre in the brainstem?
VII, IX, X
57
Where are the swallowing centres?
Brainstem
58
Which cranial nerves transmit efferent (motor( fibres?
IX, X, XII
59
Which phase of swallowing is voluntary?
Oral phase
60
What are the adaptations of the mucosa and submucosa of the small intestine?
Circular folds
Villi
Microvilli
61
In which part of the small intestine does the majority of absorption occur?
Proximal two thirds
62
What are the circular folds in the wall of the small intestine called?
Plica circulare
63
What are the villi in the small intestine covered by?
Mucosa (absorptive surface)
64
What structures form the brush border in the small intestine?
The microvilli
65
What structures are found between each villus in the small intestine?
A deep crevice leading to a crypt of lieberkühn (gland)
66
What is the function of the crypts of lieberkühn?
Secrete alkaline intestinal juice (pH 7.4-7.8)
67
What causes secretion of the alkaline intestinal juice from the crypts of lieberkühn in the small intestine?
Distension of small intestine
Irritating effects of chyme on intestinal mucosa
68
What do Brunner's glands produce?
A bicarbonate rich alkaline mucous (neutralises acidic chyme).
69
Where are Brunner's glands found?
Duodenum
70
What are Peyer's patches?
Small masses of lymphatic tissue
71
Where are Peyer's patches found?
Ileum
72
What do Peyer's patches function as?
Immune surveillance system of intestinal lumen - facilitate generation of immune response within mucosa.
73
What do I cells secrete?
Cholecystokinin (CCK)
74
Which cells secrete cholecystokinin?
I cells
75
What do S cells secrete?
Secretin
76
Which cells secrete secretin?
S cells
77
What do K cells secrete?
Gastric inhibitory peptide (GIP)
78
Which cells secrete gastric inhibitory peptide (GIP)?
K cells
79
What do enterochromaffin cells secrete?
Serotonin
80
Which ells secrete serotonin?
Enterochromaffin cells
81
What is cholecystokinin secreted in response to?
Presence of fat in small intestine
82
What causes secretin to be secreted?
Low pH of the chyme
83
What causes gastric inhibitory peptide to be secreted?
Chyme entering small intestine
84
What causes serotonin to be realised in the small intestine?
Mechanical stimulation
85
Where are CCK cells concentrated?
Proximal small intestine
86
What is the most potent stimulant for production of CCK?
Presence of partially digested fats and proteins in duodenum
87
What are the 4 functions of CCK?
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
What does CCK do to gastric emptying?
Inhibits it
89
What causes secretin to be released?
Acidity in duodenum
Presence of fatty acids
90
What is the main function of secretin?
Neutralisation of acidic chyme
91
Name 4 functions of secretin.
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
Digestive enzymes from the pancreas enter the small intestine in response to what?
Cholecycstokinin
93
How is sodium absorbed by enterocytes in the small intestine?
Co-transport with organic nutrients
AND
By exchange with protons
94
How is a low intracellular volume of sodium maintained in enterocytes?
Large number of Na+/K+ ATPases in basolateral membrane
95
How many of each ion do Na+/K+ ATPases transport?
3 Na
2 K
96
Which enzymes cleave polypeptides into oligopeptides?
Trypsin
Chymotrypsin
97
What breaks oligopeptides down into amino acids?
Carboxypeptidase (from brush border)
98
How do amino acids enter cells in the small intestine?
Enter epithelial cells by secondary active transport coupled to sodium ions
AND
through different co-transporter mechanisms
99
What is the main enzyme involved in lipid digestion?
Pancreatic lipase
100
What does pancreatic lipase do?
Breaks down triglycerides into free fatty acids and monoglycerides
101
What is formed when free fatty acids or monoglycerides join with bile salts?
Micelles
102
Why are micelles formed?
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
Which are the fat soluble vitamins?
A, D, E, K
104
How are fat soluble vitamins absorbed?
Along with free fatty acids and monoglycerides.
105
How are water soluble vitamins absorbed?
Diffusion or mediated transport (except B12 - intrinsic factor)
106
How are monosaccharides transported across the epithelium in the small intestine?
Co-transporter molecules that link monosaccharides with sodium ions
107
How is calcium absorbed in the small intestine?
Via a concentration gradient or via a carrier mechanism
108
How is calcium transported from the enterocytes into the bloodstream?
Ca2+ ATPase
OR
Na+/Ca2+ antiporter
109
Why can't the majority of dietary iron be absorbed?
Because it is in the ferris (Fe3+) form
110
Which form of dietary iron can be absorbed?
Fe2+ (ferrous form)
111
How is Fe2+ (ferrous iron) absorbed in the small intestine?
As an Fe2+-ascorbate complex with a carrier protein
112
Where in the small intestine is iron absorbed?
Duodenum
113
Where is B12 absorbed?
Terminal ileum
114
What does B12 need to bind to in order to be absorbed?
Intrinsic factor
115
Where is intrinsic factor secreted from?
Parietal cells in stomach wall
116
What volume of chyme enters the large intestine per day?
1.5L
117
What are the 3 primary functions of the large intestine?
Absorbing water and electrolytes
Producing and absorbing vitamins
Forming and propelling faeces towards the rectum
118
What is the primary function of the ileocaecal valve?
To limit the reflux of contents into the ileum
119
What results in the opening of the ileocaecal valve?
Distension of the terminal ileum
120
What causes the ileocaecal valve to close?
Distension of the caecum
121
What are the teniae coli?
Longitudinal ribbons of muscle
122
Where are the teniae coli found?
Just below the serosa
123
What happens when the teniae coli contract length wise?
They produce the haustra
124
What is the parasympathetic innervation of the caecum and ascending colon?
Vagus nerve
125
What is the parasympathetic innervation of the descending and sigmoid colon, rectum and anal canal?
Branches of pelvic nerves from the sacral spinal cord.
126
What is the sympathetic innervation of the proximal and vital parts of the large intestine?
Superior mesenteric plexus
Inferior mesenteric plexus
Superior hypogastric plexus
127
What is the sympathetic innervation of the rectum and anal canal?
Inferior hypogastric plexus
128
Parasympathetic innervation of the colon causes what?
Segmental contraction
129
Sympathetic innervation of the colon causes what?
Stoppage of colonic activity
130
Where are the majority of bacteria in the GI tract found?
Large intestine
131
What are the roles of the bacteria in the large intestine?
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
What is the internal anal sphincter innervated by?
Parasympathetic neutrons of the. myenteric plexus (Auerbach's plexus)
133
What is the external anal sphincter innervated by?
Somatic efferent neurons
134
What are the 4 main functions of the stomach?
Food storage
Food digestion
Chyme regulation
Intrinsic factor secretion
135
What 2 substances do parietal cells secrete?
Intrinsic factor
Hydrochloric acid
136
What are 3 stimuli that cause parietal cells to secrete hydrochloric acid?
Histamine on H2 receptors (biggest stimulus)
ACh on M3 receptors
Gastrin in CCK2 receptors
137
What do gastric chief cells secrete?
Pepsinogen
Gastric lipase
138
What converts pepsinogen to pepsin?
Gastric (hydrochloric) acid
139
What 3 factors stimulate chief cells to secrete pepsinogen/ gastric lipase?
ACh (parasympathetic activity)
Decrease in pH
Secretin
140
What does pepsin do?
Breaks down proteins into smaller peptides
141
What is the function of gastric lipase?
Digestion of fats in the stomach
142
Where are gastric G cells located?
Antrum of stomach
Duodenum
Pancreas
143
What do G cells secrete?
Gastrin
144
Which 4 factors stimulate gastrin release from G cells?
Vagal stimulation
Hypercalcaemia
Stomach distension
Presence of partial digested proteins
145
Which factors inhibit gastrin release?
Somatostatin
Presence of acid (low pH)
146
What do enterochromaffin cells release?
Histamine
147
What do gastric mucous-neck cells secrete?
Mucous and bicarbonate
148
What do D cells produce?
Somatostatin
149
Which cells produce somatostatin?
D cells
150
Where are gastric D cells found?
Gastric antrum
Duodenum
Pancreatic islets
151
What does somatostatin do to parietal cells?
Inhibits gastric acid secretion
152
Secretion of which hormones are inhibited by somatostatin?
Growth hormone
TSH
Gastrin
CCK
Secretin
Motilin
VIP
GIP
Insulin
Glucagon
153
What does somatostatin do to gastric emptying?
Reduces rate of gastric emptying
154
What volume of gastric acid is secreted by the stomach per day?
2-3L per day
155
What is the pH of gastric acid?
1.5 - 3.5L
156
What are the components of gastric acid?
Water
HCl
Pepsinogen
Mucous
Intrinsic factor
157
How are hydrogen ions generated within parietal cells?
Dissociation of water
158
Hydroxyl ions formed by the dissociation of water combine with what to form bicarbonate ions?
CO2
159
What catalyses the reaction that joins a hydroxyl group with CO2 to produce a bicarbonate ion?
Carbonic annhydrase
160
Bicarbonate is transported out of the base lateral membrane of parietal cells in exchange for what?
Chloride ions
161
What are the 3 phases of gastric secretions?
Cephalic phase
Gastric phase
Intestinal phase
162
What is the cephalic phase of gastric secretions triggered by?
Sight, smell and taste of food
163
What is the gastric phase of secretion initiated by?
Entry of food into the stomach
164
What are the main stimulants of the gastric phase of secretion?
Stomach distension
Chemical composition of food
165
When does the intestinal phase of secretion begin?
When partially digested food enters the duodenum
166
Which 5 factors promote gastric emptying?
Increased food volume in stomach
Gastric
Motilin
Parasympathetic innervation (vagus nerve)
Prokinetics (metoclopramide/ erythromycin)
167
Name the factors which inhibit gastric emptying.
Duodenal distension
CCK
Secretin
Somatostatin
VIP
GIP
Sympathetic innervation (coeliac plexus)
Pregnancy (increased progesterone levels)
