GI Physiology Flashcards
1
Q
What’s the challenge of digestion?
A
Most food stuff molecule too large for absorption & transportation in blood stream
2
Q
Digestion
A
Breakdown of food into simpler molecules for absorption & transportation in blood stream
3
Q
What are the four basic processes of the digestive system?
A
- Motility
- Secretion
- Digestion
- Absorption
4
Q
What is the function of GI tract?
A
- Move materials
- Digestion of food
5
Q
What does absorbed nutrients do when they enter cells?
A
- Cellular metabolism
- Used for energy or stored
- Hormone play a role in gut function as well as cellular metabolism
6
Q
Challenges to GI tract?
A
- Protect from autodigestion - ulcers
- Dehydration from diarrhea
7
Q
Which mechanisms do the GI use to repel foreign invaders?
A
- Largest contact area
- Several defense mechanism
- Mucus, enzymes, acid, lymphoid tissue (GALT)
- 80% of immunoglobulin-producing cells-small intestine
8
Q
Why are there more immunoglobin in the small intestine and not stomach?
A
Because the stomach is already acid so it already kills many bacterias
9
Q
Parts of the GI
A
- Mouth, Pharynx and esophagus: mechanical breakdown of food
- Stomach: acidic compartment
- Upper or small intestines: digestion (starts in the stomach) and absorption
- Lower or large intestines: absorption of water
- Anus: release of indigestible material
10
Q
Major regions of the stomach
A
- Fundus: mostly storage predigestion
- Body: were all the cells produce mucus, acid and hormone (gastrin), it’s release into the bloodstream
- Antrum: all the semidigested by acid and enzymes is here, is released to the smallest intestine
11
Q
Functions of the stomach
A
- Reservoir - storage
- Partial Protein Digestion
- Disinfection
- Formation of CHYME = bolus + gastric juice
12
Q
How many sphincters are there at the stomach?
A
Two
13
Q
What does the small intestine does?
A
- Mostly absorption, it’s shorter because it has more surface to absorb
14
Q
Parts of the small intestine
A
- Duodenum
- Jejunum
- Ileum
15
Q
Parts of the large intestine
A
- Colon
- Rectum
16
Q
Function of the large intestine
A
- Watery chyme is converted into semisolic feces
- Distention of rectal walls
- Triggers defecation reflex
17
Q
What are the modification to increase lumen surface area (inside)
A
- Rugae in the stomach
- Plicae in the small intestine
- Microvilli in small intestine
18
Q
What are the four layers of the GI wall?
A
- Mucosa
- Submucosa
- Muscularis: smooth muscle
- Serosa: connective tissue
19
Q
Characteristics of the mucosa
A
- Epithelial cells: single layer
- Lamina propria (capillaries + vasculature)
- Muscularis mucosae (smooth muscle whose function is to open epithelial cells to increase surface area)
20
Q
What happens in the crypt?
A
There are the stem cells
21
Q
Which ones are the dominant cells in the intestine?
A
- Absorptive cells: move nutrients to ECF
- Endocrine cells
- Globet cells: secrete mucus
- Other secretory cells: mast cells that produce histamine
22
Q
What is the average life span of cells?
A
A few days
23
Q
How is the cell junction of the stomach?
A
Tight junctions
24
Q
How is the cells junction in the intestine?
A
Leaky
25
Parts of the mucosa?
- Epithelial cells
- Lamina propria
- Muscularis mucosa
26
How are the epithelial cells of the mucosa?
Single layer
27
How is the lamina propria of the mucosa?
- Has subepithelial connective tissue
- Contains nerve fibers
- Contains blood and lymphatic vessels
- Wandering immune cells
- Macrophages and lymphocytes
- Peyer's patches: collection of lymphoid tissue
28
How is the lamina propria of the mucosa?
- Separates mucosa from submucosa
- Contraction of this smooth muscle: alter effective surface area
29
What is under the mucosa?
Submucosa
30
What is submucosa?
A thick connective tissue layer
31
What does the submucosa contain?
- Nerves
- Glands
- Blood vessels
- Submucosal plexus
32
What is under the submucosa?
Muscularis
33
What is muscularis?
Two layers of smooth muscle
34
Layers of muscularis?
- Inner layer is circula- constriction/dilation
- Outer layer is longitudinal- shorten/lengthen (the stomach has a third layer: oblique)
- Myenteric plexus
35
What is under muscularis?
Serosa
36
What is the serosa layer?
The visceral peritoneum
37
How is it form the serosa layer?
- Connective tissue & simple squamous epithelium
- In some areas it is continuous with the mesenteries
38
What is motility?
- Moving food from mouth to anus
- Mechanical mixing: maximizes exposure to digestive enzymes and absorptive epithelium
- Smooth muscle: contracts spontaneously
- Single unit smooth muscle
- Connected by gap junctions
39
What are slow waves in GI smooth muscle?
Spontaneous slow waves of graded depolarization in the circular muscle of the muscularis externa.
40
Where do slow waves originate in the GI tract?
In the interstitial cells of Cajal (ICCs), a network of pacemaker cells.
41
What is the basic electrical rhythm (BER)?
The frequency of slow waves generated by ICCs, varies in different areas of the GI tract, propagation because of electrical coupling via gap junctions, affected by neural and hormonal input
42
Does the BER remain constant throughout the GI tract?
No, it varies in different areas
43
How do slow waves propagate in the GI tract?
Through electrical coupling via gap junctions
44
What factors affect the height of the BER?
Neural and hormonal input
45
How does the parasympathetic system influence GI smooth muscle activity?
It excites GI smooth muscle, increasing BER height
46
How does the sympathetic system influence GI smooth muscle activity?
It inhibits GI smooth muscle, decreasing BER height
47
How are the tonic contractions in motility?
- Sustained
- Occur in smooth muscle sphincters and stomach
- Keep bolus from moving backwards
48
How are the phasic contractions in motility?
- Last a few seconds
- Peristalsis moves bolus forward
- Segmentation mixes
49
What is the function of peristalsis in the GI tract?
It promotes forward movement of food
50
What type of contraction is peristalsis?
A progressive wave of contraction
51
What triggers the peristaltic reflex?
Distention of the intestinal wall
52
Which nervous system mediates peristalsis
The enteric nervous system
53
What factors influence peristalsis?
Hormones, paracrines, and the autonomic nervous system
54
What is the function of segmental contractions in the GI tract?
They promote mixing of contents and keep them in contact with absorptive epithelium
55
What type of contractions are segmental contractions?
Mixing contractions
56
What happens to the circular and longitudinal muscles during segmental contractions?
The circular muscle contracts while the longitudinal muscle relaxes
57
What occurs in the receiving segment of segmental contractions?
The opposite pattern: the circular muscle relaxes while the longitudinal muscle contracts
58
How must be the volume of fluid entering the GI tract compared to the volume leaving?
Must be equal
59
how many liters are absorbed in the small intestine?
7.5 L
60
What are the major ions involved in GI secretion?
Na⁺, K⁺, Cl⁻, HCO₃⁻, and H⁺
61
What is the purpose of zymogens in the GI tract?
They are inactive proenzymes that prevent autodigestion
62
Where are zymogens activated?
In the GI lumen
63
Why are zymogens stored in an inactive form within cells?
To prevent autodigestion and allow safe storage
64
Give an example of a zymogen and its activation site
Pepsinogen (activated in the stomach) and pancreatic enzymes
65
What can be secreted a the pancreas and duodenum?
Bicarbonate
66
What is secreted in the intestinal an colonic
Cl-
67
Path to secrete Cl-?
1. Na+, K+ and Cl- enter by cotransport
2. Cl- enters lumen through CFTR channel
3. Na+ is reabsorbed
4. Negative Cl- in lumen attracts Na+ by paracellular pathway. Water follows
68
What is secreted into mouth, stomach and intestine?
Digestive enzymes
69
What is secreted in stomach and globet cells in the intestine?
Mucous cells
70
What type of secretion is saliva?
Exocrine
71
What does the liver secretes?
Bile
72
Digestion
Enzymatic breakdown aided by mechanical breakdown, bile
73
By what is determined the location of enzymes?
pH
74
How much is absorbed in the stomach, and were is absorbed more
Everything is absorbed in small intestine, very litte by the stomach
75
Were is absorbed the nutrients to?
Into the hepatic portal system
76
Were does the digested fat go?
Lymphatic system
77
Were is absorbed the ions and water?
Large intestine
78
Types of epithelial cells in the stomach, and what do they release?
1. Mucus cells: mucus
2. Parietal cells: HCl
3. Chief cells: pepsinogen
4. G cells: gastrin
79
What does mucus does?
Protects the stomach lining from macidic gastric juice and mechanical damage
80
What is the function of hydrochloric acid (HCl) secreted by parietal cells?
Lowers stomach pH, activates pepsinogen into pepsin, and kills pathogens.
81
What is the function of pepsinogen secreted by chief cells?
It is an inactive precursor that gets activated into pepsin, which digests proteins.
82
What is the function of gastrin secreted by G cells?
Stimulates secretion of gastric acid (HCl) and promotes stomach motility.
83
Where does digestion occur?
Mouth and small intestine
84
Which one is the key enzyme for absorption?
Amylase
85
Where are disaccharidases located in the small intestine?
In the brush border of the intestinal epithelium
86
What do disaccharidases do?
Break down disaccharides into monosaccharides
87
How is glucose absorbed at the apical membrane?
Via Na+/glucose symport
88
How is glucose transported at the basolateral membrane?
By facilitated diffusion
89
How is fructose absorbed in the small intestine?
By facilitated diffusion across both membranes
90
What ion is key for glucose and galactose absorption?
Sodium (Na+)
91
What nutrient do intestinal cells use as fuel?
Glutamine
92
How is GLUT2 activated?
By high levels of glucose
93
What is the function of SGLT (SGLT1) in the small intestine?
Cotransports glucose or galactose with Na+ across the apical membrane of enterocytes
94
What is the function of GLUT2 in the small intestine?
Facilitated diffusion of glucose, galactose, and fructose across the basolateral membrane into the bloodstream
95
What is the function of GLUT5 in the small intestine?
Facilitated diffusion of fructose across the apical membrane of enterocytes
96
How is SGLT1 activated in the small intestine?
By the sodium (Na+) gradient maintained by the Na+/K+ ATPase on the basolateral membrane
97
How is GLUT2 activated or recruited to the apical membrane?
High luminal glucose levels can trigger its translocation to the apical membrane (in some species); otherwise, it is normally located on the basolateral membrane
98
How is GLUT5 activity regulated?
Expression is upregulated by high luminal fructose; it operates via passive facilitated diffusion
99
What is lactose?
A disaccharide sugar making up 2–8% of milk.
100
What causes lactose intolerance?
Lack of the enzyme lactase in the small intestine
101
What happens when lactose is not digested?
It passes into the colon undigested
102
Why does undigested lactose cause symptoms?
It increases osmolarity in the colon, drawing water in and causing cramping and diarrhea
103
What are common symptoms of lactose intolerance?
Abdominal cramping, bloating, and diarrhea
104
What is the solution for people with lactose intolerance
Consuming lactose-free dairy products
105
Into what are dietary proteins digested?
Small peptides and amino acids
106
What are the two main types of enzymes for protein digestion?
Endopeptidases and exopeptidase
107
What do endopeptidases (proteases) do?
Break peptide bonds within the protein chain to form smaller peptides
108
What do exopeptidases do?
Remove amino acids from the ends of peptides
109
How are proteases secreted?
As inactive proenzymes (zymogens)
110
Where are proteases secreted from?
The stomach, intestine, and pancreas
111
What is the function of peptidases?
To break peptides into smaller peptides or amino acids
112
What do endopeptidases do?
Cleave peptide bonds within a peptide chain to produce smaller peptides
113
What do exopeptidases do?
Remove single amino acids from the ends of peptides
114
What are the two types of exopeptidases?
Aminopeptidases and carboxypeptidases
115
What does an aminopeptidase do?
Cleaves amino acids from the N-terminus (amino end) of a peptide
116
What does a carboxypeptidase do?
Cleaves amino acids from the C-terminus (carboxyl end) of a peptide
117
Where does protein digestion occur?
In the stomach and small intestine
118
How are amino acids absorbed?
Via multiple specific amino acid transporters in the intestinal epithelium
119
How are small peptides absorbed?
Through peptide transporters; some are digested intracellularly into amino acids
120
What happens to small peptides inside the cell?
They can be further digested into amino acids or transported into the bloodstream
121
What is transcytosis in the context of protein absorption?
The transport of intact peptides across enterocytes into the bloodstream
122
What is the link between transcytosis and food allergies?
Transcytosis of intact proteins or peptides may trigger immune responses, leading to food allergies
123
Cell junctions in the stomach
Tight junctions
124
Intestine cell junctions
Leaky
125
What is the primary form of dietary lipids?
Triglycerides
126
Are triglycerides water-soluble?
No
127
What two secretions aid in fat digestion?
Bile and colipase
128
What is the function of bile in fat digestion?
Emulsifies fats into smaller droplets to increase surface area for enzyme action. It contains salts, pigments, cholesterol that are stored in the gallbladder
129
What is the role of colipase?
Helps lipase bind to fat droplets for efficient triglyceride breakdown
130
Where does fat digestion occur?
Mouth, stomach, and small intestine
131
What enzyme begins fat digestion in the mouth?
Lingual lipase, secreted by serous glands of the tongue
132
What enzyme contributes to fat digestion in the stomach?
Gastric lipase, secreted by chief cells
133
What are lingual and gastric lipases collectively called?
Acid lipases (function in low pH)
134
What is the main enzyme for fat digestion in the small intestine?
Pancreatic lipase
135
What are the products of triglyceride digestion?
One monoglyceride + two free fatty acids (FFA)
136
Which enzyme digests phospholipids?
Pancreatic phospholipase
137
How are fatty acids and monoglycerides absorbed?
By simple diffusion across the intestinal epithelium
138
What is the function of bile in digestion?
It assists in the breakdown and absorption of fats
139
What defence mechanisms exist to repel foreign invaders?
Mucus, enzymes, acid, lymphoid tissue (GALT - gut-associated lymphoid tissue)
140
Interocytes
Single epithelial cells in small intestine that are important for nutrient absorption
141
Smooth muscle
Contracts spontaneously and regulated by enteric nervous system and hormones, dictated by the type of food you eat
142
Single unit smooth muscle
Responsible for pacemaker/spontaneous contractions, generates basal electrical rhythm
143
Interstitial cells of Cajal (ICCs)
Network of cells that produce the slow waves in circular muscle of muscularis (spontaneous slow waves of graded depolarization)
144
Trypsinogen is converted to trypsin by:
Enteropeptidase (which is stored in the brush border membrane of the small intestine)
145
What enzyme is necessary for cells that secrete acid/bicarbonate?
Carbonic anhydrase
146
Where does bicarbonate secretion occur?
Pancreatic duct cell or duodenal cell
147
Where does chloride secretion occur?
Intestinal and colonic crypt cells (chloride ions enter lumen through CFTR channel), also causes sodium secretion by paracellular pathway
148
Amylase
Found in the mouth, breaks down glucose polymers to disaccharides
149
Disaccharidases
Found in the intestinal epithelium (on the brush border membrane), breaks down disaccharides into monosaccharides
150
Bile salts
- Bile acid combines with amino acids
- This is what coats fat droplets
151
Chylomicrons
Formed in intestinal cells/enterocytes by combination of absorbed fat with cholesterol and proteins, chylomicrons are then released into the lymphatic system via lacteals
152
Nucleic acid
- Not a significant part of most diets
- Digested by pancreatic and intestinal enzymes
- Breaks down into nucleotides, then bases + monosaccharides
- Absorption of bases via active transport, monosaccharides enter via the same mechanisms as glucose uptake
153
Absorption of vitamins A, D, E, K
(Fat soluble), absorbed with fats
154
Absorption of water soluble vitamins
Mediated transport
155
Absorption of vitamin B12
Absorption depends on intrinsic factor secreted by stomach, if deficiency of B12 absorption = pernicious anemia
156
Absorption of minerals
Active transport (linked to their concentrations in the body because they can contribute to the electrical gradient)
157
CNS integrates:
Long reflexes
158
ENS integrates
Short reflexes (motility, secretion, growth)
159
Cephalic phase of digestion
Reflex outside of ENS, i.e. CNS-mediated reflexes
160
Gastric pits
Invaginations in the stomach that increase surface area
161
Gastrin family of GI hormones
- Gastrin and cholecystokinin (CKK)
- Gastrin is synthesized by G cells in the stomach
- CCK is synthesized in intestinal mucosa
162
Secretin family of GI hormones
- Secretin
-Vasoactive intestinal peptide (VIP)
-Glucose-dependent insulinotropic peptide (GIP)
- Glucagon-like peptide 1 (GLP-1)
163
Secretin
- Synthesized in intestinal mucosa
-Released in response to acidic chyme in small intestine
- Causes release of pancreatic buffers (bicarbonate ions) to neutralize small intestine environment
164
Vasoactive intestinal peptide (VIP)
- Synthesized in the ENS
- Contracts/relaxes muscle
165
Glucose-dependent insulinotropic peptide (GIP)
- Released in response to carbohydrates
- Synthesized in intestinal mucosa (epithelium interocytes of duodenum)
- In the presence of glucose, it stimulates insulin release (a trophic hormone, acts on pancreas)
166
Glucagon-like peptide-1 (GLP-1)
- Functions similarly to GIP
- Stimulates insulin-release by pancreas
167
Motilin
Regulates motility, peristalsis, segmentation contractions
168
Cholecystokinin
- Released in response to fatty food
- Synthesized in intestinal mucosa
- Stimulates gallbladder contractions for bile release
- Delay of gastric emptying (helps fat digestion)
- Enhance satiety
169
Gastrin
- Synthesized in the stomach
- Released following parasympathetic activity after a meal or by stimulation from enteric nervous system
- Stimulates acid secretion by direct action on parietal cells in stomach or indirectly through histamine
170
Procolipase
Split into colipase (signals to act on fat) and enterostatin (released into duodenum)
171
Enterostatin
Satiety signal for fat ingestion
172
Histamines
- Secreted into ECF on the basolateral side, effects acid secretion
- Secreted by enterochromaffin cells in gastric-mucosal lumen
173
Serotonin
Increased luminal pressure, stimulates reflex
174
What are the gastric events following a meal?
Cephalic phase (brain involvement, long reflex), gastric phase (short reflex), intestinal phase
175
Reflux esophagitis
Heartburn
176
What is the negative feedback signal that modulates acid and pepsin release?
Somatostatin release by D-cells (triggered by increased H+) - causes inhibition of cells that stimulate digestion
177
Stomach ulcers
Erosion of stomach lining by gastric juices
178
Peritonitis
Inflammation of the peritoneum caused by perforating ulcers (perforation of mucus layer), can be fatal
179
Effect of aspirin and other non-steroidal anti-inflammatory drugs
Inhibit mucus and bicarbonate secretion
180
Helicobacter pylori
Stomach bacteria that can precipitate an immune response that destroys the stomach lining making it more vulnerable
181
Solutions for stomach ulcers
- Buffer stomach acid (milk, antacids)
- Proton pump inhibitors (omeprazole)
182
What comprises the intestinal phase?
- Reflexes that direct digestion action
- Limit chyme entrance rate and motility (chyme in the small intestine inhibits gastric motility and secretion)
- Neutralize HCl, add bile and enzymes
- Most of digestion/absorption occurs in the small intestine
- Large intestine concentrates waste for excretion
- Mass movement triggers defection
- Absorption of bile salts, water, and some nutrients
- Microbe-mediated digestion of complex carbohydrates
