Transport along the GI tract Flashcards
How does gastric accommodation occur?
→ Decrease in cholinergic activity
→ VIP / NO promote accommodation
Where do tonic contractions start from?
→ The top of the antrum
What does the stomach do to store food and why?
→ The proximal stomach relaxes to store food at low pressure while it is acted on by acid and enzymes mechanically
Why is gastric emptying carefully regulated?
→ It ensures adequate acidification/neutralization
→ Action of enzymes, mechanical breakdown
→ avoid swamping the duodenum
What is a disorder of gastric emptying?
→ Gastric stasis
What is gastric emptying dependent on?
→ Propulsive force generated by the tonic contractions of the proximal stomach
→ The stomachs ability to differentiate types of meals ingested and their components
What decreases the force and rate of gastric emptying?
→ Fatty, hypertonic and acidic chyme in the duodenum
How do liquids pass?
→ in spurts
How do solids pass?
→ Broken down into 1-2mm sizes
How do large indigestible materials pass?
→ Cleared by migrating motor complex
→ or vomiting
How are liquids emptied?
→ Rapidly disperse
→ Empty without a lag time
What is the rate of liquid emptying dependent on?
→ The rate of emptying is influenced by nutrient content
→ Nutrient rich liquids are retained for longer
How many phases are there in solid emptying?
→ 2 phases ( lag time and linear phase)
What is the duration of the lag time related to in solid emptying?
→ Duration of the lag time is related to the size of the particle
How are solids emptied?
→ Trituration of larger particles to smaller ones
→ Pylorus regulated the passage of material
→ (60 mins for a typical solid-liquid meal)
What is the order of highest gastric motility to lowest?
→ carbohydrates > proteins > fatty foods > indigestible solids
What are the determinants of gastric motility?
→ Types of food eaten
→ Osmotic pressure of duodenal contents
→ Vagal innervation upon over distension
→Hormones
What hormones inhibit emptying?
→ Somatostatin
→ CCK
→ Secretin
→ GIP
What does injury to the intestinal wall and bacterial infections cause?
→ Decrease in motility
How does the osmotic pressure of duodenal contents affect gastric motility?
→ Hyperosmolar chyme decreases gastric emptying
What is BER?
→ Intrinsic basic electrical rhythm
What does BER allow?
→ smooth muscle cell to depolarize and contract rhythmically when exposed to hormonal signals
What do the stomach cells produce from resting potential?
→ Electrical depolarizations
→ Move ripples towards the antrum
What control is the fundus under?
→ Vagal excitatory control
What is an ICC?
→ Cajal cells
→ Pacemaker cells in the wall of the stomach, small intestine and large intestine
What produces the BER?
→ Regular migrating ripples from the ICC
What are the depolarizations due to?
→ Entry of K+ and Ca2+
What mediates a decrease in fundic motor activity?
→ Cholecystokinin → Secretin → VIP → Somatostatin →Duodenal distension → Gastrin releasing peptide
What increases fundic contractions?
→ Motilin
What initiates and maintains peristalsis in the small intestine?
→ Hormonal and nervous factors
What decreases activity in the small intestines?
→ Secretin
What does antral over distension cause?
→ Inhibitory signals
→ Vago- vagal reflex
What does distension of the fundus cause?
→ Excitatory effects in the antrum
→ Antrum contracts
What happens during duodenal over distension and chemical stimulation?
→ Vago-vagal reflex and hormones
What does the pyloric sphincter contract in response to?
→ Antral or duodenal rhythms
What do fatty acids in the duodenum cause?
→ Contraction of the pylorus
What promotes the relaxation of the pyloric sphincter when the duodenum has no food in it?
→ NO and VIP
What causes the pylorus to relax?
→ Descending inhibitory reflex
What are the types of movements in the intestine?
→ Segmentation (mixing contractions)
→ Peristalsis
→ Migrating Motor Complex
→ Mass movements
What are the phases of motor activity in the small intestine?
→ Phase 1 : quiescent/ quiet period
→ Phase 2 : Irregular propulsive contractions
→ Phase 3 : Burst of uninterrupted phasic contractions
Where does segmentation originate?
→ In the pacemaker cells (ICC)
What is segmentation and why is it needed ?
→ Divisions and subdivisions of chyme
→ Brings chyme into contact with intestinal walls
What does segmentation cause?
→ Slow migration of chyme towards the ileum
How long are the contractions in the jejunum/duodenum?
→ 10-12 contractions per min
How long are the contractions in the ileum?
→ 8-9 contractions per min
What is peristalsis?
→ The propagating contraction of successive sections of circular smooth muscle preceded by relaxation/dilatation
What is the difference between segmentation and peristalsis?
→ Peristaltic contractions spread the food out allowing digestive enzymes to mix with it
→ Push food towards the anus (global movement)
→ Segmenting contractions primarily churn the food but also propel it towards the anus
When does the migrating motor complex occur and where does it start?
→ Between meals when the stomach/intestine are empty → Starts in the lower portion of the stomach
What is phase 3?
→ Burst of high frequency large amplitude contractions that migrate along the length of the intestine and die out
What are the functions of the MMC?
→ Intestinal housekeeper
→ Indigestible residues are moved out by large contractions and wide opening of the pyloric sphincter during phase III
→ Removes dead epithelial cells by abrasion
→ Prevents bacterial overgrowth
→ prevents colonic bacteria entering small intestine
What inhibits motility in the intestine?
→ Increased sympathetic activity
What stimulates motility in the intestine?
→ Increased parasympathetic activity
What decreases motility?
→ Pain and fear
What is the large intestine for?
→ Storage while water is absorbed from the contents
→ Intensive mixing and slow movement of waste and indigestible material aborally
What are the fermenting chambers for in the large intestine?
→ Hydrolysis of fibre and indigestible nutrients
What mixes the contents in the large intestine?
→ Segmental or haustral contractions
What are the features of motility in the large intestine?
→ Intensive mixing
→ Fermentation
→ Slow propagating aboral flow
What is mass movement?
→ Powerful contraction of mid-transverse colon that sweeps colon contents into rectum
What is diarrhoea?
→ Frequent discharge of liquid feces
→ 3x a day
What is constipation?
→ Difficulty/ constraint in opening bowels
What are adaptations of the small intestine to absorb food?
→ Epithelial folds
→ Villi + microvilli
→ Increase SA
What is absorbed when it enters the upper small intestine?
→ Dietary nutrients
→ Water
→ Electrolytes
What is in the crypt cells?
→ Stem cells
What are the two modes of transport across enterocytes?
→ Transcellular
→ Paracellular
How can carbohydrates be absorbed?
→ In the form of monosaccharides
What are complex carbohydrates reduced to and how?
→ Disaccharides and by amylases
What converts disaccharides to monosaccharides?
→ Brush border enzymes
What is the equation for the breakdown of sucrose?
Sucrose + sucrase → Glucose + fructose
What is the equation for the breakdown of lactose?
lactose + lactase →Glucose + galactose
What is the equation for the breakdown of glucose oligomers?
Glucose oligomers + glucoamylase → glucose
How are glucose and galactose absorbed?
→ rapidly
→ by a secondary active transport process
Describe the way glucose and galactose are transported?
→ On the basolateral side Na+ is transported into the blood
→ Gradient is created for Na+ to come into the lumen
→ Driving force of Na+ allows glucose to come with it
What is the glucose/galactose co-transporter called?
→ SGLT 1
What are the transporters for glucose and fructose called?
→ F - GLUT-5
→ G - GLUT-2
How are polypeptides produced from proteins?
→ Action of pepsin
How are di and tri peptides formed from polypeptides?
→ Action of pancreatic proteases
How are amino acids completely digested?
→ Di-peptidases in the brush border
How is pepsin secreted and how is it activated?
→ Secreted as a zymogen (pepsinogen)
→ HCl converts it into pepsin
How are amino acids transported?
→ Na+ coupled carrier system
→ Separate carriers for different types of AA
→ Some di+tri peptides are transported using an inwardly directed H+ gradient
What forms the majority of dietary lipids?
→ Triglycerides
What do triglycerides include?
→ phospholipids
→ Cholesterol
→Fat soluble vitamins (A,D,E,K)
What are dietary triglycerides broken down into?
→ Simpler units to facilitate absorption
Where are most TGs digested?
→ In the small intestine
What digests a small fraction of TGs?
→ Salivary lipase
What must TGs be dissolved in before they can be digested?
→ Aqueous phase
What is the digestion and absorption of lipids facilitated by?
→ Emulsification
→ Micelle formation
How much fat does gastric lipase break down and what is the remainder digested by?
→ 10-30% of fats
→ Pancreatic lipase
What does lipase action require?
→ Emulsification of TGs by bile salts
Where does pancreatic lipase bind?
→ To the surface of the small emulsion particles
What does the duodenum detect?
→ Lipids which must be emulsified
How do bile salts work?
→ They make the lipid molecules much smaller
→enzymes from the pancreas can break down the lipid and get to the core
How are lipids transported?
→ Micelles fuse with the gut mucosa → Resynthesis of TGs occurs on the SER → Lipoprotein is added → Chylomicrons are formed → This is transported and ends up in the blood →Goes through lymphatic system
Where do digestive components of lipids end up?
→ Do not end up directly in the blood they go via the lymphatic system
Where do bile salts go?
→ Recycled by enterohepatic circulation
What disorders can lead to fat malabsorption?
→ Crohns disease
→ Liver disease
→ Gallstones
→ Pancreatitis
What is one of the causes of steatorrhea?
→ Having gall stones
