Gastrointestinal System Flashcards
(165 cards)
1
Q
What is the primary function of the GI system?
A
To bring nutrients into the internal environment so that they can be used.
2
Q
Name the four specialised functions of the GI system.
A
Motility
Secretion
Digestion
Absorption
3
Q
Describe the general structure of the GI system.
A
Long tube with outgrowths, closed off at both ends by sphincters and lined by epithelium.
4
Q
Define microvilli.
A
Finger-like extensions of the plasma membrane that increase surface area.
5
Q
Describe how the epithelium changes down the GI system and why.
A
Stratified squamous protects from food abrasion- mouth/ oral cavity, esophagus
Simple columnar for secretion and absorption- stomach, small and large intestines
Stratified squamous protects from faeces abrasion- anal canal
6
Q
Describe a unicellular gland.
A
Only one in the body- goblet cells.
- columnar epithelial cell
- apical aspect packed full of mucous granules
- basal aspect contains nucleus
7
Q
Describe the two types of multicellular glands and give examples.
A
Simple- gland with single duct (stomach, small intestine)
Compound- gland with 2 or more ducts, increased SA for secretion (salivary glands)
8
Q
Name the four layers of the gut tube.
A
Mucosa
Submucosa
Muscularis externa/ proper
Adventitia
9
Q
Describe the structure of the mucosa.
A
Three layers:
- epithelium
- lamina propria (FCT), includes vessels and lymphatics
- muscularis mucosae (thin layer of smooth muscle)
And sometimes glands
10
Q
Describe the structure of the submucosa.
A
Layer of connective tissue containing glands, blood vessels and the submucosal nerve plexus.
11
Q
What is the submucosal nerve plexus?
A
Plexus of nerve fibres regulating secretion of glands in the submucosa. Part of the enteric nervous system (ENS)- acts independently of CNS input.
12
Q
Name a function of the muscularis mucosae.
A
Contracts to squeeze secretions out of glands.
13
Q
Describe the muscularis externa/ proper.
A
Contains:
- inner circular smooth muscle layer (controls diameter of tube)
- myenteric plexus (ENS) between muscle layers- regulates motility
- outer longitudinal smooth muscle layer (shortens tube)
14
Q
Describe the adventitia. What does it touch?
A
Made up of FCT. Connects with the serosa- an additional outer covering of organs.
15
Q
Describe the peritoneum.
A
A serous membrane- layer of epithelial cells that secrete serous fluid. Two layers:
- parietal layer lines body wall
- visceral layers lines organs
Between the layers is the peritoneal cavity.
16
Q
What does the term retroperitoneal refer to?
A
Posterior to the peritoneum.
17
Q
What does the fluid-filled peritoneal cavity allow?
A
Frictionless movement for organs to slip over each other.
18
Q
Define the terms mesentery and omentum.
A
Mesentery= double layer of visceral peritoneum that connects an organ to body wall Omentum= double layer of visceral peritoneum that connects an organ to another organ
19
Q
What do mesenteries and omenta prevent?
A
Tubular portions of the GI tract tangling into knots.
20
Q
What else is in mesenteries, apart from visceral peritoneum?
A
Blood vessels, nerves, lymphatics, adipose.
21
Q
Where does digestion begin and how?
A
Mouth/ oral cavity
- mechanical- teeth and pushing tongue against hard palate
- chemical- enzymes produced by salivary glands
22
Q
Describe the mechanism of swallowing food.
A
Form bolus with food material, push through fauces (opening) with tongue.
23
Q
Describe the three pairs of salivary glands connected to the oral cavity via ducts.
A
Parotid- anterior + inferior to the ear (serous fluid with amylase)
Sublingual- inferior to the tongue (mucous only)
Submandibular- inferior to mandible (both serous fluid with amylase and mucous)
24
Q
Describe the structure of salivary glands.
A
- compound secretory glands
- acini (cells in clusters) that secrete amylase
- cells in the duct secrete bicarbonate to buffer/ neutralise acidity
25
Describe the structure of the esophagus.
Long tube ~ 25cm that extends from pharynx to stomach. Highly folded mucosa and submucosa has the capacity to expand.
Stratified squamous epithelium
26
Describe the muscularis externa/ proper of the esophagus.
First third- skeletal muscle
Second third- mixture of skeletal and smooth muscle
Last third- smooth muscle
Moves food bolus
27
How does the esophagus provide lubrication? Why doesn't it have goblet cells?
Have glands with ducts to the surface. Because goblet cells would get scratched off by the food bolus, and have a different shape to stratified squamous epithelium.
28
How does the esophagus get to the stomach?
Travels down the thoracic cavity, through the esophageal hiatus in the diaphragm and into the abdominopelvic cavity.
29
How is reflux of acidic material from the stomach prevented? What does reflux cause?
Lower esophageal sphincter LES contracts.
| Damage to the epithelium, heartburn.
30
Define the four regions of the stomach.
```
Cardia= area esophagus opens into
Fundus= most superior, close to diaphragm, lots of glands
Body= central, forms the bulk of the stomach
Pylorus= gatekeeper to the duodenum
```
31
Describe the omenta that connect to the stomach.
Lesser omentum: connects the lesser curvature of the stomach to the liver
Greater omentum: connects the greater curvature of the stomach to the transverse colon
32
Name two properties of the greater omentum (other than its location).
Adipose provides cushioning for underlying organs (gives yellow colour).
Provides first line of defense by isolating infections in peritoneum.
33
What differs between the muscularis externa of the esophagus and of the stomach?
Stomach has an extra inner oblique layer.
| - generates strong contractions to drive food toward pylorus region
34
What differs between the muscularis externa of the distal and proximal regions of the stomach?
It is thicker in the distal region (pylorus + lower body). increased contraction allows for more mechanical and chemical digestion.
35
Describe the folds of the stomach.
Rugae are temporary folds that expand to allow the stomach to store food.
36
How are the rugae able to flatten out?
Connective tissue of their submucosal core contracts/ expands.
37
What are the permanent folds in the stomach?
Gastric glands= folded simple columnar epithelium
| - folded to increase surface area for secretion
38
What do the glands of the stomach secrete?
Acid and enzymes for digestion
Mucous for protection
Hormones for regulation
39
Which cells line the surface and neck of the gastric glands?
Goblet cells/ mucous epithelial cells
40
Which cells line the deeper aspect of the gastric gland? Name their functions.
Parietal cells
G cells (secrete hormones)
Chief cells
41
Describe the structure and function of parietal cells.
Abundant mitochondria to produce ATP to pump H+ ions
Folded structure to increase SA for cellular respiration
Central nucleus
Secrete acid and intrinsic factor (important for vit B absorption)
42
Describe the structure and function of chief cells.
Abundant rough ER for protein production
Zymogen (enzyme) granules occupy apical aspect
Nucleus pushed to basal aspect
Secrete pepsinogen (precursor for pepsin- digests proteins)
43
How is stomach function regulated?
Endocrine cells in mucosa, gastrin and ghelin into the bloodstream.
ENS (submucosal and myenteric plexuses)- local reflexes
CNS- long neural reflexes to modulate function
44
How is food release into the small intestine controlled?
Pyloric sphincter relaxes to allow a 'spurt' of chyme through, then contracts.
45
Which layer of muscle is the pyloric sphincter?
Thickened circular muscularis externa.
46
How is the small intestine digesting and protecting itself from the acidic chyme?
Receives enzymes for digestion and bicarbonate for acid neutralisation from the pancreas. Submucosal glands and some goblet cells in the mucosa produce mucous.
47
Describe the shape and location of the pancreas.
- head in C-shaped duodenum
- tail towards spleen (LHS).
- posterior to stomach
- duct extends into duodenal lumen
- retroperitoneal, covered by visceral peritoneum on anterior surface
48
Why is the pancreas retroperitoneal?
It's bound to the posterior abdominal wall because it doesn't need to move for its function- just producing secretions.
49
Why is most of the duodenum retroperitoneal?
Doesn't need to move for its function- just receiving chyme and enzymes.
50
What is the heaptopancreatic ampulla?
Dilated area where the bile duct from the liver meets the pancreatic duct.
51
Where do secretions empty into the duodenal lumen from? What controls this release?
The duodenal papilla- a projection into the lumen from the hepatopancreatic ampulla.
Hepatopancreatic sphincter
52
Describe the structure and function of pancreatic acinar cells.
Apical zymogen granules
Basal nucleus
Abundant rough ER
Secrete enzymes
53
Name the three main regions of the small intestine. Where are they located?
Duodenum- next to stomach, retroperitoneal
Jejunum- between duodenum and ileum, intraperitoneal
Ileum- between jejunum and large intestine, intraperitoneal
54
Why are the jejunum and ileum intraperitoneal?
So they are free to perform motility patterns.
55
What prevents the small intestine from becoming tangled?
Mesentery
56
What runs in between the two visceral peritoneum layers of the mesentery?
Mesenteric arteries, mesenteric veins, nerves and lymphatics.
57
How does the small intestine combat and protect itself from the acidity of the chyme it receives?
Goblet cells in mucosa, mucous-producing glands in submucosa of duodenum.
It receives bicarbonate from the pancreas which neutralises pH, as well as producing a small amount of its own in the epithelium.
58
What is the function of the small intestine in the GI tract and what is the key factor required for this?
Further chemical digestion, and absorption.
Large surface area
59
Name four ways the small intestine is structured to have a large surface area.
- 6m in length
- plicae circulares
- villi
- microvilli brush border
60
Describe the structure and function of plicae circulares.
Permanent large folds of the small intestine: a core of submucosa with overlying mucosa. They run around the circumference of the lumen to increase SA and slow passage of material by making it spiral around. This allows more time for digestion and absorption.
61
Describe the basic structure and function of villi.
Permanent folds of the mucosa, with a core of lamina propria. The muscularis mucosae isn't folded with the rest of the mucosa, but it allows the villi to wiggle back and forth, exposing them to luminal contents for absorption.
62
Which vessels do villi contain?
Lymph lacteal: absorbs products of fat digestion (would clog up blood vessels)
Capillary network: absorbs amino acids and monosaccharides
63
Which specific epithelial cells of the villi carry out absorption?
Enterocytes/ simple columnar epithelial cells.
64
Where do products of small intestine absorption drain into after leaving the villi?
Venules carry deoxygenated, nutrient-rich blood to the mesenteric veins, which drain into the hepatic portal vein.
Lymph lacteals drain into the cisterna chyli, the thoracic duct, then left subclavian vein.
65
Describe the structure and function of the microvilli brush border.
Fingerlike projections formed by folding the apical surface of enterocytes (microvilli).
Surrounded by glycocalyx: network of glycoproteins and branched filaments that tethers enzymes to the border.
This allows contact digestion: enzymes digest anything that comes into contact with the border, breaking it down so it can be absorbed.
66
Describe the difference between transcellular and paracellular pathways of molecules through the epithelium.
Transcellular= down epithelial cell, through plasma membrane via transmembranous proteins or simple diffusion
Paracellular= beside epithelial cell, through tight junctions, only small molecules can diffuse through, passive diffusion requires gradient
67
Name five cells of the small intestine epithelium and their functions.
Enterocytes- absorption
Goblet cells- secrete mucous
Stem cells- replace other epithelial cells
Paneth cells- secrete antibacterial enzymes e.g. defensins, lysozymes
Endocrine cells- secrete regulatory hormones into blood
68
Name the three regions of the large intestine.
Cecum
Colon
Rectum
69
What are the names of the colon as it turns?
Ascending colon
Right colic/ hepatic flexure into transverse colon
Left colic/ splenic flexure into descending colon
Sigmoidal colon
70
Which parts of the colon are retroperitoneal, and which are intraperitoneal?
Retroperitoneal: ascending and descending
Intraperitoneal: transverse and sigmoidal
71
Which structure sits superficial to the large intestine?
Greater omentum
72
What regulates passage of food material into the cecum? What other function does it have?
Ileocecal valve- from ileum of small intestine to cecum of large intestine
Prevents backflow of faeces into the small intestine
73
What is the function of the vermiform appendix?
Contains a large reserve of intestine bacteria.
- digest indigestible material
- produce vitamins and hormones
- provide energy for cells
- protect against pathogens
74
Why is it best to treat appendicitis right away?
If the appendix becomes too inflamed, it can burst through the peritoneum, leading to fluid loss and infection.
75
What is the purpose of a barium enema, and what might it show?
Enables an X-ray image to pick up soft tissue of large intestine to assess function. If the ileocecal valve isn't functioning, the barium will travel to the small intestine as well, and will show up in the image.
76
How does the large intestine differ from the small intestine? (3)
No villi- mucosa is invaginates into intestinal glands
Epithelium dominated by mucous secreting cells
Teniae coli
Haustra
Omental appendages
77
Describe the types of cells lining the intestinal glands and why? Which layers of the mucosa invaginate to form the glands?
Function of the colon is to absorb water and electrolytes. This dries out the faeces, so epithelium is mostly made of goblet cells, with some absorptive cells.
The epithelium invaginates into the lamina propria, while the muscularis mucosae isn't involved.
78
Describe the teniae coli.
Three bands of longitudinal smooth muscle- modified from the muscularis externa layer. Generates very strong contractions to help move faeces through the large intestine.
79
How do the teniae coli affect the gross structure of the large intestine?
They pucker the large intestine into small pouches called haustra, which are separated by semilunar folds.
80
What are omental appendices?
Sacs of fat on the exterior surface of the large intestine
81
Where does the epithelium changes from large intestine to anus, and how does it change?
At the anal columns, from simple columnar to stratified squamous.
82
How is passing of faecal material out of the anus regulated?
Movement stimulates stretch receptors, and the internal anal sphincter relaxes (smooth muscle). The external anal sphincter (skeletal muscle) only relaxes once the decision is consciously made.
83
Describe the location and structure of the liver.
Upper right quadrant of the abdominopelvic cavity. Divided into hexagonal units called lobules. Each lobule has rows of hepatocytes surrounding a vein and producing bile:
- liver sinusoids (leaky capillaries) divide rows
- bile canaliculi divide cells
84
Describe the portal triad.
Branch of the hepatic artery, branch of the hepatic portal vein, and a bile duct.
85
Describe the flow of blood and bile, and what happens in the liver?
Blood flows toward central vein, and the venous blood is processed by hepatocytes which remove toxins and produce bile. Bile is secreted into the canaliculi and travels to the bile duct
86
Where does the central vein drain into?
The hepatic vein, then into the inferior vena cava.
87
Where does the bile duct travel to?
The gallbladder, where it is stored and concentrated. Bile duct ultimately joins pancreatic duct at the hepatopancreatic ampulla, before it enters the intestinal lumen.
88
How much of the cardiac output does the liver receive? How is it transported in?
25%
1/3 from hepatic artery
2/3 from hepatic portal vein
- both travel within the lesser omentum
89
What are the effectors and receptors of GI regulation? What are we regulating, and through which systems?
```
Receptors:
- Stretch receptors in wall of tract
- Change in pH, osmolarity, aa's, sugars, fats
Effectors:
- Smooth muscle
- Glands
```
The conditions of the intestinal lumen. Through nervous (CNS and ENS) and hormonal regulation.
90
What is the role of the CNS in regulation of GI function?
Co-ordinates activity over long distances. PNS stimulates motility and secretion, SNS inhibits motility and secretion. It also modulates ENS activity.
91
What is the role of the ENS in regulation of GI function?
Regulates secretion (submucosal plexus) and motility (myenteric plexus). Involved in local reflexes for peristalsis and segmentation.
92
Name four critical hormones of GI regulation.
Gastrin
Gastric inhibitory peptide GIP
Secretin
Cholecystokinin CCK
93
Name four functions of motility in the GI tract.
Movement at a controlled rate to aid storage and chemical digestion.
Mechanical digestion to increase SA.
Mixing food to aid chemical digestion.
Exposure to absorptive surfaces.
94
How is GI regulated?
Smooth muscle that acts spontaneously (needs no external input). CNS regulates strength of contraction, ENS regulates frequency of contraction.
95
Describe contraction frequencies throughout the GI tract.
Stomach- 3 per min
Duodenum- 12 per min allows chyme to mix with bicarbonate
Ileum- 9 per min allows more time for abosrption
96
Describe the fasting motility pattern.
'Migrating motor complex'
Occurs 4 hours after a meal, and repeats every 2 hours until more food is eaten. Occurs mainly at night.
Flushes residual secretions and undigested material through the tract.
- 1h inactivity
- 50 mins uncoordinated activity
- 10 mins coordinated activity
97
Describe the feeding motility pattern.
Involves:
- storage in the stomach and colon
- propulsion (peristalsis) through esophagus, stomach, and intestines
- mixing: retropulsion in stomach, segmentation in intestines
98
How do the stomach and colon store food?
Through smooth muscle relaxation, which changes the volume without changing the pressure.
99
Describe peristalsis.
Contraction of circular smooth muscle layer to push bolus through GI tract.
100
Describe segmentation.
Contraction of circular smooth muscle layer at successive points along tract, then alternate points. This mixes food together at a local level, rather than pushing it onwards through the GI tract.
101
Describe the purpose of chewing.
Reduces the size of food to allow ingestion into the esophagus. Mixes it with saliva to maximise taste.
102
Is chewing voluntary?
Yes- it's controlled by skeletal muscle. But it's mostly under reflex control- e.g. strength, frequency and side.
103
In which parts of the stomach do storage, mechanical digestion, mixing and controlled movement occur?
Storage- fundus and body (smooth muscle distends)
Mechanical digestion; antrum
Mixing- antrum
Controlled movement- pyloric sphincter
104
Why is controlled movement into the small intestine important?
The small intestine has a limited capacity for food.
105
How small does the stomach become when fasting?
~50mL in volume
106
How is peristalsis performed in the stomach?
Initiated on greater curvature and spreads to antrum. 3 contractions per minute, 1st 60 mins after meal is gentle, 60-300 mins is more intense activity.
107
Describe retropulsion.
Movement of chyme in the opposite direction to propulsion, cause by peristalsis and the closed pyloric sphincter.
108
Which factors affect gastric emptying?
Size of meal- larger= faster emptying
| Composition of meal- fluids are faster, fats slower and difficult to digest
109
Describe the feedback loop for gastric empyting?
Duodenum contains fatty, hypertonic, acidic chyme.
- duodenal enteroendocrine cells sense this, and stimulate secretion of secretin and cholecystokinin
- chemoreceptors and stretch receptors respond to chyme by triggering enterogastric reflex
- short reflex (ENS)
- long reflex (PNS and SNS)
These all decrease contractile force of the stomach and rate of gastric emptying.
110
Describe large intestinal motility.
Stores faeces, so long periods of inactivity. Mass movement 1-2 times a day following meals- a peristaltic wave initiates defecation.
111
Where is fluid sourced for exocrine secretions into the GI tract? How is it maintained?
Blood plasma- 3L
| We excrete 8L a day, so it must be reabsorbed.
112
When and where does most salivary secretion occur?
Submandibular glands. During stimulated secretion- thought, sight, smell or presence of food.
113
What is salivary secretion composed of (3), and what are the functions of these components?
Mucus- lubrication
Dilute solution of sodium bicarbonate and sodium chloride- dilute food and provide optimal pH for digestive enzymes
Digestive enzymes- alpha-amylase and lingual lipase
114
Which conditions must be present for alpha-amylase?
Alkaline pH, it's denatured when it gets to the stomach.
115
What does salivary secretion aid?
Digestion- enzymes
Hygiene- irrigates the mouth and prevents xerostomia (dry mouth)
Talking, chewing and swallowing
116
How is salivary secretion regulated?
Predominantly by autonomic nervous input
- thought, sight, smell of food, or presence of food in the mouth
- PNS stimulates secretion of large amounts of fluid
- SNS stimulates secretion of small volumes of viscous fluid
117
Describe the composition of gastric secretion between and during meals.
Between meals- slow rate of secretion- mucous
During meals:
- mucous
- parietal cells secrete HCl and intrinsic factor
- chief cells secrete pepsinogen
118
Describe the functions of gastric secretion components.
Mucous- protects from abrasion and acid
HCl- Dilutes and suspends food, denatures protein, activates pepsinogen to pepsin, creates optimum pH for pepsin activity, protects against microbes
Intrinsic factor- essential for vit B12 absorption in the ileum
Pepsinogen- changes to pepsin and starts protein digestion
119
How is pepsinogen converted to pepsin?
Pepsinogen is secreted by chief cells at the bottom of the gastric glands, then travels upwards to the surface, passing parietal cells that secrete HCl. HCl converts it to pepsin.
120
How is acid sourced by parietal cells? Where is CO2 sourced?
Carbonic anhydrase forms carbonic acid (H2CO3) from CO2 and H2O. Carbonic acid dissociates to give H+ and HCO3- (bicarbonate).
CO2 is sourced from interstitial fluid (diffuses across serosal membrane), and is produced as a natural byproduct of metabolism.
121
How is acid secreted by parietal cells?
The H+-K+ATPase (across apical membrane) pumps H+ ions into the lumen in exchange for K+ ions. K+ ions return to lumen through a membrane channel.
122
How is Cl- sourced by parietal cells?
An anion counter transporter in the serosal membrane imports Cl- ions from the interstitial fluid into the cell, and ejects HCO3- out.
123
How is Cl- secreted by parietal cells?
Through a Cl- channel in the apical membrane.
124
Name the three phases of regulation of gastric secretion.
```
Cephalic phase (20%)
Gastric phase (70%)
Intestinal phase (10%)
```
125
What is the cephalic phase of regulation of gastric secretion stimulated by?
Higher centres- thought, smell, sight of food
| Chewing action and taste
126
What does the cephalic phase of regulation of gastric secretion stimulate, and how?
PNS acts through the submucosal plexus, via the vagus nerve, which stimulates parietal, chief, and goblet cells, as well as the secretion of gastrin from G cells (also stimulates parietal and chief cells).
127
What is the gastric phase of regulation of gastric secretion stimulated by?
Stretch receptors in wall of stomach, and chemoreceptors sensing products of digestion in the stomach lumen and elevated pH.
128
What does the gastric phase of regulation of gastric secretion stimulate, and how?
Local nervous reflex- stretch and chemoreceptors stimulate submucosal and myenteric plexuses
- stimulate goblet, chief, parietal, and G cells
- increases peristalsis.
External nervous reflex- PNS (vagus nerve to plexuses)
Hormonal regulation- gastrin stimulates chief and parietal cells
129
What is the intestinal phase of regulation of gastric secretion stimulated by?
Distention of duodenum- stretch receptors
| Arrival of acid chyme, and lipids and carbohydrates in the duodenum- chemoreceptors
130
What does the intestinal phase of regulation of gastric secretion stimulate, and how?
Presence of lipids and carbohydrates stimulate CCK and GIP hormones. Decreased pH stimulates secretin. These hormones inhibit chief and parietal cells, and peristalsis.
Enterogastric reflex is stimulated by distention of the duodenum. This inhibits the myenteric plexus, decreasing peristalsis via neural feedback.
131
What are the exocrine components of pancreatic secretion and their functions?
Enzymes, produced by acinar cells for chemical digestion.
Alkaline fluid, secreted by duct cells, containing bicarbonate to neutralise acid and make duodenum optimal pH for enzymes.
132
What stimulates the secretion of pancreatic enzymes? How are they secreted and activated, and why?
Cholecystokinin CCK
As precursors that are activated by trypsin, once it has been activated by membrane-bound enteropeptidase.
If they were secreted as active enzymes, they'd start digesting proteins of the pancreas before getting to the duodenum.
133
What stimulates the secretion of alkaline fluid from the pancreas?
Secretin, which is stimulated by the arrival of acid chyme in the duodenum.
134
What are the components of biliary secretion, and their functions?
Bile salts- fat digestion
| Bicarbonate-rich fluid- neutralises acid
135
How is biliary secretion regulated?
Bile is secreted constantly by the liver, is stored in gallbladder, and delivered to duodenum with arrival of food.
CCK stimulates gallbladder contraction and hepatopancreatic ampulla relaxation. Secretin plays a mild role as well.
Bile stimulates its own secretion via the enterohepatic circulation- 95% bile reabsorbed into the ileum and transported back to the liver via the hepatic portal vein. Reabsorption stimulates more bile secretion.
136
What are the components of intestinal secretion, and their functions?
```
Mucous- lubrication (only secretion in large intestine)
Isosmotic fluid (NaCl and NaHCO3)- neutralises acid and dilutes food to aid digestion
Digestive enzymes- aid chemical digestion
```
137
How does the small intestine produce secrete digestive enzymes?
The epithelial layer sheds into the lumen every 7 days- and with it the enzymes bound to the brush border.
138
Which polymers do we source carbohydrate from?
Starch and glycogen.
139
Which bonds do starch and glycogen consist of?
alpha 1-4 glycosidic bonds between glucose monomers
140
Name three disaccharides.
Sucrose
Lactose
Maltose
141
What is protein digestion required for, and where do we source them?
As a source of amino acids, not energy.
| 50% diet, 50% endogenous proteins: enzymes and immunoglobulins secreted into the intestine.
142
What benefits does fat digestion bring to the GI function?
Slows gastric emptying
Source of fat soluble vitamins (A D E K)
Source of energy, but not essential
143
Describe the structure of triacylglycerols.
Glycerol backbone with three fatty acid chains of varying lengths.
144
How does mechanical digestion aid chemical digestion?
Breaks up food to increase the surface area for enzymes to work (only on surface).
145
What is the optimal pH of salivary, gastric, and intestinal enzymes?
Salivary and intestinal- alkaline
| Gastric- acidic
146
Why do we secrete a second form of amylase into the small intestine?
Salivary amylase is degraded by the acidic pH of the stomach.
147
Why can't we digest cellulose?
We don't have an enzyme to hydrolyse the beta 1-4 glycosidic bonds.
148
Name the two general stages of chemical digestion.
Luminal digestion- involves enzymes secreted into the lumen
| Contact digestion- involves enzymes produced by enterocytes and attached to the brushborder
149
Describe luminal and contact digestion of carbohydrates.
Luminal- salivary and pancreatic amylase converts polysaccharides to oligo- and disaccharides
Contact- sucrase, lactase, and maltase convert disaccharides to monosaccharides
150
Describe luminal and contact digestion of proteins.
Luminal- pesin, trypsin, chymotrypsin, carboxypeptidase convert proteins into polypeptides
Contact- peptidases convert polypeptides into individual amino acids (and di and tri)
151
Describe the four stages of lipid digestion.
Emulsification: motility pattern in stomach (retropulsion) and small intestine (segmentation) breaks lipid into smaller droplets
Stabilisation: bile salts stabilise the emulsions in the small intestine, and increase surface area
Hydrolysis: Colipase anchors lipase to droplet surface, so it can covert triacylglycerols into monoglyceride and free fatty acids
Micelles form to transport these fatty acids and monoacylglycerols.
152
Name two other enzymes that have a minor role in lipid digestion.
Lingual lipase, and gastric lipase (don't have long enough to play a major role).
153
Define absorption in the GI tract.
Passage of substances from the lumen, across the epithelial lining of the intestine, into the interstitial fluid, then into the blood or lymph.
154
The main site of absorption is the small intestine. What absorption occurs in the stomach and large intestine?
Stomach has minimal absorption of lipid soluble substances e.g. alcohol, aspirin
Large intestine absorbs the last 9% of water and sodium from the chyme.
155
What factors impact absorption?
Motility
Surface area
Size of food particles
Removal of substance from interstitial fluid
156
What prevents buildup of substance in the interstitial fluid?
The presence of blood vessels close by.
157
Which parts of fluid loss does our drinking of water replace?
Sweat, urine, breath, and faeces
158
How is water absorbed in the GI tract?
The osmotic gradient from the lumen to the bloodstream is set up by the absorption of salts and nutrients. Water moves passively.
159
How is sodium absorbed in the GI tract?
Some passive movement via paracellular pathway.
Most actively transported via transcellular pathway:
- Na+ channel
- Na+/H+ exchanger
- Na+ glucose cotransporter
- Na+ amino acid cotransporter
Out of the cell against its concentration gradient by Na+K+ATPase (active transport)
160
How is glucose absorbed in the GI tract?
Transcellular pathway
- Na+ glucose cotransporter across apical membrane
- Glucose carrier across basolateral membrane
161
How are di- and tri-peptides absorbed in the GI tract?
Transcellular- via H+ dependent cotransporter
| Cytoplasmic peptidases break them down into amino acids, and an amino acid carrier transports them out of the cell.
162
How are amino acids absorbed in the GI tract?
Some passively via paracellular pathway.
Most transcellular:
- Na+ amino acid cotransporter
- Amino acid carrier out of the cell (basolateral membrane)
163
How are products of fat digestion absorbed?
Delivered to brush border by micelles, where fatty acids and monoglycerides diffuse into the cell. They are resynthesised into triglycerides and packaged into chylomicrons. Chylomicrons are exocytosed and enter the lacteals.
164
Are bile salts absorbed in the GI tract when lipids are?
No, lipids are absorbed in the jejunum, and bile salts travel to the ileum before Na+ bile acid cotransporters carry them across the apical membrane.
(Also some passive absorption in the large intestine)
165
Describe B12 absorption, and B12 deficiency.
Binds to intrinsic factor and is absorbed in the ileum by a specific transporter.
Pernicious anemia- can't produce enough RBCs
