GUT alimentary transport 1: along the GI tract Flashcards
what are the 2 mechanisms that help with emptying the gastric reservoir?
The transport of undigested food from the gastric reservoir into the antral pump is caused by 2 mechanisms:
-Tonic contractions
-Peristaltic waves
How does storage of food occur?
the proximal stomach relaxes to store food at low pressure whilst it is
acted upon by acid, enzymes and mechanically
Why is emptying of partially digested food regulated?
i. Neutralisation of acidic chyme
ii. Emulsification of fats
iii. Appropriate functioning of pancreatic enzymes
iv. Mechanical breakdown
v. Too much volume is not handled by the duodenum to avoid swamping it
Steps involved in gastric motility and emptying
-Gastro-gastric reflex
-Pyloric activity
-Gastro-gastric reflexes provide balance between gastric reservoir and antral pump. Distension of the reservoir stimulates antral contractions. Distension of the antrum enhances and prolongs relaxation of the reservoir
-Pyloric activity is modulated by antral inhibitory and duodenal excitatory reflexes
a)Duodenal stimuli allows ascending excitatory reflex causing pyloric contraction and increasing tone
-presence of acidic chyme is important
b)Descending inhibitory reflex causes pyloric relaxation
-via transmitters NO/VIP
What feedback system regulates gastric emptying?
Gastric emptying is regulated by negative feedback systems
What reflex is activated due to antral over-distension?
Vago-vagal reflex
What reflex is activated due to duodenal over distension and chemical stimulation?
Vago-vagal reflex and
hormones
What does the pyloric sphincter contract in response to?
The pyloric sphincter contracts in response to antral or duodenal rhythm
What does fatty acids in duodenum cause?
The pyloric sphincter contracts in response to antral or duodenal rhythm;
fatty acids in duodenum cause contraction of pylorus
What neurotransmitter involved in all preganglionic vagal efferents in both accomodation and emptying?
Acetylcholine and VIP/NO
What is gastric emptying dependent upon?
- Propulsive force generated by the tonic contractions of the proximal
stomach - Stomach’s ability to differentiate types of meals ingested and their
components
What are the effects on gastric emptying of fatty,
hypertonic, acidic chyme in the duodenum?
decrease the force and rate of gastric emptying
When is particle size a limiting factor in gastric emptying?
Particle size is a limiting factor in the fed but not fasting state
How do liquids pass in gastric emptying?
Liquids pass in spurts
What are solids broken down into in gastric emptying?
Solids are broken down to 1-2mm sizes
What happens to large indigestable material?
Large indigestible materials remain; cleared by MMC or vomiting
What are the 2 phases for solids being emptied into duodenum?
2 phases (lag time and linear phase); duration of lag time is related to size of particle
What is emptied and what remains in proximal stomaach?
Liquid part emptied and solid component is retained in proximal stomach
What regulates passage of material?
Pylorus regulates passage of material
What is the rate of emptying of liquids influenced by?
Rate of emptying is influenced by nutrient content (nutrient-containing
liquids retained longer)
what do fatty foods, when being emptied into the duodenum, liquefy at and at what rate do they empty?
Liquefy at body temperature; float on top of liquid layer and empty slowly
What are fats potent inhibitors of?
Fats are potent inhibitors of gastric motor events and gastric emptying
When is indigestible solids not emptied?
Not emptied in immediate post-prandial period
What is needed to empty indigestible food?
Migrating motor complex (MMC) activity is needed to empty them
How does hyperosmolar chyme impact gastric emptying?
hyperosmolar chyme ↓ gastric emptying
What does vagal innervation upon over-distension do to gastric motility?
Vagal innervation upon over-distension ↓ gastric motility
What do hormones like somatostatin, secretin and CCK do to gastric emptying?
Hormones (somatostatin, secretin, CCK): inhibit emptying
What does injury to intestinal wall and bacterial infections do to gastric motility?
Injury to intestinal wall and bacterial infections ↓ motility
What are the steps involved in myogenic control of gastric motility?
- Intrinsic basic electric rhythm (BER)
- Stomach muscle cells produce electric depolarisations from resting
potential - Ripples move towards the antrum
- Slow waves from ICC – regular recurring migrating ripples (3
waves/min) known as BER (a rhythm of depolarisation-
repolarisation)
What neurohormones mediate a decrease in fundal motor activity?
- Cholecystokinin (CCK)
- Secretin
- VIP
- Somatostatin
- Duodenal distension, duodenal acid
What increases fundic contractions?
Motilin increases fundic contractions
How is movement through small intestine controlled?
-Hormonal and nervous factors initiate and maintain peristalsis and mixing
-Localised distension of the duodenum
What stimulates contraction in small intestine?
CCK, gastrin and motilin
What inhibits contraction in small intestine?
Secretin inhibits contractions
What speed small bowel transit?
CCK and motilin speed small bowel transit
What does motilin cause, during fasting, in the small intestines?
Motilin causes the cyclical bursts of gastroduodenal contractions during
fasting
What are the contraction characteristic of the fed state caused by in the small intestine?
The contractions characteristic of the fed state are caused by gastrin and
CCK
What do VIP and glucagon do to motility in small intestine?
VIP and glucagon slow transit/decrease motility
What are the different forms of motility in the intestine?
-Segmentation (mixing contractions): stationary contraction & relaxation
* Peristalsis (propulsive): in stomach (3 waves/min)
* Migrating Motor Complex
* Mass movements (evacuation)
What are the three phases of motor activity?
- Phase 1: quiescence/quiet period
- Phase II: irregular propulsive contractions
- Phase III: burst of uninterrupted phasic contractions (peristaltic rush)
Where does segmentation originate?
Originates in the pacemaker cells (ICC)
What does segmentation do to chyme and what does it bring it in contact with
divisions and subdivisions of chyme, bringing chyme in contact with intestinal walls
What does segmentation cause?
Segmentation causes the slow migration of chyme towards ileum
How many contractions are there in segmentation in the duodenum/jejunum?
10-12 contractions/min
How many contractions are there in segmentation in the ileum?
8-9 contractions/min
What is peristalsis?
Peristalsis is a propagating contraction of successive sections
of circular smooth muscle preceded by a dilatation
What is the summary of gastric emptying?
1.Presence of fatty, hypertonic, acidic chyme in duodenum
a)This stimulates duodenal entero-endocrine cells which allows for the secretion of secretin, CCK and GIP.
-Causes contractile force and rate of stomach emptying to decline
b)chemoreceptors and stretch receptors are stimulated which target via
-short reflexes with enteric neurons and long reflexes through the CNS centers which increase SNS activity and decrease parasympathetic activity. Both of these Causes contractile force and rate of stomach emptying to decline
When does MMC occur?
Occurs between meals when the stomach / intestine are “empty”
What phase of motor activity is of interest during MMC?
Only phase III is of interest
What is MMC?
Burst of high frequency, large amplitude contractions that migrate along the
length of intestine and die out
What is the interval between phase 3 ?
Interval between phase IIIs is 90-120min
Where does MMC start and go?
Starts in lower portion of stomach
Antrum → duodenum → jejunum → ileum
What does MMC move out of the stomach?
Indigestible residues moved out of stomach by large contractions and wide
opening of the pyloric sphincter during phase III
What does MMC remove?
Removes dead epithelial cells by abrasion
What does MMC prevent?
- Prevents bacterial overgrowth
- Prevents colonic bacteria from entering small intestine
What does segmentation in the small intestines during fed state do?
segmentation, mixes and stirs contents with enzymes,
prevents unstirred layer formation
What does peristaltic contraction in the small intestines during fed state do?
Peristaltic contractions (slow waves): these move the contents in an oral to
anal direction (law of the gut) - local reflex mediated via ENS but can be
enhanced or suppressed by extrinsic innervation (i.e.
parasympathetic/sympathetic); ↑sympathetic and parasympathetic
inhibit and stimulate motility, respectively
What does pain and fear do to motility?
Pain and fear ↓ motility
Where does absorption of water and ions occur in the large intestine?
Ascending colon
Where does bacterial fermentation occur in the large intestine?
Descending colon
Where is the storage of waste and indigestible material?
Sigmoid colon
Where is the elimination of waste in the large intestine?
Rectum
What are the different modes of motility of the large intestine?
- Segmental or haustral contractions-mix contents/ key role for
taenia coli longitudinal muscle - Peristalsis: slow in large intestine in comparison to small intestine; moves
contents towards the anus; distension initiates contraction
3.Mass movement: powerful contraction of mid-transverse colon that sweeps
colon contents into rectum (responsible for colonic evacuation).
What is diarrhoea?
watery stool; increased frequency of bowel emptying -
abnormal water absorption in the intestine
What is constipation?
reduced frequency and difficulty in opening bowel;
hardened faeces
