GI system (https://youtu.be/ueMJWsPy3hA?si=t7tvHTopi6nYReA5) Flashcards
1
Q
What are the functions of the GI tract?
A
Digestion, absorption and excretion of organic nutrients, minerals and water. It also acts as host defense
2
Q
Where does digested food go?
A
water soluble components go into the blood and fat soluble components go into the lymphatic system
3
Q
What kinds of compounds are not digested?
A
drugs, cholesterol, steroids and some fiber
4
Q
Why Is the Gi tract a potential portal for harmful substances
A
Because it is continuous with the exterior of the body
5
Q
What components make up the GIT?
A
Mouth, Pharynx, Esophagus, stomach, small small intestine, large intestine and accessory organs
6
Q
What makes up the small intestine?
A
Duodenum, jejunum, ileum
7
Q
What are the accessory Organs?
A
Pancreas, liver, gall bladder and salivary glands
8
Q
What is the difference in composition between the GIT?
A
upper part is skeletal muscle and lower part is smooth muscle
9
Q
What structures help to increase SA in the GIT
A
Vili, microvilli and crypts
10
Q
What layers make up the GIT
A
Mucosa, submucosa, muscularis externa and serosa
11
Q
What layers make up the mucosa?
A
Epithelium, lamina propria and Muscularis mucosa
12
Q
What type of arrangements are present in the epithelial layer?
A
Basolateral and apical arrangement; different transport proteins are present at the apical surface compared to the basolateral surface
13
Q
What do tight junctions do in the epithelial?
A
Confine the proteins to specific membrane regions
14
Q
What is the epithelial function?
A
- Provides selective uptake of nutrients, electrolytes and H2O
- Regeneration of the epithelial occurs every ~5 days due to migration of daughter cells towards the villous
15
Q
What are the 2 types of selective transport across the epithelium?
A
Paracellular- tight junctions allow water and small ions between cells
Transcellular- 2 step process that requires transport proteins on the apical and basolateral surface of the cell; substances pass through cells
16
Q
What makes up the lamina propria?
A
Connective tissue, small blood vessels, lymphatic vessels , nerve fibres and immune/inflammatory cells
17
Q
What is the muscularis mucosa?
A
A thin layer of smooth muscle which is important for villli movement, it is not involved in GIT contraction
18
Q
What is the submucosa made of?
A
A plexus of nerve cell bodies, connective tissue, blood and lymphatic cells
19
Q
What is the function of the submucosa?
A
Relay of information to and from the mucosa
20
Q
What is the structure of the Muscularis?
A
-Thick inner layer of circular muscle. Fibres allow for narrowing of the lumen.
-Myenteric nerve plexus regulates muscle function
-Thinner outer layer of longitudinal muscle has fibres oriented to shorten the tube
21
Q
What is the structure of the Serosa?
A
Connective tissue encases intestine and forms a connection point to the abdominal wall
22
Q
Why is blood circulation in the GIT critical?
A
It carries away water soluble absorbed nutrients to be used by other body structures
23
Q
What is the circulation of blood between the gut and liver called?
A
Portal circulation
24
Q
Where does the portal vein drain blood from to
A
From the digestive tract into the liver
25
What is the importance of portal circulation?
-Removes harmful substances
-Processes nutrients
26
What makes the liver different from other organs?
It receives blood from the portal and arterial circulation
27
What governs the GIT processing
The volume and composition of the lumen contents
28
What initiates reflexes in the GIT
-Distention of walls by contents
-osmolarity of contents
-Ph of contents
-[ ] of specific contents (monosaccharides, FA, peptides and AA)
29
What propagates GI processes
-mechanoreceptors (mechanical stimulators)
-Osmoreceptors (change in osmolarity)
-Chemoreceptors ( binding of chemicals)
30
what is involved in the Intrinsic (enteric) regulation of GI processes?
-Dense and complex network of neurons that function independently of the CNS
-controls activity of the secretomotor neurons; motility and secretory
-Myenteric plexus and submucosal plexus
31
What is the function of the Extrinsic GI regulation?
regulation through the autonomic nervous system
-parasympathetic vs sympathetic
influences motility and secretion of GIT
-hunger
-sight / smell of food
-emotional state
32
What does each plexus affect
-Myenteric influences smooth muscle
-Submucosal influence secretion
However, nerves in each can influence each other
33
What is the function of the extrinsic neuronal regulation of GI processes?
Regulation through the autonomic nervous system influences the motility and secretion
-hunger, smell , emotions
34
Explain the long (extrinsic) neural pathway
1) extrinsic factor (sight, smell taste etc)
2) travels to CNS by afferent neurons
3) to nerve plexus within GI walls though efferent neurons
4) smooth muscle/glands are targeted
5) response occurs in GI lumen causing reflex
35
Explain the short (intrinsic) neural pathway
1) chemoreceptors, osmoreceptors or mechanoreceptors within the GI wall are sent to the nerve plexus
2) smooth muscle/glands are targeted
3) response occurs in GI lumen causing reflex
4) The stimulus is sent to release more receptors
36
What are the 4 types of chemical messengers?
Endocrine: hormone released from blood targets 1+ cells in the body
Neurocrine: NT from nerve targets neuron or effector cell in close proximity
Paracrine: Local cell releases paracrine to cells in close proximity
Autocrine: local cell releases autocrine to act on the same cell that secreted the substance
37
What are the 4 most common GI hormones?
Secretin, Cholecystokinin (CCK), gastrin and glucose dependent insulinotropic peptide (GIP)
-they are all peptides that participate in a feedback control system
38
Gastrin
-released from antrum (G cells)
-stimulated by peptides/ amino acids in stomach
-increases [HCL] and motility
39
Secretin
-released from the small intestine ( S cells)
-stimulated by acid in small intestine
-decreases [HCl] and stomach motility, increases HCO3 - and H2O
40
CCK
-released from small intestine (I cells)
-stimulated by digested fat/protein in small intestine
-decreases [HCl], stomach motility and increases pancreatic enzymes and bile expulsion
41
GIP
-released from small intestine (K cells)
-stimulated by glucose / fat in small intestine
-increases [insulin]
42
What is intestinal motility?
The contraction and relaxation of the 2 outer smooth muscle layers if the GIT
-peristalsis (propulsion)
-segmentation (mixing)
43
What are slow waves (basal electrical rhythm)?
constant depolarization and repolarization cycles in the GIT
44
How are slow waves propagated?
Through circular and longitudinal muscle layers through gap junctions
45
What affects contraction from slow waves?
Absence of neural/hormonal (food) input decrease contractions
46
what are the 3 phases of of GI system control?
1) cephalic (head)
-receptors stimulated by sight, smell, taste, chewing food and emotional state
-parasympathetic fibres activate neurons in GI nerve plexi
2) Gastric (stomach)
-receptors stimulated by distention, acidity, amino acids and peptides
-mediated by short/long neural reflexes
3) Intestinal
-receptors stimulated by distention, acidity, osmolarity and digestive products
-mediated by short/long neural reflexes and by hormones (secretin, cck, gip)
47
What part of the brain controls food intake? Where is the feeding center vs the satiety center?
The hypothalamus
feeding center- lateral region
satiety center- ventromedial region
48
What are orexigenic factors and what do they do?
They are factors that increase food intake
-NPY is a NT in the hypothalamus that stimulates hunger
-Ghrelin is made/released from endocrine cells in the stomach during fasting to stimulate the release of NPY
49
What are anorexigenic factors and what do they do?
They are factors that decrease food intake (LIMP)
Leptin-from adipose
Insulin- from pancreas
Melanocortin-from hypothalamus
Peptide YY-from intestine
50
What factor is believed to be common in obese people?
Leptin resistance which decreases feelings of satiety
51
Where does water regulation occur?
The thirst center in the hypothalamus
52
What 4 things stimulate water intake?
1) increased plasma osmolarity
-vasopressin (ADH) released to conserve water in the kidneys
2) decreased plasma volume
-requires significant volume loss, activates renin angiotensin system which produces angiotensin 2; increasing thirst
3) dry mouth
-stimulates thirst
4) prevention of over-hydration
53
What are the three pairs of salivary glands?
1) parotid - watery (serous) secretion *WATERY
2) Submandibular -serous/mucous
*WATERY and MUCOUS
3) Sublingual- mucous *MUCOUS
~1500mL of saliva/day
54
What is saliva composed of?
- water (97-99%)
-Electrolytes (K+/HCO3-)
-Digestive enzymes (Amylase/lipase)
-Glycoproteins (mucin- mucous + water)
55
What are the functions of saliva?
-moisten/lubricate food
-initiate digestion (amylase/lipase)
-diffuse food into tastebuds
-prevent microbial colonization
-aids in speech
-neutralizes acid (contains HCO3-)
56
What three cells make up salivary glands?
Acinar-protein and electrolyte secretion (leaky tight junctions allow passage of water/ions)
Ductal-create alkaline and hypotonic nature (tight junctions don't allow easy passage of substances)
Myoepithelial-push saliva from acinus into duct
57
How is saliva formed?
1) Primary secretion- acinar cells secrete initial saliva which is isotonic (contains water, electrolytes and proteins)
2) myoepithelial cells contracts to move saliva from acinus to duct
3) Ductile cells make initial hypotonic saliva alkaline
- loss of Na+ and Cl-
-addition of K+ and HCO3-
58
What is basal level salivary production? What does it increase to when stimulated?
-0.5mL
-increases 10 fold
59
What pathway stimulates salivary secretion?
Sympathetic and parasympathetic but para-sym is predominant pathway
60
What stimulates and inhibits saliva secretion for parasympathetic pathways?
stimulated by:
-smell and taste
-pressure receptors in mouth
-nausea
inhibited by:
-fatigue/sleep
-fear
-dehydration
-drugs
61
What are the sympathetic affects on saliva?
-increased saliva
-increased protein secretion from acinar cells
-increased flow in myoepithelial cells
62
How does saliva aid in digestion?
amylase in saliva initiates breakdown of amylose; however, majority of carbs are digested in the small intestine
63
what is xerostomia?
Dry mouth
-can lead to decreased oral pH , tooth decay and difficulty swallowing
-water and fluoride are considered treatments
64
Where does the swallowing reflex begin?
It is initiated by pressure receptors in the pharynx walls (food/liquid entering) and signals are sent to brainstem
65
What structures are needed for swallowing?
Larynx, Glottis and epiglottis
66
What is the function of the esophagus?
Transfer of food from the mouth to stomach (18-25cm long)
-skeletal muscle upper 1/3 and smooth muscle for the rest
-food passes rapidly
-stratified epithelium (20-30 cells thick) protects it from rough food contents
67
What is the structure of the esophageal sphincters?
upper-skeletal muscle
lower-smooth muscle
they remain closed except when swallowing, vomiting and burping
68
what are the phases of swallowing?
69
What is heart burn?
Pain resulting in acid reaching the esophagus, it can:
-stimulate peristalsis
-increase saliva
and can occur from:
-an inefficient sphincter
-a big meal
-pregnancy
70
where is the stomach located and what is its function?
It is located between the esophagus and small intestine and helps:
-store food
-mechanically and chemically breakdown food
-control rate that food enters the small intestine
-secrete IF for b12 absorption
71
what components make up the stomach?
Fundus and body
-thin layer of smooth muscle that secretes mucous, pepsinogen and HCl
Antrum
-thicker smooth muscle that secretes mucous pepsinogen and Gastrin
72
What is an exocrine?
A chemical messenger that is secreted into the ducts then to an epithelial surface without passing into the blood
73
What are the three major exocrines in the stomach and their function?
Mucous-protects epithelium from self digestion
HCl- hydrolyzes proteins and sterilizes food
Pepsinogen- digests proteins
74
What are the minor secretions in the stomach and their functions?
IF - B12 absorption
Gastrin- endocrine that stimulates HCl production and stomach motility
Histamine-paracrine that stimulates HCl
Somatostatin-paracrine that inhibits HCl
75
What are the 6 major cells of the gastric gland?
mucous-secrete mucous
parietal cells (oxyntic cell)
-gastric glands in fundus/body
-canaliculi increase SA
- secrete HCl and IF
chief cells
-gastric gland in all regions
-secrete pepsinogen, an inactive precursor to pepsin (which accelerates protein digestion)
enteroendocrine
-gastric gland in antrum
-also known as G-cells
-secrete gastrin (HCl production and motility)
ECL
-Gastric glands in all regions
- secrete histamine (HCl stimulant)
D-cells
Gastric gland in all regions
- secrete somatostatin (decrease HCl)
76
How is acid secreted from parietal cells?
The use of many mitochondria allow for high energy required to secrete acid - more defined canaliculus structure shown when acid is secreted
77
How much HCl does the stomach secrete a day?
~2L of 0.1M
78
what is the pH in the lumen compared to the cytosol?
lumen-1
cytosol-7
79
What is the process of acidification in the stomach lumen?
1) H+/K+ ATPase
-luminal membrane
-H+ into lumen in exchange for K+ (active transport)
2)Carbonic anhydrase (CA)
-forms H2CO3 from H2O and CO2
-H+ and HCO3- are bi-products which get secreted into the lumen
3) Cl-/HCO3- exchanger (secondary active transport)
- HCO3- leaves, Cl- comes in
-maintains neutral pH
4) K+ channels
-K+ recycled back to lumen by diffusion
loss of positive charge compensated by Cl- secretion
5) Cl- channels
-Cl- enters lumen and binds with H to make HCl
-diffusion through channel
-compensated by loss of K+
80
What do chief cells secrete?
-Pepsinogen, an inactive precursor to pepsin
-it is stimulated by the enteric nervous system and parallels the release of HCl
81
What activates pepsinogen
Acidic pH of the stomach lumen (irreversibly inactivated when it enters the small intestine)
82
What are the three phases of gastric secretion?
1) Cephalic - excitatory mainly via vagus (sight, smell taste, etc.)
2) Gastric - excitatory mainly via vagus (food in stomach)
3) Intestinal (food in duodenum) - mainly inhibitory due to presence of acid, fat, digestion products and hypertonic solutions in the duodenum
83
What secretions directly stimulate the release of HCl from the parietal cell?
Histamine, Ach and Gastrin
84
How is acid production downregulated?
High [HCl] inhibits gastrin production and stimulates production of somatostatin
85
What does Somatostatin do to the parietal cell?
It inhibits the secretion of acid
86
What does Ach do ?
Increases Acid secretion by:
-inhibiting the secretion of somatostatin from D-cells
-stimulating the release of gastrin from G-cells which stimulates histamine
-Stimulating release of histamine from ECL
87
How much can the stomach increase without increasing pressure?
1.5L
88
What is the effect of peristaltic waves on the stomach?
food stimulates weak contractions in the stomach and powerful contractions in the antrum
89
What is the result of the closure of the pyloric sphincter?
-small amounts of contents released into the duodenum
-mixing of contents with enzymes and acid
90
What allows for contraction in the stomach?
Pacemaker cells with spontaneous depolarization and repolarization
91
What types of things may cause vomiting?
GIT disturbances, Motion sickness, Alcohol / toxins, pressure on CNS, etc.
92
What are some benefits and risks of vomiting?
Benefits:
-removal of harmful substances
-prevention of consuming harmful substances again
Risks:
-dehydration
-electrolyte imbalance
-erosion of tooth enamel
93
What are ulcers?
Damaged areas to the GIT mucosa caused by imbalance of agressive and protective factors (acid/pepsin vs mucus/HCO3-)
94
what are causes and treatments for ulcers?
Cause:
-Infection
-smoking
-alcohol
-stress
-gastrinomas
Treatment:
-Antibiotics
-Histamine
-Prostaglandin type drugs
95
Is the pancreas an exocrine or endocrine gland?
Both
The exocrine pancreas:
-produces secretions that go into the gut
-is a source for majority of enzymes required for meal digestion
-is critical for secreting HCO3-
-Produces enzymes in excess
The endocrine Pancreas:
-is not directly involved in digestion
-produces hormones that regulate the entire body (Insulin)
96
What is endocrine?
ductless gland that secretes across the epithelial basolateral surface into the blood
97
What is the structure of pancreatic ducts?
Very similar tp salivary glands
acinar cells produce and secrete digestive enzymes (exocytosis)
duct cells secrete H2O and HCO3-
98
What is the composition of pancreatic juices?
-Isotonic alkaline solution
-Electrolytes ( HCO3-, Na+, K+)
-Digestive enzymes (secreted by acinar cells)
99
How is HCO3- produced by Duct cells?
1) Cl - (CFTR) channel opens
2) Cl- in lumen exchanged for HCO3- in the cell
3) H2O and Na+ follow
4) exchange of H+ for Na+ maintains neutral pH of cytosol
-maintained through secondary active transport
100
What is the alkaline and acid tide?
After a meal:
-parietal cells in stomach produce acid and HCO3- is pumped across the basolateral surface into the blood (alkaline tide)
-Duct cells in the pancreas produce/secrete HCO3- and H+ is pumped across basolateral surface into the bloodstream (acid tide)
HCO3- and H+ from pancreas eventually meet up in the portal vein which help maintain acid-base balance
101
What does protease digest?
Proteins into peptides and AA
102
What do amylolytic enzymes digest?
Starch into sugars
103
What do lipases digest?
Triglycerides into free FA and monoglycerides
104
What do nucleases digest?
Nucleic acids into free nucleotides
105
Where are inactivated enzymes activated?
In the duodenum
106
What is enterokinase enzyme?
a brush border enzyme that cleaves trypsinogen into trypsin
-trypsin is a protease that activates other proteases
107
What 3 ways is autodigestion prevented?
1) storage of inactive enzymes
2)Secretion of trypsin inhibitors
3) Trypsin will degrade itself if activated prior to reaching the intestine
108
What mutation causes cystic fibrosis?
The Cl- channel in the pancreas involved in HCO3- secretion
-CFTR (Cl-) channel mutations create watery mucous instead of thick mucous in the lungs
109
What major proteases are secreted by the pancreas and what are they activated by?
110
What 3 active lipolytic enzymes are made from Prephospholipase A2?
111
What influence does CCK have on the pancreas?
-FA and AA in the small intestine trigger CCK secretion into the blood which increases digestive enzyme secretion and gall bladder contraction (bile release for fat breakdown)
-absorption of AA and fats stops CCK stimulation
112
What influence does secretin have on the pancreas?
-acid entering the duodenum from the stomach causes the pH to be reduced triggering the release of secretin
-the release of secretin stimulates liver and pancreatic duct cells to increase HCO3- secretion
neutralization of stomach acid stops secretin release
113
What influence does CCK and Secretin have on the Stomach?
They both inhibit gastrin secretion which reduces stomach motility (slows stomach emptying) acid secretion
114
What 3 phases are involved in pancreatic secretion?
115
how much of cardiac output does the liver receive?
25%
116
Where is bile produced and where does it go?
Produced in the liver and can go straight to the intestine or stored in the gallbladder
117
What is the structure of the hepatic lobes?
hexagonal with central vein through center and portal triads in the corners
-portal triads are composed of hepatic artery, portal vein and bile ducts
118
What are the three cell types in the liver?
Epithelial- hepatocytes ~70% and bile duct (cholangiocytes) 3-5%
Kupffer-macrophages
Endothelial-lining the sinusoids (fenestrated)
119
What are the major functions of the liver?
Exocrine gland - formation/secretion of bile
Metabolism/storage of nutrients - matches supply to demand (glucose/glycogen)
Deactivation/detoxification -drugs, hormones, waste, etc
Production of circulating proteins- blood coagulants, lipoproteins
120
What is bile made of?
121
What is the role of bile in fat digestion?
Bile is essential in order to emulsify large lipid droplets which prevents them from re-aggregating before they are digested
122
Why cant fat be digested without being emulsified?
Because pancreatic lipase is a water soluble enzyme and will only work at the surface of lipid droplets
123
What 2 things are required for emulsifictaion?
1) mechanical disruption (GI motility)
2) emulsifying agent to prevent re-aggregation
-bile acids
-phospholipids
124
What do bile acids form with phospholipids?
Mixed Micelles and products of lipase digestion (free FA and monoglycerides)
- these are soluble clusters of amphipathic molecules
125
What is the function of micelles?
They keep monoglycerides and fatty acids in small soluble aggregates
free forms of FA and monoglycerides are available to diffuse across the epithelium, micelles act as an intermediate (holding station) for insoluble lipids
126
How is bile formed?
127
How is bile recycled?
128
what is circulation of bile acids called?
enterohepatic circulation
129
How is bile circulated?
-Bile is reabsorbed at the ileum to allow for it to be present when needed
-secretion rate is able to greatly exceed synthesis
-95% of bile is recovered from circulation, ~0.5g gets lost in feces
130
What does dietary fiber do to bile acids?
It sequesters them preventing them from cycling back to the liver
-they get lost in the feces
this causes bile acids to be synthesized from cholesterol
131
Can drugs and toxins undergo enterohepatic circulation?
yes- can be beneficial (loperamide) or a disadvantage ( reduces bioavailability)
132
How is bile regulated during intestinal phase?
133
What are gallstones?
cholesterol stones- high [colesterol] in bile leads to cholesterol precipitating out
-cholesterol is water insoluble and kept in bile solution through micelles
pigment stones-excessive red blood cell breakdown (hemolysis) increases [bile pigment] -less common
134
What are consequences and treatment of gallstones?
135
What three sections make up the small intestine and their function?
duodenum - closest to stomach, mixes pancreatic digestive enzymes, absorbs nutrients and releases secretin/CCK
jejunum- digestion and absorption
ileum- closest to large intestine digests bile acids and vit B12
Digestion and absorption of protein, fat, carbs, electrolytes, water, mineral and vitamins-Majority of chyme entering small intestine is digested by the duodenum and jejunum (first 25%)
136
How is SA increased in the small intestine?
Folded lumen with microvilli
137
What cells are found in the villi?
Absorptive cells
goblet cells
endocrine cells
paneth cells
138
What cell types are derived from stem cells in the vili?
Absorptive- bring nutrients into the cell; brush border enzyme
Goblet-secretion of mucus onto the lumen, microvilli project into lumen
Enteroendocrine- releases hormones
Paneth- reduces bacterial load by secreting antibacterial proteins
139
What is a brush border enzyme?
an enzyme anchored to the brush border (microvilli of epithelial covering villi of small intestine) with catalytic activity in the lumen
-breaks down carbs and peptides for transport
140
What does sucrose break down into? what enzyme is needed?
glucose + fructose
sucrase (brush border enzyme)
141
What does lactose break down into? what enzyme is needed?
Glucose + Galactose
lactase (brush border enzyme)
142
How are carbs digested?
Starch---->maltose, maltotriose, alpha-limit dextrins by amylase
Maltose,maltotriose, alpha-limit dextrins------>Glucose by brush border enzymes (alpha-dextrinase)
must be broken down to monosaccharides for absorption
143
How is Glucose/galactose absorbed in the intestine?
Glucose must bind with Na+ on the cotransporter in order to pump glu/gal inside the cell; it can then be pumped into blood by the glu transporter
-the glu is able to enter blood due to the [gradient] created by the Na+/K+ ATPase
https://www.youtube.com/watch?v=8Fp91-ostBE
144
How is Fructose absorbed into the intestine?
Fru is diffused through the glu transporter; no active transport is needed
https://youtu.be/YT0klheJUxQ?si=c6Q_KkwIKhSTBKeY
145
Explain the pathway of a carbohydrate
146
What is the effect of lactose intolerance?
incomplete lactose digestion resulting in:
-decreased water absorption producing osmotic gradient
-lactose going into large intestine and being fermented by bacteria (gas)
147
What is the order of protein breakdown?
Protein---->pepsin by trypsin/chymotrypsin (proteases)
pepsin----> free AA by:
-carboxypeptidase (pancreatic protease)
-Aminopeptidase(brush border enzyme in small intestine)
-other brush border enzymes
148
How are proteins absorbed?
small peptides or AA are able to be absorbed
-secondary active transport coupled to Na+ for AA
-Secondary active transport coupled to H+ for small peptides; peptidases in cytosol then breakdown into AA
there are many secondary active and facilitative transporters for different AA
https://youtu.be/HL7-fYBtJ1Y?si=2TsjQ7_vtYNDkb-i
149
How is fat digested and absorbed?
-Fat is emulsified and then micelles form (smaller emulsified droplets). Lipase is then able to breakdown fat into FA and monoglycerides which diffuse across across the intestine
-Absorption by the enterocyte is followed by triglyceride formation in the ER, aggregation of triglycerides coated with amphipathic proteins allow lipid droplets to be secreted by golgi
-extracellular fat droplets are known as chylomicrons which are absorbed by the lymphatics
https://youtu.be/jud8LN1Scfg?si=Gg4e7dh9_FqZekLM
150
What do chylomicrons contain?
triglycerides, phospholipids, fat soluble vitamins and cholesterol
151
Why do chylomicrons not enter the capillaries?
They are large and lacteals in the lymphatic are leakier
-lymphatics enter into the systemic circulation
152
How is Fe stored in the enterocytes (intestinal lining cells)
It is incorporated into the protein ferritin
153
How is Fe transported in the blood?
It is attached to the plasma protein transferrin
154
How is Iron regulated in the intestine?
excess Fe increase ferritin production resulting in reduced absorption of Fe (increased storage)
depletion of Fe decreases ferritin resulting in increased absorption (decreased storage)
155
How is Fe excreted from the body?
There is no mechanism for Fe excretion so it accumulates on tissues
-can lead to toxicity; skin pigmentation and heart failure
156
Why is fluid control critical for GI function?
-to permit contact between food and digestive enzymes
-aid in diffusion of digested nutrients to absorption site
-fluidity needed for transit without damage to epithelium
157
How much fluid is handled by the GI tract each day?
8-9 L; majority is reabsorbed in the small intestine
-100mL lost in feces
158
Where is water absorbed vs excreted?
absorbed at the villi
secreted from the crypts
159
How is water absorbed?
The intestinal epithelium establishes an osmotic gradient and water follows through tight junctions (paracellular)
-Na+ gradients generated from secondary active transport
160
How is water secreted?
Cl- gradients generated by Na+/K+/2Cl- transporter (NKCC1)
161
What is cholera?
infection of vibrio cholerae bacteria from ingesting contaminated contents
-causes vomiting and diarrhea ~20L of stool per day (0.1L is normal)
-the bacteria produce a toxin that increase cAMP production resulting in excessive activation of Cl- channel and increased secretion of Cl- into gut lumen (water follows)
162
What is segmentation?
mechanical breakdown and mixing of food with digestive enzymes
163
What determines the frequency and force of segmentation contraction?
frequency set by basic electrical rhythm- more contractions in duodenum (12/min) compared to ileum (9/min)
force set by neurohormonal input
164
What occurs after segmentation?
peristaltic activity called the migrating myoelectric complex (MMC)
-pushes undigested material from small intestine to the large intestine
-prevents bacteria from remaining in the small intestine
165
How is the migrating myoelectric complex (MMC) regulated?
intestinal hormone motilin; released by cells in small intestine
-feeding will inhibit the release of motilin
166
What is the structure of the large intestine?
-shorter than small intestine but thicker
-ileocecal valve retains large intestine contents
167
What are the large intestine functional domains?
appendix - no function in humans
colon (ascending, transverse, descending, sigmoidal) - reabsorb water, store waste and indigestible materials
rectum - fece reservoir
anus- 2 sphincters to control defecation
168
How is SA increased in the large intestine?
Crypts only; SA is much lower than small intestine
169
What cell types are present in the large intestine?
stem cells- can differentiate into 4 different epithelial cell types same as small intestine
absorptive (enterocytes) - similar to small intestine except no brush border enzymes
Goblet cells- add mucous
few panath and endocrine cells; large bacteria ecosystem
170
What is the importance of gut microbiome?
Liberates short chain FA from dietary fibre (butyrate, propionate) which are then absorbed
produces vitamins that are absorbed
healthy gut has >1000 species
171
How is water absorbed in the large intestine?
same as the small intestine except there is no glucose dependent channel
172
How is water secreted in the large intestine?
Same as the small intestine
173
Describe motility in the large intestine
174
describe the defecation reflex
