Introduction to the alimentary tract Flashcards
What are the digestive functions of the stomach?
- Accommodation & storage
- Mechanical and enzymatic breakdown
- Slow delivery of chyme to the duodenum
What is stored in the stomach?
Food stored here during first stage of digestion
What does the relaxation of the fundus and body of the stomach?
Fundus and body of stomach (thinner muscle tone) relaxes, allowing large volume
(~1.5L) of food storage
What does vagal reflex inhibit and what factors are pressent?
• Vagal reflex inhibits smooth muscle tone – mechanoreceptors → fundic relaxation
– Vasoactive intestinal peptide (VIP) and nitric oxide (NO)
What does the antral region do and eventually lead to what?
• Antral region mixes/grinds food with gastric secretions → Digestion
What does colon/recturm store?
Storage of indigestive residues and faecal matter
How many liters of gastric juice does the stomach store and what does it help in?
• Stomach stores 2-3 litres of gastric juice/24hr (mucus, pepsinogen, intrinsic factor, lipase) which help in digestion and absorption of food
What does mucus act as and how does it do so?
– Mucus (secreted by goblet cells and mucus neck cells) – acts as a lubricant by acting as a barrier that protects the stomach and colon especially from gastric acid
What does lipase convert?
Lipase converts triglycerides to fatty acids and glycerols
Role of pepsin
Protein digestion
What is HCl important in?
Important in defense
Why are intrinsic factors needed?
for vitamin B12 absorption
What are paracrine secretions often called?
Often called local hormones
Where are paracrine secretions secreted from and where do they act?
- Secreted from cells in the mucosa, but unlike hormones, the chemical acts locally on adjacent cells via the interstitial fluid
Exocrine secretions(Salivary glands)
- Mucus
- Lipase
Exocrine secretions(Gastric glands)
- HCl
- Pepsin
- Mucus
Exocrine secretions(Pancreas)
Bicarbonate ions, enzymes
Exocrine secretions(Liver)
Bile salts, bile acids
What is the target tissue of gastrin?
Stomach
What is the target tissue of secretin?
Duodenal mucosa
When is secretin secreted?
• In response to high acidity within the duodenum
What is the target tissue of • Pancreozymin-cholecystokinin?
duodenal mucosa
When is pancreozymin-cholecystokinin secreted?
• In response to fatty foods, and allows the contraction of the gall bladder
What is the target tissue of insulin?
Pancreas(beta-cells)
What do exocrine, endocrine and paracrine secretions allow?
Allow active difestion and control of digestion, gastric motility
Where does absorption mainly occur?
Mainly occurs in small intestine
Why does absorption mainly occur in small intestine?
– Highly vascularised
– Moist and thin
– Large surface area
Where does absorption of fluids occur?
• Absorption of fluid occurs in the small intestine and colon
Where does motility occur?
Storage like proximal stomach
What does the movement of the muscular wall allow?
Movement from one region to another (law of gut); mass evacuation
Mechanical degradation, e.g. gastric antrum
Mixing lumen contents, e.g. small intestine
Transport of nutrients, water and of urea and electrolytes
Digestion and absorption
What is the largest lymphoepithelial organ?
Intestines
What are the defence mechanims involved in the gut?
- Sight, smell and taste alerts us to harmful food substances
- Vomit reflex
- Acid in stomach (HCl) kills most harmful bacteria
- Mucus secretions
- Natural bacterial flora prevents colonisation of harmful bacteria
- Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to mount a response to food-borne antigens - analyse and respond to pathogenic microbes
- Peyer’s patches: located in the lamina propria layer (lies beneath the epithelium) of the mucosa and extending into the submucosa of the ileum
What is the liver involved in the metabolism of?
• It is involved in carbohydrate, nitrogen and lipoprotein metabolism
What does the parasympathetic innervation of the gut stimulate?
Stimulates salivation
What does the sympathetic innervation of the gut inhibit, relax and contract?
Inhibit salivation
Relaxes the bladder
Contracts the rectum
What does the gut have of its own and what is it called?
- The gut has its own “little brain”
o Called the enteric nervous system
What does afferent mean?
To the brain
What does efferent mean?
From the brain
What does the ANS enable in the stomach?
Enables food storage in the stomach
What does ANS enable the movement of food into?
ANS enables movement of food into the duodenum
Steps involved in the movement of food into the duodenum
- Ripples of contraction move the food towards the antrum (thicker muscle layer)
- Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave
- Repulsion of chyme causes the opening of pyloric sphincter
- Small partially digested material is squirted through the pyloric sphincter into duodenum
The repulsion of what allows mixing/grinding?
• Repulsion of antral contents backwards towards the body allows mixing/grinding
What happens in the sieving effect?
viscous and solid matter are retained in the stomach
What three mechanisms of communication mediate responses in the GI tract?
Endocrine
Neurocrine
Paracrine
What does vago-vagal reflex describe?
describes a type of reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk
What is the vago-vagal reflex a reflex circuit within?
• It is reflex circuit within the GIT
What is the vago-vagal reflex pathway via?
• Pathway is via the brain stem (medulla)
When is the vago-vagal reflex active and in response to what?
the vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing
Where does the vago-vagal reflex go and leads to what?
o The reflex goes from the stomach to the brain and then back to the stomach
Leads to the active relaxation of the smooth muscles of the stomach
What does the vago-vagal reflex describe?
- Describes the reflex control of responses to gut stimuli via the NTS and DMVN
What is the NTS?
Nucleus tractus solitarius
• main site of termination of vagal afferents
What is the DMVN?
Dorsal motor vagal nucleus
• main site of origin of vagal efferents (motor) supplying the gut
What two nerve fibers are intrinsic to the gut and there functions?
- Myenteric plexus (Auerbach’s plexus): motor function
2. Submucosal plexus (Meissner’s plexus): intestinal secretions
What does the ENS relexly do?
• Reflexly regulate GI functions entirely within the wall of the gut
What is the ENS connected to and by what?
• Connected to CNS by parasympathetic and sympathetic fibres
What are the neurotransmitters involved in the ENS?
• Neurotransmitters: ACh, NO, NA, 5-HT, GABA, ATP
Where are the auerbach’s and meissner’s plexuses located?
- Auerbach’s and Meissner’s plexuses are in wall of colon
What does parasympathetic activity do in the ENS?
Increases contraction of proximal colon
What does sympathetic activity do in the ENS?
Decreases colonic movement
What happens in the colo-colonic reflex in the presence of food?
presence of food or food products/distension of the stomach increase motility of the colon in response
What is the most prominent plexus?
Auerbach’s Plexus
What does cholinergic innervation in auerbach’s plexus do?
Increases gastric motility and secretion
What does adrenergic stimulation din auerbach’s plexus do?
Decreases gastric motility and secretion
What is rate of emptying dependent on?
dependent upon the material’s ability to be absorbed
What do fatty acids in the duodenum decrease and how?
in gastric emptying by increasing the contractility of pyloric sphincter
What is peristalsis slower in?
• Slower in large intestine compared to small intestine
What is peristalsis and what does it move?
• Wave of propulsive contractions and moves contents of gut towards the anus
What initiates the contraction in peristalsis?
• Distension initiates contraction
What controls the movement in peristalsis?
• Vagal inhibitory and excitatory fibres control movement
What does the regulation of peristalsis require?
Regulation of Peristalsis Requires Neural Reflexes
