Secretion of small intestine:
Extensive array of compound mucous secreting glands located
in the 1st few centimeters of duodenum , these glands secrete
mainly alkaline mucous in response to :
1- tectile or irritating stimulus of the overlying mucosa
2- vagal stimulation
3- gastrointestinal hormones especially secretin
The function of mucous secreted by the glands is to protect the
duodenal wall from digestion by gastric juice ., their rappid &
intense response to irritating stimuli is especially geared to this
purpose , in addition , the secretin stimulated secretion by
glands contains a large excess of bicarbonate ion which add to
the bicarbonate ion from pancreatic secretion &bile in
neutralizing acid entering duodenum from stomach .
bruner´s gland is inhibited by sympathetic stimulation ,
therefore , such stimulation is likely to leave duodenal bulb
unprotected &is perhaps one one of the factors that cause this
area to be site for peptivc ulcer in abou 50% of cases
Secretion of intestinal digestive juice by crypts of
leiberkuhn:
crypts of leiberkuhn are small pits located on the entire surface
of small intestine between the villi , the intestinal surface of
both the crypt &villi are covered by an epithelium composed of
2 types of cell : amoderate number of goblet cells which
secrete mucus that lubricate &protect the intestinal surface , &
largre number of enterocytes which in the crypts secrete large
quantity of water &electrolytes &over the surface of the villi
reabsorbe water &electrolytes ,the intestinal secretions are
formed by the enterocytes of the crypt at rate about 1800ml/
day , have slightly alkaline PH(7.5- 8), this circulation of fluid
from the crypt to the villi supplies a watery veichle for
absorption of substances from the crypt as it come in contact
with villi which is the primary function of S.I.in addition to the
watery secretion , the goblet cells in the epithelium secrete
moderate amount of mucus which provide its usual function of
lubrication &protection of intestinal mucosa .
Mechanism of secretion of watery fluid:
The exact mechanism that cause the marked secretion of
watery fluid by the crypts of leiberkun is not known, it is
believed to involve at least 2 active secretory processees:1active secretion of bicarbonate ion &2-active secretion of
chloride ion . Finally all these ions together osmotic movement
of water .
Enzyme in the secretion of S.I.”
When secretion of S.I.are collected without cellular debris they
have almost no enzyme , howevere, the the enterocytes of the
mucosa , especoially those that cover the villi , contain
digestive enzyme that digest specific food substances while
they are being absorbed through the epithelium .. These
enzymes are 1-several peptidase (for splitting peptides in to
amino acids )
2- some enzymes for splitting disacharides in to
monosacharides
3- intestinal lipase splitt neutral fat in to glycerole &fatty acids .
Most if not all of these enzymes are mainly in the brush border
, yhey are believed to hydrolysis of food on the outside surface
of the microvilli before absorption of the end products .
Regulation of small intestinal secretion :
1- local stimuli: the various local nervous reflexes in
response to the presence of chyme in the intestine
especially reflexes intiated by tactile or irritative stimuli
&by increase enteric nervous activity associated with
gastrointestinal movements , therefore,the greater amount
of chyme the greater secretion .
2- hormonal secretions:some of hormones that promote
GITsecretion also increase S.I. secretion , especially
secretin &cholecystokinin in general , the local enteric
reflex mechanism play the dominant role.
Secretion of large intestine :
1-mucus secretion :the mucosa of the large intestine has many crypts of
lieberkun but unlike that of S.I.there are no villi ,also the epithelial cells
contain no enzymes , they consist mainly of mucous cells that secrete only
mucus , therefore, the great secretion in the large intestine is mucus , this
mucus containe large amount of bicarbonate ion caused by active
transporte through other epithelial cells that lie between epithelial cells that
secret mucus .
The rate of secretion regulated by:
1- tactile stimulation 2- local nervous reflex 3- stimulation of pelvic nerves
which carry parasympathetic innervation to cause marked increase in
mucus scretion .
Mucus in large intestine 1- protect the wall against excoriation
2- provide the adherant medium for holding fecal matter together . 3- protect
the intestinal wall from bacterial activity
4 it plus the alkalinity of secretion .
2- secretion of water &electrolytes :when the L.I. becomes irritated like in
bacterial infection , the mucosa secretes large quantities of water
&electrolytes in addition to the normal viscid solution of alkaline mucus this
act to dilute the irritating factors &cause rappid movements of fluid results in
diarrhea,also washes away irritant facters