Small intestine onwards Flashcards
how long does chyme take to pass through the small intestine
2-5 hours
what controls motility
slow wave activity
what does contraction require
slow waves must be large enough to generate action potentials
segmentation contractions
commonest form of motility but are only weakly propulsive
what is peristaltic activity increased by
stretch of the gut wall
what is the ileocaecal valve
the junction of the caecum and colon where the lips are reffered to as the ileocaecal valve
what does high pressure in the colon do to the iloecaecal valve
reduce movement through the valve
what is the iloecaecal sphincter formed of
smooth muscle
what happens when the terminal ileum is distended
a peristaltic reflex is coordinated by the ENS and pushes intestinal contents onwards
what is the gastroileal reflex
ileal motility enhanced in response to signals from a full stomach
what is the colonoileal reflex
inhibits movement through the sphincter when colon is full
approximation of nutrient ratios absorbed everyday
carbs>fat>amino acids>ions>water
what do mucosal folds of kerckring do
increase the surface area of the small intestine
what else increase the surface area
vili and microvilli on the epithelial cells forming a brush border
what do the crypts of lieberkuhn do
found between the villi and secrete fluid and contain stem cells for the replacement of desquamated epithelial cells lost from gut
difference between folds of kerckring and crypts of lieburkuhn
Folds are on the surface of the small intestine whereas the crypts are found between the vili
what does stomach acid help do in regard to proteins
denature it, therefore making it vulnerable to attack by pepsins e.g. digesting collagen
what is the most important enzyme type in protein digestion
pancreatic proteases
inactive zymogen trypsinogen is converted to….
trypsin by enteropeptidase on the brush border of the upper small intestine
what is the brush border
villi and microvilli on the epithelial cells of the small intestine
what does trypsin activate
trypsinogen, chymotrypsin, elastase and carboxypeptidases
what do these enzymes do
digest proteins to peptides
what happens after the proteins are digested to peptides
they are further digested by brush border peptidases
what happens to the products of digestion
they are taken up into the cells by facilitated diffusion or secondary active transport.
what happens to some amino acids within the cells
glutamine oxidised for energy within the rapidly dividing intestinal cells
what does the salivary and pancreatic amylase do to starch
cleave the internal a-1,4 bonds but cannot touch the a-1,6 branching liinks or the a-1,4 bonds next to them
what is the result of salivary and pancreatic amylase
smaller chains of glucose molecules (oligosaccharides) mostly maltose
what are the enzymes on the brush border of the duodenum and jejunum
glucoamylase and a-dextrinase, lactase, sucrase and trehalase
what do glucoamylase and a-dextrinase do
break down the a-1,4 and 1,6 bonds respectively within oligosaccharides
what do lactase, sucrase and trehalase do
digest lactose, sucrose and trehalose respectively
how is glucose and galactose is taken up by
SGLT1 on the apical membrane of epithelial cells in the duodenal and jejunal villi
fructose is taken up by
GLUT5 a facilitated diffusion transporter
how is fructose exported
glut2 on basolateral membrane
where is sodium absorption the highest
in the small intestine, its movement into cells is down electrochemical gradient
what is sodium absorption coupled to
movement of monosaccharides via SGLT-1 and some amino acids
when does potassium become concentrated
as water is absorbed and this provides driving force for paracellular uptake by the small intestine
where is there net secretion
from the colon via apical potassium channels
where and when is chloride absorbed
throughout digestive tract via paracellular pathway and exchange with bicarbs
what is the absorption of water
throughout the digestive tract both through ingesta and gastrointestinal secretions e.g. saliva
what is the reserve capacity
the gut can take up two or three times more water than 7.5litres if needed
what is the standing gradient model
water uptake across an epithelium holds that sodium is pumped into the intercellular clefts by primary active transport involving the na+/k+ ATPases, anions follow
what happens in the standing gradient model when the anions follow
a solute gradient is set up, highest near the tight junctions, decreasing towards the open ends where it becomes equal to the concentration in the bulk phase
what happens as a result of the high solute concentration within the intercellular clefts
water enters from the adjacent cells and from the lumen via leaky tight junctions, putting up the pressure in the clefts, driving flow across the basement membrane and removed by blood capillaries
what happens as a result of the small intestinal epithelium being leaky
absorption is isosmotic, much less water is absorbed in the colon but this can be against a larger osmotic gradient since tight junctions are tighter
what helps improve solubility of calcium and iron
gastric acid
what helps reduce insoluble fe3+ to fe2+
vitamin c in the stomach
what are the mechanisms for calcium absorption
draw this . lol .
include paracellular uptake which is important if calcium intake is high.
active form of vitamin d3 can upregulate the expression of several proteins involved in transcellular uptake INCLUDING CALBINDIN
what is the role of iron reductase and where is it located
on duodenal brush border, reduces fe3+ to fe2+ which is taken up via the proton fe2+ cotransporter
what is the proton fe2+ cotransporter called
DMT1
how else is iron taken up
as haem
how is iron transported out of the cell
via ferroportin
how does iron transport around the blood
bound to the protein transferrin
in the presence of hepcidin from the liver what happens
efflux is reduced and excess iron is trapped in the cell bound to the protein ferritin
when is ferritin lost
when epithelial cell is shed
how are vitamins absorbed
transported in lymph, most water soluble ones taken up in small intestine by diffusion or active transport.
how is vitamin b12 transported
absorbed via receptor mediated endocytosis in the ileum and exported from the cells, vitamin b12 travels bound to protein transcobalamin II
structure of the liver
thousands of lobules, between this are portal triads. mixed blood from hepatic artery and portal vein drains via hepatic sinusoids towards central vein which empties into the hepatic vein
what lines the sinusoids
hepatocytes
what are bile canaliculi
tiny channels which drain the bile produced by hepatocytes towards the branches of the bile duct
bile flows…. to the blood
countercurrent
outline the functions of the liver
carb, protein and lipid metabolism, bile formation, storage of vitamins, destruction and detoxification of some hormones, filtration of blood, blood reservoir
describe the destruction and detoxification of hormones by the liver
the first pass metabolism of toxins absorbed by the gut ensures that they do not reach the rest of the circulation
what is important about the filtration of blood by the liver
removal of effete erythrocytes and any gut bacteria which have entered the hepatic portal vein
describe carb metabolism by the liver
liver takes up monosaccharides from the portal vein by facilitated diffusion using GLUT2
what happens to glucose?
converted to glycogen which accumulates in the liver and in the muscle (much more)
can muscle export glucose directly
no
what happens to excess glucose
converted to triglycerides by the liver and exported as lipoproteins, stored as fat in adipocytes
can long chain fatty acids be made into glucose
no, but short chain fatty acid propionate can
how can intracellular proteins be metabolised by the liver
intracellular proteins can be decomposed via lysosomal enzymes and returned as amino acids to the blood
how can extracellular proteins be metabolised by the liver
digested by macrophages
what does the process of transamination allow for
interconvert amino acids, pyruvate, and TCA cycle intermediates by the process of transamination allowing for non essential amino acids to be synthesized
what are essential amino acids
must be included in the diet
what happens to excess amino acids
oxidised for energy directly or converted to glucose or ketone bodies
what is the exception for urea and glutamine
produced and exported directly
what is bile
an excretory product of the liver
what does bile contain
bile acids, phospholipids, cholesterol and bile pigments
roles of bile 3
1 promotion of fat absorption as bile acids are surfactants therefore used with phospholipids. 2 excretion of waste esp cholesterol and heavy metals. 3 protection as IgA, mucus and tocopherol have protective roles
what are primary and secondary bile acids
primary are made from cholesterol in the liver whereas secondary bile acids by gut bacteria
what is the role of the gall bladder
bile is collected and concentrated before expulsion into the digestive tract
where is the sphincter of oddi
at the entrance of the duodenum
what happens between meals in the gall bladder
sphincter of oddi contracts and bile is diverted into the gall bladder
what happens during a meal
cck promotes gall bladder contraction and the relaxation of the oddi sphincter emptying the bile into the duodenum
what happens to bile once it enters the duodenum
epithelial cells in terminal ileum take them up by secondary active transport, rest absorbed passively in the colon
how are bile acids returned to the liver
via the hepatic portal vein, in hepatocytes some secondary bile acids are re-converted to primary
how can cholesterol be controlled with bile acids
as bile is made of cholesterol, inhibition of bile acid up take by the gut can be a strategy for reducing cholesterol levels
what is the bile pigment
bilirubin is yellow coloured breakdown product of haem made in the spleen, bone marrow and liver
how is bilirubin transported
bound to albumin and is taken up by the liver where it is rendered soluble by conjugation with glucuronic acid
how is bilirubin broken down
bacteria in colon and ileum convert it into urobilinogen
what happens to urobilinogen
some reabsorbed into the blood and either resecreted in bile or secreted in the urine, rest lost in faeces
urobilinogen can be converted into …
yellow urobilin if oxidised on exposure to air in urine or brown stercobilin which is responsible for colour of faeces
why are bile salts amphiphatic
hydrophobic domains bind to surface of a fat globule while the hydrophilic domains face outwards as stabilization
what happens to triglycerides and lipids in the duodenum
they are emulsified into emulsion droplets which increases the surface area for attack by lipases
what enzyme is responsible for the hydrolysis of triglyceride within an emulsion droplet
pancreatic lipase
what does pancreatic lipase do
with the helper protein colipase it forms two free fatty acids and one 2-monoglyceride
what are mixed micelles
tiny aggregates of long chain fatty acids, monoglycerides, phospholipids and cholesterol, they are produced as a result of the action of lipases
what do mixed micelles do
ferry the products of fat digestion to the brush border, these enter the epithelial cells by diffusion or transport protein, dietary fat almost completely absorbed by the end of the ileum
how does cholesterol enter enterocytes
via special transporters, its absorption can be reduced by plant sterols
how is fat exported from the gut epithelial cells
fat digestion products and cholesterol bind to fatty acid binding proteins FABP within the epithelial cells of the small intestine, they then go to the ER where they are converted back to triglycerides
what happens to the triglycerides at the ER
combined with apolipoproteins phospholipids and cholesterol to form CHYLOMICRONS a lipoprotein particle
what happens to chylomicrons
exported from the golgi apparatus and released via exocytosis to enter the central lacteals of the villi. the chylomicrons within lymph enter the venous circulation via the thoracic duct
outline fat metabolism
lipoprotein lipase bound to capillary walls catalyzes the hydrolysis of triglycerides within chylomicrons
what happens to the fatty acids after they were produced by lipoprotein lipase
transported across endothelium and diffuse into the cells where they are resynthesized to triglycerides
when are VLDLs secreted
secreted by the liver when fasting as a means of exporting triglyceride and hepatic cholesterol to tissues. they contain phospholipids and apoplioproteins too
what are the ketone bodies
collectively they are the three products: the ketoacids acetoactate and b hydroxybutyrate or acetone
what is the longitudinal muscle layer of the colon gathered into
three bands, the taeniae coli
what are haustra
regular but non permanent divisions along the colon which arise from contractions of the circular smooth muscle
does the colon have villi
no
where are faeces stored
transverse colon
what are the roles of the large intestine
store mix and process contents. expose contents to mircobes. expel waste as faeces
what is the large intestine motility
ICCs within the circular muscle of the colon generate slow waves which can be prolonged by ACh
what is transit time through the colon
may be 1-2 days as colonic movements are low amplitude contractions to allow fluid absorption
how can forwards movement of contents be accelerated
through high amplitude propagating contractions allowing mass movements promoted by gastrocolic reflex
what is the internal anal sphincter
thickening of circular smooth muscle just inside the anus, has its own myogenic tone
what is the external anal sphincter
made of striated muscle therefore under somatic motor control. it is tonically contracted
what happens when the sigmoid colon is filled by a mass movement
Sensory nerves from the rectum send signals to the sacral
spinal cord, which responds via autonomic fibres in the
pelvic nerves, resulting in highly propulsive movements.
- The internal anal sphincter relaxes, a reflex that seems to
involve both intrinsic and extrinsic neurons.
- The external sphincter may be voluntarily relaxed.
- Relaxation of pelvic floor muscles lowers the anus, which
straightens the angle between rectum and anal canal.
- Defaecation is aided by the Valsalva manoeuvre
what is hirschsprung disease
ens ganglion cells are lacking in descending colon and internal anal sphincter»_space;> colon dilates to a large size and may perforate = congential megacolon
diarrhoea or constipation?
the consistency of the stool reflects the balance between overall GI secretion, ability of gut to hold water and absorption.
what are opioid receptors
in gastrointestinal tract which are stimulated by endogenous transmitters and promote decreased propulsion, decreased secretion and increased sphincter toner
what are the gut flora
bacteroides, bifidobacterium and eubacterium species.
describe gut bacterial metabolism
bacteria will metabolise and carbs in the colon to produce volatile fatty acids which are energy source for colonic cells
what is a high fibre diet believed to do
provide substrate for the metabolism of beneficial gut bacteria, relieve constipation, promote satiety, protect against bowel cancer
what happens to lactose in those that are lactose intolerant
undigested lactose gets through the colon, colonic bacteria thrive on it producing metabolites leading to excess gas, water retention and diarrhoea
what is the breath hydrogen test for lactose intolerance
hydorgen is some of the gas produced by the colonic bacteria
what is the importance of gut microbes
normal intestinal bacteria help maintain health digestive immune system
how are normal healthy gut microbes restored after diarrhoea or antibiotics
vermiform appendix may be a store of benefical microbes . the mucosal walls of the appendix contain masses of gut associated lymphoid tissue as a local defence against infection.
what is flatus
gas in the digestive tract, it contributes to borborygmi
what are the proportions of gases in flatus
50% n2, small amount of o2 and 25% h2, 15% co2 and 10% methane
how much faeces is produced per day
120 grams of which 75% is water > bacteria > fat/protein> inorganic matter and remainder is indigestible fibre
