digestion and absorption Flashcards
(21 cards)
The pancreas
it is both an exocrine and endocrine gland
endocrine - pancreatic islets responsible to produce insulin and glucagon > help to regulate blood glucose levels
exocrine - pancreatic acini
acinar cells responsible to secrete pancreatic juices
pancreatic juice component: enzyme (alpha amylase, lipase, proteolytic enzymes and nucleases), mucin and electrolytes
pancreatic secretion
acinar cell secretion
- enzyme (alpha amylase, lipase, proteolytic enzymes and nucleases)
- failure to produce these enzymes = malnutrition
ductal cell secretion
- high in hco3-, hco3- helps to regulate the pH of the upper intestine, neutralizes the acidic chyme when it enters into the small intestine
- failure = duodenal ulcer
aqueous component of pancreatic juice
high hco3- than ECF
the hco3-/cl- exchanger, helps to secrete hco3- from the plasma and absorbs cl- to make up the aq component
enzymatic component of pancreatic juice
pancreatic alpha amylase
- helps to break down starch, glycogen and complex ch20 except for plant cellulose
pancreatic lipases
- helps to break down fatty acids
- water soluble lipids = does not require bile salt secretion from the liver
- water insoluble lipids = require additional emulsification by bile salts from liver
pancreatic proteolytic enzymes
- trypsin, chymotrypsin and carboxypeptidase A and B
control of pancreatic secretion
controlled by the parasympathetic system (increase pancreatic secretion) and the sympathetic system (decrease/inhibit pancreatic secretion)
3 phases of pancreatic secretion
1) cephalic phase
the thought, smell, taste of the food would stimulate the secretion of pancreatic juices even before the arrival of the chyme at the small intestine
the pancreatic juices secreted in this phase is of substantial amount of enzymes and hco3-
2) gastric phase
the arrival of the bolus into the stomach would stimulate gastric distention and production of food breakdown products (gastrin)
gastrin stimulate the production of pancreatic juices high in enzymatic content
3) intestinal phase
70% of the remaining pancreatic secretion would be stimulated for release into the duodenum when the chyme is release from the stomach through the pyloric sphincter > this would be sensed by the hormones (CCK and secretin) which would then stimulate the release of pancreatic juices
CCK and secretin are secreted from the cells of the duodenum and the upper jejunum
CCK and secretin work synergistically together and they complement one another
with more CCK –> the pancreatic juice will have higher enzyme content
with more secretin –> the pancreatic juice will have higher bicarbonate content
secretin has no effect on CCK secretion and vice versa
Ach potentiates the effects of both CCK and secretin
control of secretin and CCK secretion
- response to entrance of chyme into small intestine
- amino acid (tryptophan and phenylalanine), fatty acids are stimuli for CCK secretion
- low pH would induce secretion of secretin to increase the bicarbonate component of the pancreatic juice
digestion of carbohydrates (@ luminal and @ mucosal)
begins in the mouth due to the salivary amylase > continues in the stomach as the amylase is mixed with the bolus > enters into the small intestine where it would be digested further by the pancreatic amylase before absorption (luminal)
@ luminal = pancreatic alpha amylase further digest carbohydrates, concentrated in the duodenum
alpha amylase digest carbohydrates into a Varity of oligosaccharides, which will then be further broken down into monosaccharides before it can be absorbed (rate limiting step)
@ mucosal (the lining of the small intestine also have enzymes to break down the carbohydrates) they are called disaccharidase
disaccharidase breaks down oligosaccharides into monosaccharides (fructose, glucose and galactose)
enzyme activities are greatest at brush border of jejunum
absorption of carbohydrates
absorption of carbohydrates is the most prominent at the duodenum and the upper jejunum
glucose, galactose and xylose uses the sodium dependent transportation mechanism for absorption
fructose have to first convert to lactic acid and glucose and uses the facilitated transportation mechanism for absorption.
this is to help to maintain the concentration gradient for diffusion
the monosaccharides are transported across the basolateral membrane by facilitated transport
they can also get transported when electrolytes passes through the gap junctions in between the cells
monosaccharides absorption are not regulated so there is no limit of how much gets absorbed
failure to absorb the carbohydrates can result in diarrhea and intestinal gas production
lactose intolerance
- autosomal dominant trait
- the tolerance for lactose decrease with age
- lactase enzyme is used to break down lactose for absorption
- lactase becomes sensitive when the person is going through an infection/inflammation that affects the intestine
protein digestion
begins in the stomach (pepsinogen > pepsin, denature proteins into amino acid and short chain poly peptide for absorption)
10-20% of proteins get digested in the stomach
@ luminal = pancreatic proteolytic enzymes
@ mucosal = enterocytes peptidases
50% protein is digested and absorbed by the duodenum and jejunum
10% reaches the colon and gets digested by the microorganism, presence of protein in stools is by the bacteria and cellular debris
protein absorption
uses the sodium dependent transport system
biliary secretion (bile function & bile formation, storage and release)
bile functions
- allow for digestion and absorption of fats
- excretion of water insoluble substances ie: cholesterol and bilirubin (produced by hemoglobin)
bile formation
- produced by hepatocytes of the liver and the bile duct cells
bile storage
- bile is stored in the gall bladder during interdigestive periods until it gets stimulated for release
bile release
- stimulated by CCK release causing gall bladder contraction
composition of bile
1) bile acids
primary bile aids: cholic and chenodeoxycholic
- synthesized from cholesterol
- more water soluble than cholesterol (due to hydroxylation)
- actively reabsorbs at the ileum
secondary bile acids
deoxycholic and lithocholic acids
- dehydroxylation of the primary bile acids by the intestinal bacteria so that they can get reabsorbed back (recycle)
2) bile pigment
bilirubin = yellow colour
- metabolites of hemoglobin, for excretion
- bacteria convert the bilirubin to urobilin which makes the stool brown in colour
3) phospholipid (lecithin)
- amphipathic
- increase cholesterol solubilization in bile micelles (excretion route for cholesterol excretion)
- lecithin also helps to stabilize the cholesterol to ensure that it stays dissolve in the bile
4) electrolytes
- high in bicarbonate than in plasma
conjugation of bile acids
bile acids are conjugated with glycine to give bile salt
- enhances hydrophilicity
- bile salts are more amphipathic so it is easier to form bile micelles that aids in fat absorption but the bile micelles are poorly reabsorb and retains longer in the gut
- bile salts are also resistant to hydrolysis by pancreatic enzymes
enterohepatic circulation
liver > small intestine > liver
recycle bile from small intestine to liver and back again
majority gets absorbed only small amount excreted in feces
rate of synthesis is determined by rate of return to liver
digestion of fats (intra gastric & intra intestinal)
luminal phase -breaking down of fats
intra gastric
- lipid digestion begins minimally in the stomach due to intragastric lipolysis producing gastric lipase
- lipolysis increase to 90& if pancreatic lipase secretion decrease
intra intestinal
- works best with the combination of alkaline pH, bile salts, lecithins, lipolytic enzymes
- pancreatic lipases (glycerol ester hydrolase, cholesterol esterase, phospholipase A2)
emulsification of lipids
- fats must be soluble in water for digestion and absorption
- bile is a detergent that helps to emulsify the fats and the presence of lecithin helps with enhancing the emulsification effects of bile
bile micelle formation
conjugation of bile acids with glycerin to give bile salts used to make bile micelles
absorption of fats
mucosal phase (bringing in the fats)
once the lipids are bounded to the bile micelles, it ensures the fats are emulsified and stabilized (rate limiting steps because of the migration of the micelle from chyme to microvilli surface)
once in contact with the brush borders of the small intestine, the lipids will get absorbed
- almost all ingested lipids are absorbed, presence of fats in stool are from the colonic bacteria
- cholesterol absorption is the slowest
- bile salts are actively reabsorbed at the terminal ileum
post-absorptive phase (delivery of the fats)
re-esterification of the lipids
