Basic Science Flashcards
Describe the stages of HCl formation
CO2 from blood>H2CO3 (unstable carbonic acid)
H2CO3<> H2 + HCO3
H2 for H/K pump + HCO3/Cl pump (HCO3 out, HCl in)
HCl + H2O @basolateral membrane
What controls ACh, gastrin and histamine secretion?
ACh=neurocrine
Gastrin=endocrine
Histamine=paracrine
What stimulates acid secretion?
Sight
Smell
Taste
Thought of food
3 phases of gastric acid secretion
Cephalic
-stopping eating=dec. vagal activity
Gastric phase
- dec. phase (inc. HCl)= dec. gastrin
- -ve feedback
Intestinal phase
-acid in duodenum> secretin release>dec. gastrin + dec. gastrin stim of parietal cells
Fat/CHO in duodenum > GIP release>gastrin and parietal HCl sec.
Role of intrinsic factor in B12 absorption
B12 cannot cross the membrane as its a charged molecule with intrinsic factor
If there’s no IF then B12 will pass right through and lead to pernicious anemia
No B12=RBCs can’t mature
Role of gastric secretions
Produced by surface epithelium cells and mucous neck cells
Cytoprotective role
-protects from mechanical injury
-neutral pH (HCO3)=projects against gastric acid corrosion and pepsin digestion
Role of enterogasterones
Act collectively to prevent more acid build up in duodenum
- inhib gastric secretion
- reduce gastric emptying
- inhib mobility/ contract pyloric sphincter
What’re the enterogastrones?
Secretin
CCK
GIP
Pepsinogen secretion formation
Pepsinogen= the zymogen=secreted by chief cells
Low pH (< 3) converts it
Pepsinogen promotes more pepsin making
Pepsins inactivated at neutral pH
What’re the 6 components of bile?
Bile acids Lecithin Cholesterol Bile pigments -bilirubin from haemoglobin Toxic metals -detoxify by adding proteins Bicarb -neutralisation of the acid chyme
What is bilirubin?
The predominant bile pigment, extracted from blood by hepatocytes and secreted into bile=bile is yellow
Why is urine yellow and faeces brown?
Reabsorbed bilirubin excreted in urine=yellow urine
Bilirubin modified by bacterial squash enzymes=brown faeces
What’re bile acids synthesised from and where are they synthesised?
In liver from cholesterol
Before secretion bile acids are conjugated with glycine or taurine= increased solubility of bile salts
What’re the 3 layers of the gallbladder wall?
Mucosa -folded=ruggae=expansion Muscularis -smooth muscle=contraction Serosa -connective tissue
How does the gallbladder concentrate bile?
Absorbs the Na and H2O
What effect does CCK have of on the sphincter of Oddi and what causes CCK release?
Fat in duodenum=CCK release
CCK=sphincter of Oddi relaxes, gallbladder contracts to squeeze out bile
Discharge of bile into duodenum=fat solubilisation
Summarise the action of CCK
Fat/amino acids in duodenum=CCK release CCK=decreases gastric emptying CCK=increases pancreatic enzyme secretion Gallbladder contraction DIGESTION
Summary of actions of secretin
Acid in duodenum=secretin Decreases gastric acid Decreases gastric emptying Increases duodenal HCO3 secretion Increases pancreatic HCO3 secretion Increases HCO3 secretion NEUTRALIZATION
What’s the function of the duodenum, ileum and jejunum?
Duodenum=gastric acid neuralization, digestion and iron absorption
Jejunum=95% of nutrient absorption
Ileum=NaCl/H2O absorption=chyme dehydration
What does a virus cell absorb?
NaCl Monosaccharides Amino acids Peptides Fats Vitamins Minerals Water
What does a crypt cell secrete?
Cl
Water
Where is intestinal fluid secreted from?
~1500ml/day from epithelial cells lining crypts of Lieberkuhn
Why’s H2O secretion important for nutrient digestion?
Maintain contents in a lumenal state
Promotes mixing of nutrients
Aids nutrient presentation to absorbing surface
Dilated and washes away potentially injurious substances
How are K, Na + 2x Cl transported?
Co transported using an electrical gradient
Where is the chloride channel located and what does the Cl do?
Apical membrane
Makes an osmotic gradient so water can passively move through the cell
What’re the events that lead to the Cl Chanel opening?
ATP>cAMP> phosphorylation of PKA>Cl channel opens
What is the purpose of segmentation?
Primary mechanism during digestion
Contraction then relaxation of short segments
Mixes contents with enzymes
Chyme comes into contact with absorbing surface
How are segmentation contractions generated?
Pacemaker cells in longitudinal muscle
Intestinal basic electrical rhythm (BER) produces oscillations in membrane= potential>threshold>AP>contraction
AP determines strength of contraction
BER decreases as you go down the pathway
What does parasympathetic, sympathetic and autonomic stimulation do to gut contraction?
Parasympathetic=vagus=increases
Sympathetic=decreases
Autonomic=no effect
What is the migrating motility complex?
Pattern of peristalsis from antrum to ileum
As one wave ends, another begins
What does the arrival of food mean for the MMC?
=stops MMC and initiation of segmentation
What does the MMC act to?
Move undigested food into the large intestine
Limit bacterial colonisation of small intestine
What hormone is involved in the initiation of the MMC?
Motilin
How is intestinal movement mediated?
Myenteric plexus
Describe the gastroileal reflex
Gastric emptying=increase in segmentation activity in ileum
- opening of ileocaecal sphincter
- entry of chyme into large intestine
- distension of colon
- reflex contraction of ileocaecal sphincter
- -prevents backflux into small intestine
What’s the 2 muscular layers of the colon?
Circular layer= complete
Longitudinal layer=incomplete
Where is the muscularis externae thicker than in the rectum?
Anal canal
What’s the epithelium in the anal canal?
Simple columnar
How are chymes dehydrated?
Actively transports Na from lumen into blood>osmotic absorption of water>dehydration of chyme>solid faecal pellets
What is fermented by bacteria in the large intestine?
Short chain FA’s=energy source in ruminants
Vit K=blood clotting
Describe the mechanisms controlling defeacation
Wave of intense contraction towards rectum
Distension of rectal wall made by mass movements of faecal matter into rectum>defaecation reflex>urge to defaecate
What is the innervation of the defaecation reflex?
Parasympathetic via pelvic splanchnic nerves
No sympathetic
Transit time to mouth, oesophagus, stomach, small intestine and colon
Mouth=1 min Oesophagus= 4-8s Stomach=2-4hrs Small intestine=2-4hrs Colon=10hrs-several days
PH of mouth, stomach, small intestine and colon
Mouth=6.5-7.5
Stomach=1.5-4
Small intestine=7-8.5
Colon=5.5-6.5
In what conditions can a facultative anaerobes grow
Can grow with or without O2 but may not thrive
In which conditions can obligate grow?
Can’t survive in O2
Is gut microbiota higher or lower in western individuals?
Lower
Do fatty diets have more or less diversity?
Less
Jobs of bacterial diversity?
Metabolism of dietary components Produce essential metabolites Defence against: -competition -barrier function -PH inhibition -immune system priming Grow in fibre Can use host derived (endogenous) substrate for growth
Why should fibre be included in diets?
Contain phytochemicals, anti-oxidants and vitamins Increases bacterial fermentation: -more phytochemicals -maintain pH -increase commensal population -supply of short chain FAs
What’re the 3 main FAs and their functions?
Butyrate
-epithelial growth and regreneration
Propionate
- gluconeogenesis in liver
- satiety signalling
Acetate
- transported in pheripheral tissues
- lipogenesis
Why’s re the products of/ presence of bacteria important for a healthy gut?
Barrier effect
- large numbers prevent colonisation of ingested pathogens
- inhibit overgrowth of pathogenic bacteria
Active competition exclusion
PH inhibition
What must the immune system be able to do?
Respond to foreign/pathogenic agents
Actively down-regulate immune responses to self proteins, dietary antigens and commensal microbiota
Functions of the SI with regards to digestion?
Breaking food down to its components
Decontaminated dirty food
Controlled hydrolysis to avoid fluid shifts
Absorption against nutrients
What digests proteins in the SI?
Trypsin
Chromotrypsin
How is fat digested in the SI?
Pancreatic lipase
- absorbs glycerol and free FAs
- via lacteal and lymphatic system
How’re carbs digested in the SI?
Pancreatic amylase
- breakdown to disaccharides
- final digestion by brush border disaccharides
How does the SI achieve its barrier function?
Regulating what gets in and out Low bacterial populations It’s a toxic environment -digestive enzymes -bile salts -presence of IgA
How does is the barrier of the SI maintained?
Immune sampling
Monitoring the presence of pathogens
Translocation of bacteria
Gut associated lymphoid tissue (GALT)
