Gastrointestinal absorption:
1.There are many folds called valvulae connivents
which increase surface area of absorption of mucosa
about 3 folds located on the intire surface of S.I.from the
point at which CBD empties into the duodenum down to
the ileocecal valve .
2.Villi which project about abouit 1 millimeter from the
surface of mucosa , the presence of villi enhance
absorptive surface about 10 folds.
3.Each intestinal epithelial cells is characterized by
brush border consisting of microvilli this increase surface
area about another 20 fold .
so by combination of the folds , villi &microvilli the
absorbtive surface area increase to about 1000 folds.
Absorption of water :
Water is transported through the intestinal
membrane by diffusion this diffusion obey the law
of osmosis , when the chyme is dilute , water
absorbed through the intestinal mucosa in to the
blood of villi by osmosis , the water can be
transported in the opposite direction from the
plasma in to the chyme , when hyperosmtoic
solution are discharged from stomach into the
duodenum , water transferred by osmosis to
make the chyme isomotic with plasma.
Absorption of ions:
Active transporte of Na+:
20-30 gram of Na is secreted daily in to the intestinal
secretion , in addition , the normasl person eat
5- 8 gram/ day sodium , so the S.I. must absorb 2535 gram /day sodium which equal to about 1/7th of all
sodium present in the body , normally less than
0.5% of intestinal sodium is lost in feces each day
.the sodium also plays an important role in the
absorption of suger & amino acids , the basic
mechanism of Na+ absorption from the intestine are
the same as those for absorption of Na+ from the
renal tubules&G.B.The motive power for Na+
absorption is provided by active transport of Na+
from inside the epithelial cells in to the paracellular
spaces , it requires energy, the energy process is
catalized by ATPase enzyme in the cell membrane
This active transport reduces the Na+
concentration inside the cell to about 50
mEq/liter), but the Na+ concentration in the
chyme is normally about 142mEq/L sothat Na+
moves from the chyme through the brush border
of epithelial cell in to the cytoplasm of epithelial
cell &replaces the Na+ that is actively
transported, the next step in the transport process
is osmosis of water in to the paracellular spaces ,
this is caused by the the elevated concentration
of ions in the paracellular spaces.
Aldosteron greatly enhance sodium absorption :
When a person becomes dehydrated , large
amount of aldosterone are secreted from adrenal
gland within 1-3 hours , the excess aldosterone
enhance the enzyme &transport mechanism of
Na+ absorption by epithelial cells of intestine .
The increased Na+ absorption lead to increase in
absorption of Cl – ion , water & some other
substances , this effect of aldosterone is
important to prevent loss of water & electrolytes.
- Absorption of bicarbonate in the duodenum &
jejunum: :
large quantity of bicarbonate must be reabsorbed
from the upper S.I , because of the secretion of
large amount of bicarbonate through the
pancreatic &bile secretion this absorption is in
indirect way as the fallowing :
When Na+ ion are absorbed , moderate amount
of H+ ion are secreted into the lumen of the gut
in exchange with some of Na+, these H+ ion
combined with bicarbonate ion to form carbonic
acid, which then dissociated into water &carbone
dioxide , the water remain as part of the chyme in
the intestine , but CO2 is absorbed to the blood ,
then expired through the lung .
Absorption of other ions :
Calcium actively absorbed in the duodenum ,
controled by the needed of Ca ion in the body
.parathyroid hormone activate vitamine D
enhances Ca absorption .
Iron ion actively absorbed in S.I.the absorption
controlled by the body neeed to iron, especially
for Hb formation .
Potassium , magnesium, &phosphate :
Actively absorbed .
Absorption of nutrient:
Absorption of carbohydrate:
All CHO are absorbed in the form of
monosaccharide &only small fractions absorbed as
disaccharides , glucose absorption is the most
absorption of monosaccharides (80%), the
remainder (20%) are galactose & fractose, because
glucose is the final digestion products of most
CHO. In food .
Glucose is basically transported by Na+- co
transport mechanism:
In the absence of Na+transport through the
intestinal membrane , virtually , no glucose can be
absorbed , because absorption of glucose occur
with active transport of sodium.
There are 2 stages in the transport of sodium
through the intestinal enterocyte:
1st is active transport of Na+ through the basolateral
membrane into the paracellular spaces , this leade
to decrease the sodium inside the cells , then cause
diffusion of Na+ through the brush border of the
enterocytes from intestinal lumen to inside of these
cells by facilitated diffusion .the sodium 1st
combined with transporter protein , but this will not
transport Na+ to inside the cell until it also
combined with other substance such as glucose ,
therefore,both sodium &glucose are transported
together to the interior of the cell.
Inside the enterocytes there are other transport
protein & enzyme cause facilitated diffusion of the
glucose outward through the basolaterl membrane
of enterocytes into the paracellular spaces.
The digestion of disaccarides & trisaccharides at
the brush border enhance glucose transport.
When disach. &trisach. Composed of glucose
come in contact with the brush border , the
digestive enzyme that attached to the membrane
of the brush microvilli cause hydrolysis of
disach.&trisacch. into glucose , this digestion
increase the concentration of glucose in the area
adjacent to the absorptive membrane of
enterocytes , the high concentation of glucose
then plays an additional role in enhancing
glucose transport to interior of enterocytes&then
all the way through the enterocytes into the
paracelluilar spaces .
Absorption of other monosaccharides
Galactose transported by the same mechanism as glucose .
Fractose by facilitated diffusion , most of fractose converted
to glucose all the way through the enterocyts.
Absorption of proteins :
Most of protein are absorbed through the S.I.enterocyts in the
form of di&tripeptides &free amino acids by the same way
that sodium co- transport of glucose occurs that is called cotransport or 2ndry active transport of the amino acids or
peptides .
A few a.a .are transported by facillitated diffusion in the same
way that fractose is transported.
Absorptoion of fat :
The end products of fat digestion ( monoglycerids
&free fatty acids ), become dissolved in the central
lipid portion of bile acid micells , in this form , the
monog. & free f.a. are carried to the surface of
microvilli in the brush border , penetrating among
the moving , agitating microvilli , here both monog.
&f.a diffuses immediately through the enterocyts
cell membrane to the interior of the enterocyts
because these lipids are soluble in the enterocyts
membrane as in the micells , this leave the bile acid
micells in the chyme & absorbe more monog. & f.a . ,
thus the micells perform a ferrying function , which
is highly important for fat absorption . In the
absence of bile acid only 40- 50% is normally
absorbed & about 97& of fat absorbed of
abundasnce bile acid micells.
Small quantities of the undigested tri & diglycerids
(which are highly soluble in the lipid mkembrane
of enterocytes are normally absorbed because
bile acid micells will not dissolve them & will not
ferry them to the enterocyte membrane , after
entering the enterocytes, the f.a. & monoglycerids
are taken up by the SER & here they recombined
to form new triglyceride , a few of monog. Are
digested into glycerol & f.a.by intracellular lipase ,
then these free f.a. are reconstituted by SER into
triglyceride by using new glycerol that synthesis
required energy from ATP& complex of enzyme.
Formation of chyloimicrone :
The reconstited trglycerids aggrigate 1st within the
E.R. & then in the golgi apparatus into globuls that
contain absorbed cholesterol, absorbed
phospholipid , a small amount of newly synthesized
cholesterol & , phospholipid then phspholipid
arrangev themselves in these globules in which the
polar portion located on the surface , this provide an
electrically charged surface that make these globule
missible with the fluid of the cell, in additition
several types of apoproteins also synthesized by
E.R. coat part of the surface of globule , in this form
the globuls released from golgi apparatus &
excreted into the basolateral spaces & pass to the
lymph these globuls then called chylomicrons
Transport of chylomicrons in the lymph:
When the chylomicrons wend their way into the
central lacteals of the villi they are propelled
along with the lymph by the lymphatic pump
upward through the thoracic duct to be emptied
into the great vein of the neck , between 80- 90%
of alll fat absorbed from the gut is absorbed in
this manner &transported to the blood by way of
the thoracic lymph in the form of chylomicrone. .