GI tract Flashcards
What is the chewing reflex?
1) Presence of bolus causes a reflex inhibition of mastifitcation muscles
2) Jaw drops
3) Initiates a stretch reflex of the muscles
4) Contraction and elevation of the mandible and closure of the teeth
5) Compression of the bolus on the roof of the mouth again causes inhibition
What are the 3 muscles used in mastification?
1) Masseter (cheek)
2) Temporalis (tempol)
3) Pterygoid (under the cheek)
What does saliva do?
- Lubricated food, makes bolus
- Aids taste and digestion
- Contains alpha-amylase which begins to break down STARCHES
What is the structure of the oesophagus?
- Tube between the oropharynx and the stomach
- Runs posterior to the trachea and the heart
What controls entry to the oesophagus?
Upper sphincter
What is the oesophagus lined with?
- Mucosa
- Submucosa
- Connective tissue
What is Barrett’s oesophagus?
- Premalignant condition where metaplasia occurs of the cells lining the lower oesophagus
- Normal stratisfied squamous cells with goblet cells replaced by simple columnar
What are the symptoms of Barrett’s oesophagus?
No symptoms directly but associated with acid refulx:
- Heart burn
- Teeth corosion
- Substernal pain
What are the treatments for Barrett’s oesphagus?
- Proton pump inhibitor (prevent production of HCl)
- Resection (removal of part of the oesphagus)
What nerve impulses cause peristalsis?
Impulses from CNX and CNIX
How does peristalsis occur?
Primary P - continuous wave
Secondary P - Triggered by distension of the oesophagus, sweeps any remaining food down
What are the 5 regions of the stomach?
1) Cardia - entry to the stomach. Contains mucus glands. Where epithelium changes
2) Fundus - Top of the stomach. Closest to the oesophagus
3) Body (corpus)- Largest part of the stomach
4) Antrum - Bottom of the stomach
5) Plyrous - End of the stomach, lets food pass into the duodenum
Where does the stomach lie in the body?
In the upper left abdominal cavity (against diaphragm)
What is the maximum capacity of the stomach?
0.8-1.5L
Where is gastric juice secreted from?
Gastric glands which cover the lumen of the stomach
What substances are released into the stomach and why?
HCl - bactericidal
Proteases- break down proteins
What do the weak mixing waves of the stomach do and how do they occur?
- Churns food and gastric secretions
- Begin in the mid/upper stomach and become more powerful towards the plyrous
- Pushes out a few mm of chyme into the duodenum from the plyrous each turn
What does the plyrous act as and how?
A sphincter to the duodenum
- Only lets fluid-like substances through
- Opening and closing controlled by nervous and hormonal signals from the stomach and the duodenum
How is food contained within the stomach?
- When food enters it forms concentric circles with old food by the wall and new food by the opening of the osophagus
What is the vagovagal reflex?
- From the stomach to the brainstem and back when the stomach is stretched
- Reduces muscular tone and allows the stomach ti bulge to contain more food
What is the rate of stomach emptying never greater than?
The rate that chyme can be digested and absorbed in the small intestine
What 2 things increase the rate of stomach emptying and how?
1) Myentric reflex
- Increases stomach emptying when there is an increase in the food volume in the stomach, which increases the stretch within the walls
2) Gastrin
- Produced by cells in the antral mucosa
- Causes production of highly acidic gastric juice
- Mild stimulatory effect on the motor activity of the pyloric pump
In 3 ways, how does the increase in stretch of the duodenal wall decrease stomach emptying?
Directly to the stomach:
1) Enteric NS
2) CNX to the brainstem
- Slightly inhibits excitatory signals to the stomach
3) Extrinsic nerves
- Prevertebral sympathetic ganglia back to the stomach via inhibitory sympathetic nerves
What causes a peptic ulcer?
Damage to the stomach wall by stomach acid:
1) H.Pylori
2) Chronic inflammation in the antral mucosa
3) Acid secretion causing corosion of the wall
4) Ibupropen/ aspirin at high doses
What are the symptoms of a peptic ulcer?
- Burning upper abdominal pain which worsens on eating
- Indigestion
- Nausea
What are the treatments of peptic ulcers?
Proton pump inhibitors
What are the symptoms of gastric cancer?
- Pain/ burning on swallowing
- Weight loss
- Upper abdominal pain
- Full after eating small amounts
- Nausea/ vommiting
- Bleeding (dark stools)
- Tiredness
What is the limiting factor of the absorption of nutrients and water?
Surface area over which the absorption occurs over
What are the folds of kerckria?
In the duodenum and jejunum
Protruce 8mm into the lumen
Increase the surface area by 3x
What are the 3 parts of the small intestine?
Duodenum
Jejunum
Ileum
What are the roles of the duodenum?
- Where bulk digestion occurs
- Involved in the enzymatic breakdown of chyme
Regulates stomach emptying via the hormones:
- Secretin
- Cholecystokinin
What are ‘plice’?
A high abundance of folds in the jejunum and ileum, increasing the surface area
What is the ‘brush border’?
- Found in the small intestine
- Made of 1000s of microvilli
- Increase the surface area by 10 x
How is the ileum different to the jejunum?
1) Smaller lumen
2) Thinner walls
3) More fat inside the mesentery (membrance which connects the intestine to the abdominal wall)
What are ‘Peyer’s patches’?
Encapsulated lymphoid nodules which contain cells of the immune system
Present in the small intestine and the large intestine
What are the roles of the ileum?
Absorb vitamin B12 and bile salts
Also absorb products of digestion not absorbed by the jejunum
How is water transported in the small intestine?
By diffusion, or if the chyme is dilute enough, by osmosis
- Paracellular
How are ions transported in the small intestine?
Sodium is actively transported
How are nutrients transported in the small intestine?
1) Monosaccharides are broken down into:
- 80% glucose
- 20% galactose and fructose
2) Glucose and galctose into the epithelial cell by secondary active transport
3) Fructose in by facilitated diffusion
4) All into the blood by facilitated diffusion
How are fats absorbed from the lumen of the small intestine?
7 steps, so write on paper
1) Broken down into monoglycerides and free fatty acids
2) These are emulsified by bile into bile micelles
3) Micelles are carried to the brush border and penetrate the recesses in between the agitating microvilli
4) Monoglycerides and FFAs diffuse out of the micelle into the interior of the epithelial cell
- Micelle remains in the chyme, to emulisify more fats
5) MG and FFA taken up by the SER anc converted into triglycerides
6) Are released to form chylomicrons
7) Chylomicrons are released from the basolateral membrane into a thethoratic lymph duct and then into the blood
How are bile micelles soluble in chyme?
They have a highly charged exterior
What are the functions of the colon?
1) Absorb remaining water and electrolytes from INDIGESTIBLE food matter
2) Accepts and stores food which is undigested
What does aldosterone do?
Enhances Na+ absorption
Which drives Cl- and water reabsorption
How and why are the tight junctions in the colon different to those of the small intestine?
They are tighter
Prevent back-diffusion of water
Why is HCO3 present in the large intestine?
Buffers the acid produced by bacterial fermentation
What is the cecum?
- Present at the beginning of the large intestine
- Compresses material into fecal matter
- Pouch-like
How is chyme moves into the cecum?
From the ileum through the ileo-caecal valve
What is the difference between the cecum in herbivores and in carnivores?
In herbivores - large as houses the bacteria to break down cellulose
In carnivores - small/ absent
- Replaced by the appendix
What are the 4 portions of the colon?
1) Ascending
2) Transverse
3) Descending
4) Sigmoid
What is tenia coli?
3 bands of longitudinal muscle in the colon
What is haustra?
Pockets in the colon which give a segmented appearance
Describe colon motility
Segmented contractions
Where is faeces stored?
In the rectal ampulla in the rectum
What does stretching of the rectal walls stimulate?
- Activate stretch receptors
- Causing the desire to defaecate
What is the compostion of faeces?
25% solid matter:
- 30% dead bacteria
- 30% undigested roughage
- 3% protein
- 10-20% fat
- 10-20% inorganic matter
75% water
What causes the colour of faeces?
Stercobulin and urobilin
What causes the odour of faeces?
- Bacterial action
- Varies from person to person, depending upon colonic flora
What essential fatty acids must come from the diet?
- Linoleic acid
- Arachidonic acid
What essential amino acids must come from the diet?
9 amino acids including:
- Leucine
- Tryptophan
- Lysine
- Methinonie
- Threonine
- Valine
What are glucoreceptors?
- In the brain
- Hypothalamic cells which are sensitive to dropping blood glucose
How does the stomach signal to stop eating?
- Stretch receptors in the wall: say when approaching to being full
- Chemoreceptors: sensitive to the pH and small amounts of food dissolved int he fluid in the stomach
How does the duodenum signal to stop eating?
Hormone cholecystokinin-CCK released from the mucousal cell layer
How do fat cells signal to stop eating?
- Brown fat releases leptin into the bloodstream
- Signals to stop eating
Where is the ‘ON’ switch for eating?
In the lateral region of the hypothalamus
Where is the ‘OFF’ switch for eating?
In the ventomedial region of the hypothalamus
What does neuropeptide Y do?
Potent stimulator of appetite which acts outside of the hypothalamus
What do lesions to the ventromedial region of the hypothalamus cause?
Obesity
- Insulin increase
- Blood glucose falls, animal eats more
- Food converted into adipose tissue, animal stays hungry
- Animal eats more
- More insulin released
(cycle)
Where is the hormone ghrelin made?
In the cells of the stomach wall
What does ghrelin do?
Increase food intake (activates appetite)
Where is PYY(3-36) made?
In the cells in the small and large intestines
What does PYY(3-36) do?
- Decreases food intake by inhibiting appetite
- Inhibits appetite stimulating neurons by binding to receptors called NPY Y2.PYY(3-36)
- Persists for 24 hours (thats why might not want any breakfast in morning after a big meal, not because food is still in the stomach)
What is satiety
Feeling of fullness
What causes a big rise in PYY(3-36)?
- More calories you eat
- High fat and fibe (slow releasing foods)
