GIT Flashcards
What afferents are responsible for vomiting?
- Pharyngeal Stimulation (fingers at back of your throat)
- GIT or urogenital distension
- Pain, ardiac ischemia (ex: child birth)
- Biochemical disequilibrium
- Vestibular signals
- Psychogenic factors (ex: smell)
What are the EFFerents responsible for vomiting?
- Widespread Autonomic Discharge + Nausea
- Retching (ineffective contraction of abdominal muscles
- Emesis (actual vomiting)
What are the functions of the upper small intestine
- Neutralization
- Osmotic Equilibration
- Digestion
- Absorption
What are the activities of the small intestine?
- Effective mixing
- Slow Propulsion (2-6 hours) to properly absorb all nutrients
*Because more contractions and more vigorous in the proximal SI
How does the BER change along the small intestine?
Decreases from 12/min in the duodenum to 8/min in the Ileum
Compared to 3/min in the stomach
*More stretch = more spikes
What are the difference between the proximal and distal small intestine ?
- frequence is greater in proximal
- Excitability of smooth muscle is greater in proximal
- Thickness of smooth muscle in greater
So frequency and amplitude are greater in proximal
What is the most common type of contractile activity of the SI, the stomach and the esophagus?
Stomach and Esopagus = Perstalsis
SI = segmentation (circular muscles)
How is peristalsis in the SI described?
Ingrequent, irregular
Weak, shallow
Travels for short distances only (few cm)
What does the law of the intestine state?
When Radial Stretch → receptors → Neurally mediated → circular muscles ahead of BOLUS relax + longitudinal contract (shortens the tract and reduces resistance ahead)
AND
Contraction of ciruclar muscles behind so doesn’t go backwards + relaxation of longitudinal (makes tract longer, more resistance if longer)
What are the functions of the colon?
- Mixing (absorb water and ions)
- Propulsion (SLOW)
- Storage (50-60h)
How si the BER in the Colon ?
IRREGULAR
Which reflexes occur in the colon following a meal?
TRIANGLE DE FEU!!
Gastroileal reflex
Gastrocolic reflex
Ileocolicreflex
What are the phases of the MMC (migrating myoelectric motor complex)?
In the distal stomac and SI ONLY
Phase 1 (60min): no spike potentials, no contraction (nothing happening)
Phase 2 (20min): Irregular spike potentials, contraction
Phase 3 (10min): Regular spike potentials, contraction (Strongest of the phases)
*All cells coordinate to creat a wave of contractions
What 3 types of enzymes are secreted on the GIT?
Amylases
Proteases
Lipases
What are the 2 patterns of regulation of secretions?
Nervous (ANS
Hormonal (Gut peptides)
What are the secretions in the mouth?
Salivary glands:
- Parotid gland → Serous fluid
- Submandibular → Mixed fluid
- Sublingual → Mucin-rich fluid
Chemical digestion:
salivaire amylase to begin carbohydrate digestion
Tongue produces lingual lipase
What does the Saliva composition look like?
Volume: 0.5-1.5 L/day
ions: Na+, K+, Cl-, HCO3-
HYPOTONIC (more H2O)
pH = 6.5-7.0
Amylase (Ptylin):
Starch (Polysaccharides) → Maltose (disaccharide)
Mucin
Lipase
Lysozyme
How are the salivary glands activated/regulated?
Salivary Centers in Medulla → efferents (parasympathetic supply) → Salivary glands
*Sensory receptors in mounts = afferents that activate salivary centers + Eye, Nose, etc. → High centers → Salivary Centers
What are the 3 phases of secretion?
Cephalic : Phsychic (conditionned reflex) + Gustatory
Gastric/intestinal
What is the composition of the mixed gastric juice?
1.5-2 L/day
Isotonic fluid: Na+, K+, Cl-, !! H+ !!
pH 1-2
Pepsinogen
Intrinsic Factor
Mucin
Which different tubular glands secrete what kind of fluid in the stomach?
Cardiac (top entry of the stomach) and Pyloric (exit of the stomach/ in the antrum (?)) tubular glands secreted alkaline, mucin-rich fluid
Fundus and Body tubular glands secreted acid, enzymes and Intrinsic factor
What are the 3 types of cell in the tubular glands of the fundus and corpus? (of the stomach)
Parietal cells (oxyntic):
Secrete HCl, filled with mitochondrias bc need high energy to produce HCl against gradient
Hav eintracellular canaliculus (invagination inside the cell)
+ produces Intrinsic Factor!!
Chief cells:
Secreted pepsinogen (inactive), will be activated when gets in the lumen
Mucus neck cells:
Secrete mucin
*Surface eptithelial cells (not in tubules but still important):
Secreted HCO3-, mucus
How is HCl produced in parietal cells?
Carbonic anydrase produces HCO3- and H+, HCO3- is returned into the capillaries and H+ is pumped by H+/K+ ATPase into the tubular gland lumen
*H+/K+ ATPase is only found in the parietal cells, nowhere else in the body
*Whole process increase alkalinity in the venous blood bc returns a HCO3-
REVIEWWWWWW
Isotonic : 150 mEq of H+ and 150 mEq of Cl- is secreted
What are the functions of HCl in the stomach?
- Precipitate soluble proteins (allows them to remain longer in the stomach to be better absorbed)
- Denatures proteins (more readily digested)
- Activated pepsin + provides optimal pH for pepsin activity
What is the role of pepsin?
Pepsinogen is activated by pepsin to become pepsin (autocatalysis) with help of HCl acid condition (pH < 6)
Protein → Polypetides (with the help of pepsin and pH of 2-3)
What is the intrinsic factor?
Only secretion of the stomach essential to life
- A glycoprotein
- secreted by parietal cells
- Required for absorption of vitamine B12 in the Ileum!
Forms IF-B12 complex
Intrinsic factor definiency → Pernicious Anemia
What cells secreted mucin in the stomach?
- All surface Epithelial cells
- Cardiac and Pyloric tubular glands
- Mucous neck Cells (fundus and corpus)
What is the role of mucin?
It form a mucous gel that neutralizes the H+ that want to reach the epithelial cell surface
When H+ enters the mucous gel, a HCO3- is absorbed in the mucous gel from the epithelial cells and neutralized the H+ by reversed carbonic anhydrase
*Mucous gel also called Muci-Bicarb Layer
H+ could still cross the membrane by would hit the Gastric Mucosal Barrier (GMB) apical surfaces and tight junctions which is impermeable to H+ ions ONLY in the stomach
+ Rapid cell turnover (re-epitheliazation) particularly in small intestin and stomach where cells easily get damaged
What factors contribute to Ulcers?
Weak barrier (with normal HCl output) ex: Aspirin and NSAIDs weaken the barrier
Excessive HCl output (Normal barrier) ex: Gastrin-producing tumors
What is the Cephalic phase of gastric secretion responsible for?
Vagal input stimulates secretion from Parietal, Peptic and Mucous cells + VASODILATATION
(Vagal-vagal reflex can be caused by distension afferent from receptor)
Sympathetic branch inhibits secretion and causes vasoconstriction
*Neural regulation
How does HORMONAL regulation of secretions in the stomach occur?
Secretagogues: amino acids or partially digested proteins which act on gastrin-releasing cells G-cells
Secretagogues → Gastrin → cardiovascular system →activate Parietal cells → HCl
What is the importance of gastrin?
How is it secreted?
Gastrin = peptide hormone released by endocrine cell in antrum (G-cells) in response to:
- Secretagogues
- Local enteric reflexes (ex stretch in the antrum)
- Vagal-vagal reflexes
How is Gastrin secretion regulated?
SELF-REGULATED:
in atrum, G-cells produce gastrin How is it regulated → circulation → Parietal cells → HCl → activates Pepsinogen to pepsin + optimal pH → secretagogues bc protein breakdown → negative feedback
Low pH → release of Somatostatin → inhibition of G-Cells and Parietal cells
What are the roles of Histamine?
Lots of Histamine in gastric mucosa
Histamine elicits large volumes of gastric juice with lots of HCl
*Parietal cells have gastrin receptors, ACh receptors AND Histamine receptors (seperate) and they have cooperative effect on each other by increasing each other’s affinity
Histamine is cstly released and bounded to parietal cells as tonic background to sensitize them to ACh and Gastrin
When Histamine receptors blocked with H2-recepetor blockers, inhibits acid secretion in response to ACh and Gastrin as well
*H2-receptor blockers used to decrease HCl secretion
*H+/K+ ATPase blockers as well (even more efficient)
What factors inhibit the gastric secretions in the intestinal phase?
- Distension in the SI
- pH < 3.5 (in SI??)
- High Osmolarity (too much enzymes working to break down too much food)
- Chemical composition
- Fats»_space; Prots»_space; Carbs
What pre-intestinal changes occur when a meal is ingested?
- Meal reduced to semi-liquid → CHYME
- Acidified, osmotic pressure unchanges
- Limited digestion:
Some polysaccharides → Disaccharides (Salivary amylase ptyalin, pH = 7)
Some proteins → polypeptides (Gastric pepsin)
Lipids → Di-, monoglycerides, fatty acids
What are the functions of the upper intestine?
CHYME neutralization (secrete cells and alkaliine juices)
Osmotic equilibration (will be isoosmotic when reach colon)
Digestion continues
Absorption begins
What is the composition of the pancreatic juice?
Volume : 0.5-1.9 L/day
Isotonic: 300 mOsm
Main Electrolytes: Na+, K+, Cl-, HCO3-
pH 7.2 - 8.2
*Pancreas produces most powerful + largest qties of enzymes
Enzymes: 3g% protein Amylase, Protease, Lipase
What are pancreatic proteases:
Proenzyme not active until cleaved at the level of the small intestine
Trypsinogen → (enterokinase) → Trypsin → reactivates enterokinase (positive feedback!!)
*All these activations occur in SI:
Chymotrypsinogen → (trypsin) → Chymotrypsin
Proelastase → (trypsin) → elastase
Procarboxypeptidase →(trypsin) → Carboxypeptidase
Pancrease also produces Trypsin inhibitor which inactivates trypsin
What are the different pancreatic lipase?
Pro-colipase → (trypsin) → colipase (SI)
Triglyceride → (colipase + pH 8) → Fatty Acids, Di-, Monoglycerides
*Fat is insoluble in water so Bile salts help break down?
What are the functions and secretions of the Liver?
Largest gland of the body
Functions:
Storage, synthesis, detoxification, metabolism
Secretes bile for heptic duct:
- bile travels in the common bile duct
- bile released into SI at Spincter of Odd as pancreatic juice
*When spincter of Oddi is not open, juice stored into the Gallbladder
What is the Liver Bile composition?
Volume : 0.5-1.0 L/day
Isotonic fluid: Na+, K+, Cl-, HCO3-
pH: 7.8-8.2 helps neutralize acidic chyme
Solids (3% but no digestive enzymes):
- Bile acids (Bile salts)
- Bile pigments (hemoglobulin breakdown products)
- Cholesterol
- Phospholipids
*Continuous secretion by the liver
*Entranceof bile into duodenum is intermittent
Volume entering SI < 500-700 mL/day
What are the functions of the Gallbladder?
Concentrate Solids:
- hepatic bile 3%
- Gall Bladder Bile 10-20%
- Viscosity increases
Reduces pH:
Hepatic Bile 7.8-8.2
Gall Bladder bile 7.0-7.5
*Gallbladder size = 50-100 mL (small volume → concentrate the fluid)
*Gallbladder does NOT synthesize bile salts, it stores and concentrates them
What is a Cholecystectomy?
gall bladder removal
Might not have enough bile salts being produced by the live so cholesterol might precipitate and for gall stones which blocks cystic duct (hurts)
As long as no rlly fatty meal, no problem, liver secretes enough bile salts
What are the roles of the bile salts?
Where are they synthesized?
Synthesized in the liver from cholesterol
Bile acids have a non-polar surface (on the fat side) and a polar surface (on the water side) to help transport and digest fats and fat-soluble vitamines (A, D, E, K)
Functions:
Formation of Micelle (contenant de gras qui sont water soluble bc outside is polar)
- facilitate digestion, transport and absorption of FAT (including cholesterol) + FAT soluble vitamins (A, D, E, K)
- Reduce surface tension (of lipid droplets)
- Help form stable emulsions
Transport = from SI lumen to intestinal cells (absorption?)
*Keeps cholesterol in solution not in precipitates
Bile Salt pool: 3.5g
Daily synthesis: 0.5g
Daily release in intestine: 15-20g (pool recirculated several times/day between Liver → (sphincter of Oddi) GIT (SI) → (circulation) Liver) or by hepatic portal veins
*Most is reabsorbed in the portal blood and returned to the liver via ENTEROHEPATIC CIRCULATION (EHC)
How is secretion of bile salts regulated?
The more Bile salt returned via portal blood, the larger the volume of bile secreted and the smaller the amount of NEW Bile Salt secreted
*POSITIVE FEEDBACK
What are the intracolonic functions of bile salts?
Inhibit Na+ transsport and H2O absorption, we want it to absorb water and nutrients but not too much!
If excess bile salts → Diarrhea
What are the Intraportal, Intrahepatic, Intraintestinal and Intracolonic functions of bile salts?
Intraportal : regulate volume of bile secreted by liver + regulate synthesis of new bile salts
Intrahepatic: keep cholesterol in solution
Intraintestinal (SI): emulsify and transport fats
Intracolonic: prevent too much water absorption
What are the possible secretions of pancreas?
2 different types of cells:
1. large volume of juice rich in HCO3- (if you just had a very acidic meal)
2. Small volume of juice rich in enzymes (if you just had a protein and fat rich meal)
What agents regulate Bile Flow?
What is the Law of reciprocal activity?
Choleretics: agents which cause the liver to secrete a large volume of bile
Cholagogues: agents which cause an increase in Gall Bladder emptying
Law of reciprocal activity:
If the Gall Bladder is contracted the Sphincter of Oddi will be relaxed and vice versa
Where does the final stage of digestion occur?
Mediated by intestinal enzymes produced by mucosa at the site the absorption occurs
What are crypt cells?
In the SI
Crypt cells → Succus Entericus → alkaline fluid, 3 L/day produced (No digestive enzymes)
Isotonic secretion: Na+, K+, Cl-, important source of HCO3-
pH: 7.5 - 9
What are the roles of the Villi Cells
Villi do not secrete fluid!
*Complete digestion and absorb nutrients and fluids
Enterocytes in Villi → synthesize digestive enzymes → remain in brush border
small intestine enzymes ex: Enterokinase (lumen) which changes trypsinogen → trypsin
Each villus has capillary loop (carbs absorption) and lacteal (fat absorption)
What is the composition of Colonic secretions?
- Small volume
- Alkaline: 100 - 150 mEq/L of HCO3- and of K+
- LOTS of mucin
No digestive enzymes, No absorption of Nutrients
(Digestion and absorption is completed in the small intestine)
Bacterial Activity
What are 3 ways of regulation of intestinal secretions?
- Local enteric reflexes
- Vagal-vagal reflexes
- Hormonal factors
What conditions allow everything to be absorbed in the small intestine?
- very large surface area of the SI
- Intimate contact with blood vessels
*SI = only GI organ essential to life
What are the postprandial blood flow to the intestine and the lymph flow?
Blood flow = 1-2 L/min
Lymph flow = 1 - 2 mL/min
Each villus has a capillary loop and lacteal for efficient absorption
Where are the following absorbed?
Iron
Ca++
CHO (Organic molecules)
Proteins
Lipids
Na+
H2O
Vitamin B12
Bile acids
Iron → duodenum
Ca++ → duodenum
CHO (Organic molecules) →in all SI but mostly in duodenum
Proteins → in all but mostly in duodenum
Lipids → in all but mostly in duodenum
Na+ → in all but mostly in duodenum
H2O → in all but mostly in duodenum
Vitamin B12 → ONLY in Illeum
Bile acids → a tiny by in duodenum, a bit more in jejunum and almost all in Illeum
By which mechanisms does absorption take place?
- Simple Diffusion
- Facilitaed diffusion
- Active transport
- Pinocytosis
- Osmosis → Water always follows the osmotic gradient generated by the movement of ions and nutrients
What are the 4 requierements for absorption?
- Adequate digestion (enzyme activated, optimal pH)
- Adequate Site
- Adequate transit time
- Adequate co-factors/transporters
What are the protective mechanisms of the GIT? (7)
- Mucin
- Inactive protease, trypsin inhibitor
- gastric Mucosal Barrier
- Spincter prevent reflux
- Negative feedback inhibition of gastrin
- Neutralization of duodenal contents
- MMC (housekeeping)
What is the efficiency of absorption of Carbs, fats and proteins?
Carbohydrates = 99%
Fats = 95%
Proteins = 92%
*high efficiency due to effective coordination of activities
neural, hormonal, motor, secretory, enzymatic
