Transport Along And Across The GI Tract Flashcards
What is emptying of the gastric resovoir controlled by?
Tonic contractions and peristaltic waves
What are tonic contractions controlled by?
Depolarisation of pacemaker and interstitial cells
Where do tonic contractions start/end?
Starts from the fundus and sweeps the length of the stomach
What does backflow into the stomach allow?
Churning of the digestor
How is churning initiated?
Thick muscular walls of the antrum contract
How does the stomach store food?
Proximal muscles relaxes to store food at low pressure
Means acids, enzymes and mechanical things can act on it
What is a disorder of gastric emptying?
Gastric stasis (gastroparesis)
Why is gastric emptying carefully regulated?
- avoid swamping the dueodenum
- allow enough acidification/ neutralisation
- allow enough mechanical breakdown
When are indigestible materials removed from the stomach?
Every so often a housekeeping process takes place
What is the sieving effect?
Viscous and solid matter are retained in the stomach (by the pyloric sphincter)
What is gastric emptying dependant upon? (2 things)
Propulsive force generated by tonic contractions of proximal stomach
Stomachs ability to differentiate types of meals ingested and their components
What food types affect gastric emptying?
Fatty, hypertonic and acidic chyme
Why do fats affect gastric emptying?
They need to be emulsified, so they take time to clear out of the duodenum for more stuff to come through
Why does acidic chyme affect gastric emptying?
Acid needs to be neutralised before travelling too far into the duodenum
What effect do fats, acids and hypertonic chyme have on gastric emptying?
Decrease the force and rate
How long is the lag phase for a liquid meal?
None
How long is the lag phase with semisolid pulp?
10 (ish) mins
How long is the lag phase with solid pulp?
20 (ish) minutes
What is the order of food types when compared to the rate of gastric motility?
Carbs > proteins > fatty food > indigestible solids
What effect does hyperosmolar chyme have on gastric emptying and why?
Decreases because it changes the osmotic pressure of duodenal contents
What does BER do?
When hormones are released, the smooth muscle can depolarise and contract rhythmically
What does BER stand for?
Basic electric rhythm
What hormones inhibit gastric emptying?
Somatostatin, secretin, CCK and GIP
What two things can lead to decreased gastric motility?
Injury to intestinal wall and bacterial infections
Vagal innervation when the duodenum is over distended
What do stomach muscle cells do?
Produce electric depolarisations from resting potential
What type of control is the fundus under?
Vagal excitatory control
How often do ripples occur at BER?
3 waves/ min
What chemicals control gastric motility?
Cholecystokinin (CCK) Secretin VIP Somatostatin Gastrin releasing peptide
How do the chemicals (CCK, VIP, secretin and somatostatin) affect fundic motor activity?
Decrease
What chemical increases fundic contractions?
Motilin
What chemicals increase colonic/ intestinal motility?
Cholecystokinin (CCK), gastrin and motilin
What chemical decreases intestinal/ colonic motility?
Secretin
What feedback system does gastric emptying work under?
Negative feedback systems
What stimulates the vagovagal reflex?
Antral and duodenal overdistension and duodenal chemical stimulation
What contracts in response to antral or duodenal rhythm?
Pyloric sphincter
What food group in the duodenum cause contraction of the pylorus?
Fatty acids
What are the four types of motility in the intestine?
Segmentation, peristalsis, migrating motor complex and mass movements
Where does peristalsis take place?
Stomach
What is segmentation in the intestine?
Mixing contractions - stationary contraction and relaxation
What are the three phases of motor activity?
1- quiescence/ quiet period
2- irregular propulsive contractions
3- burst of uninterrupted phasing contractions (peristaltic rush)
Where does segmentation originate from?
Pacemaker cells (ICC)
What does segmentation cause?
The slow migration of chyme towards the ileum
What are the two layers of intestinal muscle and which is where?
Longitudinal (outside)
Circular (inside)
What do each of the intestinal muscles do behind the bolus of food?
Circular contracts behind the bolus of food and the longitudinal relaxes
What is the migrating motor complex?
Highly organised cyclically recurring sequence of events
When does the migrating motor complex occur (MMC)?
Between meals when the stomach and intestine is empty
Where does the migrating motor complex (MMC) start?
Lower portion of the stomach
What does the migrating motor complex (MMC) involve?
Burst of high frequency, large amplitude contractions that migrate along the length of the intestine and die out
What is the interval between different phase 3’s in the migrating motor complex (MMC)?
90-120 mins
What are the 3 functions of the migrating motor complex (MMC)?
- Removes dead epithelial cells by abrasion
- prevents bacterial overgrowth
- prevents colonic bacteria from entering the small intestine
What are contractions coordinated by in the migrating motor complex (MMC)?
Pacemaker cells under the control of the enteric nervous system
Where are contractions initiated from in the MMC?
Vagus nerve in the upper tract
What inhibits the release of motilin?
Feeding
What does ICC stand for and what are they?
Interstitial cells of Cajal
Cell types found in the submucosa and submucosal plexus
What do ICCs do?
Form synaptic connections with GI smooth muscle fibres, due to their complex network between the smooth muscle layers
How is motor activity in the small intestine (while fed) controlled?
Locally by the ENS but can be enhanced or suppressed by extrinsic innervation
What effects does increased sympathetic activity have on motor activity in the small intestine?
Inhibits motility
What effect does increased parasympathetic activity have on motor activity in the small intestine?
Stimulates motility
What outside factors can affect motility in the small intestine (and which way)?
Pain and fear decrease motility
Where are fermenting chambers found?
Large intestine
What do fermenting chambers do?
Allow the hydrolysis of fibrous and indigestible nutrients
What are the three steps in the motility of the large intestine?
Haustral contractions, peristalsis and mass movement
What are the features of haustral contractions?
Intensive mixing, fermentation and slow propogation/ slow aboral flow
What initiates peristalsis in the large intestine?
Distension
How does colonic evacuation happen?
Powerful contractions of the mid -> transverse colon that moves colon contents into rectum
What’s the cause of diarrhoea and constipation?
Disorders of motility, fluid secretion and absorption
What is diarrhoea?
Frequent (>3 times/day) discharge of liquid faeces
What is constipation?
Difficulty/ some constraint in opening and emptying bowels
What are the two intrinsic neuronal plexuses in the gut?
Submucosal (meissner) plexus
Myenteric (Auerbach) plexus
What does the meissner plexus do?
Regulates the digestive glands
What does the Auerbach plexus do?
Connected with gut motility
What are the intrinsic controls of the gut?
Nerve plexuses near the GI tract initiate short reflexes
Short reflexes are mediated by local enteric plexuses
What are the extrinsic controls of the gut?
Long reflexes arising within or outside the GI tract
Involve CNS centres and extrinsic autonomic nerves
Parasympathetic reflexes
What are the three adaptations of the small intestine for absorption?
Epithelial folds, villi, microvilli
What are crypt cells important for?
Signalling and defence
Where are the stem cells in the small intestine?
Crypt cells
What is transcellular?
Transport through a cell
What is paracellular?
Transport between cells
What are the features of paracellular transport?
Passive, selective, variable and regulated
How can carbs be absorbed?
Only as monosaccharides
What breaks complex carbs down?
Amylase
What converts disaccharides to monosaccharides?
Specific brush border enzymes (sucrase)
What converts glucose oligomers to glucose?
Glucoamylase
How are glucose and galactose absorbed?
Secondary active transport process (sodium glucose pump transport mechanism thingy)
How is fructose absorbed?
GLUT5 and GLUT2
How are proteins broken down into polypeptide chains or oligopeptides?
Pepsins or pancreatic proteases
How are amino acids transported?
On a sodium coupled carrier system (like the glucose one)
How are some di and tri peptides transported?
On a carrier system that uses an inwardly directed H+ gradient
What happens once the amino acid is in the cell?
It’s really easy to transport it into the capillary as they have specific transporters
What % of dietary lipids do TGs make up?
90%
What is the other 10% of non-TG dietary lipids made up of?
Phospholipids, cholesterol, fat soluble vitamins (A, D, E, K)
Where are most dietary TGs digested?
In the small intestine
What is the solubility of TGs?
Insoluble
What must happen before triglycerides can be fully digested?
Must be dissolved in the aqueous phase
What are the two phases in the digestion and absorption of lipids?
Emulsification and miscelle formation
What % of fats are broken down by gastric lipase?
10-30%
What are the two enzymes that break down fats?
Pancreatic and gastric lipase
What does lipase action require?
Bile salts
How does pancreatic lipase work?
Binds to the surface of the small emulsion particles
How are miscelles absorbed?
Fuse with the brush border and the mono and triglycerides are emptied into the enterocyte
What happens to the TGs once they’re inside the enterocyte?
Resynthesised and aggregate. They get surrounded by apolipoproteins and form chylomicrons
What happens once chylomicrons have been made in the enterocyte?
They are extruded into the lymphatic system via the lacteals, then into the blood via the thoracic duct
Where can you find the thoracic duct?
Near the subclavian vein
What causes the lymph drainage from the small intestine to appear milky?
Lots of lipids being absorbed -> lots of chylomicrons
What is steatorrhoea?
Excess fat in faeces
What illnesses can lead to steatorrhoea?
Gallstones, pancreatitis, crohns and liver disease
