Physiology Of Intestines Flashcards
How is water secreted by the small intestine?
Intestinal epithelium secretes Na+ Cl- and HCO3-
H20 follows by osmosis
Where is mucous secreted from in the small intestine?
Brunners glands in the duodenal mucosa
What hormones does the small intestine secrete?
What do they do?
Secretin & CCK - inhibit gastric motility and secretion and control pancreatic/biliary secretion
Motilin - stimulates migrating motor complexes via ENS and ANS
VIP (vasoactive intestinal peptide) - increases blood flow to GIP tract
GIP (gastric inhibitory peptide) - inhibits gastric secretion and stimulates insulin secretion
The endocrine pancreas secretes insulin and glucagon. What does the exocrine pancreas secrete?
Ducts cells secrete alkali - isotonic solution rich in HCO3-
Acinar cells secrete digestive enzymes: Trypsin Chymotrypsin Pancreatic amylase Lipases
How are pancreatic enzymes activated?
Trypsinogen activates inactive enzymes (zymogens) by membrane bound molecule entereokinase.
Trypsin then made which can activate other enzymes in the intestinal lumen
Enzyme and alkali secretion by the pancreas are controlled separately, how?
Enzyme: stimulated by CCK and ACH (PNS Vagus)
Alkali: secretin and potentially by CCK and ACh
What molecules are digested in the lumen of the duodenum and proximal jejunum?
What enzymes are involved?
Fats > monoglyceride and fatty acids by pancreatic lipase
Starch > disaccharides > monosaccharides by pancreatic amylase and then brush border enzymes
Proteins > peptide fragments > amino acids by trypsin and chymotrypsin then carboxypeptidase and aminopeptidase
How are fats digested and absorbed in the small intestine?
Fats are emulsified by bile salts and phospholipids
Digested by pancreatic lipase and products (monoglyceride + fatty acids) held in micelles
Micelles diffuse into unstirred layer and fatty acids and monoglycerides diffuse into cell membrane - once in the cell they’re reassembled to fats
Triglyceride droplets packaged into chylomicrons and exported across basolateral membrane to lymph
How are carbohydrates digested/absorbed?
Broken down and transported across intestinal membrane via either SLGT1 (glucose/galactose) or GLUT5 (fructose)
Then across the basolateral membrane via facilitated diffusion, GLUT2
What regulates Ca2+ absorption?
Vitamin D
What are the two types of iron and how are they absorbed?
Haem - absorbed by receptor mediated endocytosis to release Fe3+ which is then reduced to Fe2+
Free Iron - reduced from Fe3+ to Fe2+ by duodenal cytochrome b ferric reductase in the brush border membrane and transported by DMT1
In both cases Fe2+ then binds to ferroportin 1 in basolateral membrane to be exported
How is Iron absorption regulated?
By demand: intracellular iron-ferritin pool LOS as cell is slouched in faeces
If iron is required by body then becomes absorbed into the blood across ferroportin 1 (receptors are unregulated)
How is vitamin B12 absorbed?
Binds to R protein in the stomach
Released from R protein and binds to intrinsic factor in the duodenum/jejunum
Vitamin B12-IF complex is absorbed by receptor mediated endocytosis in the terminal ileum
What can malabsorption arise from?
Failure/deficiency in: Pancreatic enzyme secretion Brush border enzymes Absorption Bile secretion
Name some diseases of malabsorption and their site of pathology
Coeliac disease - duodenum/jejunum, fat absorption
Pancreatitis - pancreas, fat digestion
Crohn’s surgical resection - ileum, vitamin B12 and bile salt absorption
Lactase deficiency - small intestine, lactase hydrolysis
What are symptoms of malabsorption?
Weight loss Abdominal distension Diarrhoea Steatorrhoea Pernicious anaemia
Outline lactose intolerance
In healthy individuals lactose is digested by lactase producing normal stools
In lactose intolerance there’s a lack of lactase enzyme so lactose is broken down by gut bacteria leading to fermentation producing gases, organic acids, osmotically active molecules and leading to bloating/increased motility so alter stools
Outline what excessive vomiting or diarrhoea leads to with regards to acid-base balance in digestion
Vomiting: large loss of secreted acids = metabolic alkalosis
Diarrhoea: excessive loss of fluid and therefore NaHCO3 in faeces = metabolic acidosis
What are the functions of the large intestine?
Extract Na+ and H20 from luminal contents
Make and store faeces
Move faeces towards the rectum
What does the large intestine secrete and what is its role?
Alkaline mucous from goblet cells - protects epithelium from acid/abrasion/bacterial activity and provides adherent medium for holding faecal matter together
How is water absorbed in the colon?
Na+/K+ ATPase pumps Na+ out across basolateral membrane
H20 follows transcellulary as gap junctions too tight
What’s the role of the ileocaecal valve?
Separates terminal end of ileum from the caecum
One way valve/sphincter to prevent backflow of faecal contents from the colon and control the rate at which ideal chyme enter
What’s the role of movements of the colon?
What are the two types of movement?
Facilitate efficient absorption of water and salts and permit orderly evacuation of faeces
Haustral contractions and mass movements
Longitudinal muscle of the colon is aggregated into 3 bands cal taeniae coli - what is their role?
Combined contractions of taeniae coli and circular muscle layer cause colon to bulge into ovoid segments called haustrae
Haustral contractions mix contents to facilitate water absorption
What are mass movements in the colon?
Propel luminal contents from beginning of transverse colon to sigmoid colon
Series of modified peristaltic events
Occur 1-3 times a day
Outline the gastrocolic reflex and what is it mediated by?
Initiates mass movements in the colon to push contents into the rectum, triggering defecation reflex
Mediated by gastrin and autonomic nerves
What is the defaecation reflex?
Reflex response to distension of rectal wall, mediated by mechanoreceptors:
Contraction of rectum
Internal anal sphincter relaxes
Initial contraction of external anal sphincter
Increased peristaltic activity in sigmoid colon
Relaxation of external anal sphincter
Expulsion of faeces
What’s the valsalva manoeuvre?
Voluntary defaecation:
Full inspiration followed by forced expiration against closed glottis so diaphragm moves downwards
Abdominal and thoracic muscles contraction
Increased abdominal pressure forces faecal contents into rectum
What’s the composition of faeces?
3/4 water 1/4 solid matter: Dead bacteria Fat Inorganic matter Protein Undirected materal - bile/sloughed cells
Why is faeces brown?
Bilirubin derivatives
What’s the role of anaerobic bacteria in the gut?
Immunity against common bacteria Converts bilirubin to urobilogens Forms secondary bile acids Degrade digestive enzymes Digests mucous Synthesises vitamin K
= all disrupted by oral antibiotics
What do purgatives do?
Accelerate transit through the intestine and are used to alleviate constipation
Constipation: Define Causes Treatments Bristol stool chart? What can be associated complications?
Infrequent/difficult evacuation of faeces (<3 times/week)
Diet, hormonal, side effect of medications or illness
Change in diet/exercise habits, purgatives
Types 1 & 2
Haemorrhoids & anal fissures
Name the 4 classes of purgatives
Bulk laxatives
Osmotic laxatives
Faecal softeners
Stimulant purgatives
What are the 4 types of diarrhoea?
Secretory - increased active secretion
Osmotic - loss of water
Motility related - abnormally high GI motility so reduced time for absorption
Inflammatory - damage to mucosal lining/brush border leading to passive loss of protein rich fluids
What are the 2 ways diarrhoea is treated?
Maintenance of fluid/electrolyte balance (Na+ and K+)
Anti diarrhoea agents (anti motility)
How are proteins digested/absorbed?
Proteins -> Polypeptides by pepsin
Polypeptides -> Di/Tripeptides by trypsin/chymotrypsin
Tripeptides -> Amino acids by aminopeptidases
Amino acids co-transported into cell with Na+ then diffuse across basolateral membrane into capillaries
How are the exocrine and endocrine functions of the pancreas divided?
Exocrine (majority) by duct and acinar cells, secreting digestive enzymes and HCO3- (alkali)
Endocrine function by islets of langerhans: B cells secrete insulin, a cells secrete glucagon to regulate blood sugar levels
Outline digestive enzyme secretion by the pancreas, including its stimulation
Acinar cells secrete zymogens (inactive enzymes: proteases, lipase, amylase) which are activated by trypsin (from trypsinogen) and enter the duodenum via the ampulla of vater
CCK and CN X stimulates zymogen secretion in response to taste, sight, smell of food or stomach stretching
Outline alkaline secretion by the pancreas, including its stimulation
Inside pancreatic duct cells H2CO3 dislocates to H+ and HCO3-
H+ transported out of the cell into the blood by H+/Na+ antiporter
HCO3- exchanged via Cl- exchanger into intercalated ducts
