L32 Flashcards
what are the Sites of absorption
mouth and esophagus
stomach
intestine
what is the small intestine responasabele for in absorbtion
bulk absorption of isosmotic fluid and nutrients
what is the role of the proximal colon of the large intestine responsible for in absorption
absorption of products of fermentation
fluid and electrolytes
what is the role of the distal colon (large intestine) in absorption
selective absorption of fluid and electrolyte
what if the differnce in epithelia between the large and small intestine
small = leaky
large = mostly tight
there are 2 ways of Na absorption. what are they
nutrient dependent
nutrient independent
describe nutrient dependent absorption in the small intestine
Na+ is coupled to solute absorption of monosaccharides and amino acids
it is also specialised for the absorption of..
bile salts
vitamin
describe nutrient dependent absorption of Na in the proximal colon (large intestine)
Na+ dependent absorption of short chain fatty acids
nutrient dependent of Na happens throughout the small intestine. where does it specify happen in the large intestine
in the proximal colon
where does nutrient independent absorption of Na happen
in both the large and small intestine
how does nutrient dependent absorption of Na happen in the small intestine
through SGLT1 and coupled with amino acids
don’t need to know the amino acid transporter names
what SGLT’s are found in the GI tract
only SGLT1
describe nutrient independent absorption on Na in the small intestine
this happens in the apical membrane through NHE3 (Na/H exchanger) and a Cl/HCO3 exchanger
what is important to note about the proximal and distal colon
they are functionally distinct
what happens at the proximal colon (not Na)
fermentation of indigestible sugars and proteins
generation of short chain fatty acids
describe Na absorption in the proximal colon
fermentation of indigestible sugars and proteins and generation of short chain fatty acids is coupled to Na+ absorption
this happens through electroneutral NaCl absorption
paired Na+/H+ and Cl-/HCO3- exchangers as in small intestine
Na+ absorption is associated with absorption of short chain fatty acid
Na in the proximal colon is absorbed through a Small chain fatty acid Na coupled transporter
what is the name of this transporter
Sodium monocarboxylate
transporter
There is 7L of water that we need to reabsorbe
Bulk reabsorption of water happens in the small intestine and then small part in the colon
g
what is the role of the distal colon
regulation of composition of faeces
describe Na absorption in the distal colon
Na+ absorption varies depending on fluid and electrolyte status and aldosterone (when you are dehydrated ENaC causes more Na to be reabsorbed)
salt depleted condition → high levels of plasma aldosterone
salt replete condition → low levels of plasma aldosterone
how many L of fluid do we secrete per day (usually)
1-2 L
absorption of Na depends on location. what is it dependent on at different locations in the GI tract
duodenum
Cl secretion
HCO3- secretion
jejunum
Cl secretion
ileum and proximal colon
Cl secretion
HCO3- secretion
distal colon
Cl secretion
what do we secrete water in the intestine
We are still secreting water in the intestine to absorb that last bit of nutrients
describe absorptive diarrhea
Glucogolactase (an example) causes absorptive diarrhea because you are not absorbing glucose therefore you are not absorbing Na and therefore not absorbing water
what is the difference between absorptive and secretory diarrhea
absorptive = you are not absorbing water from the intestine
secretory = you are actively secreting water from the intestine (more than normal and happens through too high levels of cAMP)
the main mechanism that causes water secretion in the small intestine and colon is what
electrogenic Cl secretion
explain the mechanisms that cause electrogenic Cl secretion in the small intestine and colon that cause water screation
active Cl secretion drives movement
of Na+ and water
result – isotonic solution of NaCl
the NKCC1 cotransporter causes an accumulation of Cl in the cell above
electrochemical equilibrium
Cl- channel = CFTR which is activated by cAMP
and is the exit pathway for Cl
what can cause secretory diarrhea
Vibrio cholerae
describe how Vibrio cholerae can cause secretory diarrhea
Vibrio cholerae is a bacteria that multiplies in the small intestine
Vibrio cholerae produces cholera toxin which stimulates adenyl cyclase which causes more cAMP to be made which stimulates CFTR leading to more secretion of Cl leading to more secretion of bicarbonate and therefore of water
what happens when vibrio cholerae secretes cholera toxin
the toxin binds to intestine, enters villous and crypt cells and stimulates adenylate cyclase irreversibly
this causes elevation of cAMP
which stimulates Cl- secretion in crypts
and inhibits nutrient-independent Na+
absorption in villi
this causes secretory diarrhea
loss of isotonic fluid
secretory diarrhea = loss of isotonic fluid
k
The absorption of Na+ and water in the small intestine
A. is mainly facilitated by channels.
B. is nutrient dependent.
C. is entirely nutrient independent.
D. is facilitated by CFTR.
A. These are characteristics of tight epithelium and the small intestine is leaky epithelium
C. It is both dependent and independent
D. Na has nothing to do with the CFTR receptor itself
B is correct
how much sugar do we inject per day
200-300g
western Diet - 40 - 50% energy intake
developing world - 90% of energy intake
polysaccharides, disaccharides and monosaccharides are the sugars we ingest
without breaking them down what % of each one is found in our diet
polysaccharides (45 -60%) - glycogen and starch
disaccharides (30 – 40%) = sucrose, lactose and maltose
monosaccharides (10%) – glucose, fructose
what sugars are we able to absorb
monosaccharides
these make up only 10% of you sugar intake therefore we need to break down the sugars we ingest
what are the main monosaccharides absorbed and how
glucose and galactose absorbed by Na+
-dependent mechanism (SGLT1)
fructose absorbed by Na+
-independent mechanism (GLUT5)
digestion of sugars to release monosaccharides is a 2-step process. describe it
luminal digestion
mouth - a amylase
small intestine – pancreatic amylase
the products of luminal digestion are
a-limit dextrins
short oligosaccharides and some disaccharides
contact or brush border digestion = enzymes attached to apical membrane of epithelial cells which release monosaccharides
such as
glucose, galactose, fructose
what are the products of luminal digestion
the products of luminal digestion are
a-limit dextrins
short oligosaccharides and some disaccharides
Contact digestion because the components need to make contact with the plasma membrane so that they can be further broken down
This happens on the brush border membrane
j
Lumanal digestion is starch with amalse, everything is soluable therefore you get pre digestion of suggars
They then they come into contact with the brush border membrane
Lactase breaks down lactose into glucose and galactose which BOTH can be transported couped to Na through SGLT1
Sucrase breaks down sucrose into frutose and glucose
Frotose is reabsorbed through GULT5
These are enzymes tha are membrane bound
k
what does absorption of glucose, galactose and fructose result in
absorption of isosmotic fluid (parracellular)
The absorption of glucose via SGLT1 is against the
concentration gradient,
BECAUSE
basolateral glucose
absorption is mediated by facilitated diffusion.
both statements are true and causally related
down the concentration gradient because although sugar might be high in the diet it is diluted in the cyme
idk why it is causal
proteins are not a source of energy but are required for amino acids
what are our sources of protein
50% diet
25% sloughed cells
25% digestive enzymes
digestion of proteins happens in a 2 steep process. what is this
luminal
contac
describe the digestion of proteins
luminal
proteins get broken down by H and pepsin in the stomach (gastric lumen) which leaves you with proteins and oligopeptides. then in the intestinal lumen these proteins broken down by trypsin, chymotrypsin, carboxypeptidase and elastases into oligopeptides
contact
oligopeptides then get broken down by enzymes in the membrane into amino acids and then are absorbed through amino acid transport proteins
NOTE di and tripeptides can be absorbed through peptide transport proteins and are then broken down into amino acids in the cytoplasum by peptidases, prolidase, dipeptidase and trypeptidase
what does Na dependent amion acid (and peptide) absorption result in
absorption of isosmotic fluid
fat digestion and absorption is very efficient, therefore if the system is so efficient how much fat is excreated per day
ONLY 5g
fat and products of digestion insoluble in water therefore they have several stages to digestion & absorption. what are they
theres 7
emulsification and stabilization (bile acids)
digestion (via lipase)
formation of micelles
absorption
re-esterification – synthesis of triglycerides
formation of chylomicrons
Absorption into the lymphatic system
what is the purpose of Emulsification and stabilization of fats with bile acids
reduction in size to droplets
motility
important as it increases the surface area available for
digestion
where does Emulsification and stabilization happen
stomach
crude emulsion, through motitty (mechanical digestion)
small intestine
emulsion stabilized by emulsification agents such as bile salts, phospholipids, cholesterol, lecithin
what are Emulsification agents
bile salts and phospholipids, cholesterol, lecithin
bile salts are amphipathic compounds
what digestive enzymes are involved in fat digestion
minor Enzymes
gastric and lingual lipases
minor role
main enzymes
pancreatic lipase/colipase
secreted by the pancreas into the small intestine
describe fat absorption
bile salts surround fat droplets to form emulsification droplets
bile salts + pancreatic lipase = micelles (smaller)
fat inside micelles gets broken down into FFA and monoglycerides which can diffuse through the cell membrane.
when inside the cell FFA and monoglyceride is resynthesized by enzymes in the ER and then packaged into vesicles bound for the basolateral membrane. these vesicles are called chylomicrons
the chylomicrons enter the lacteal and go into the lymphatic system
what are some fat soluble vitamins
A, D and E therefore they are absorbed in the same way as fats
water soluable vitamins have specific transport processes. what are some examples of this
Na+
-dependent e.g., vitamin C
vitamin B12
intrinsic factor is released in stomach
in duodenum B12 binds to intrinsic factor
intrinsic factor/B12 complex attaches to specific receptors in terminal ileum
complex absorbed by endocytosis and degraded to release B12 into portal
blood
You have a pool,of VB12 in the liver and when you cant absorb it this pool is depleted and then you end up with anemia
j
Protein
A. digestion starts in the mouth.
B. digestion occurs mainly in the stomach.
C. digestion is reduced during the gastric phase.
D. absorption is facilitated by brush border membrane peptidases
A. starts in stomach with pepsin
B. protein digestion MAINLY occurs at the brush border
D. is correct
