Download Lec.9 Thursday 21 Apr. 16 Dr.baybeen Al

Lec.9
Thursday 21 Apr. 16
Dr.baybeen Al-selevany
Lower GIT: Small intestine & Large intestine
Small intestine
Objectives
 Physiological anatomy
 Functions
 Intestinal secretion
 Intestinal motility
Physiological Anatomy: 3 parts
duodenum
jejunum
25 cm. length
2.60 cm. length
ileum
Small intestine is a tube structure that begins at the pyloric sphincter
superiorly and extends inferiorly to ileoceal valve {an area where the last
segment of small intestine (ileum) joins the first area of large intestine
(cecum)}. Food remains in small intestine about 3-5hours.
Functions of small intestine: The two processes occurs mainly in small
intestine.1.digestion which begins in the mouth and stomach and is
completed in the lumen and mucosal cells of the small intestine, and 2.
2.absorption the products of digestion are absorbed along with most of
the vitamins and fluid..Absorption due to presence of villi and microvilli
in intestinal mucosal cells that serve to increase the surface area of
absorption.
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3. Immunity: In addition to digestion and absorption, small intestine
performs immunity function. At the distal small intestine there are
collections of lymphoid tissue called Peyer's patches, they serve to
destroy bacteria and prevent their entery into the circulation.
Intestinal Secretion
About 9 L. of fluid / day enter small intestine, 2 L. from dietary
source and 7 L. of gastrointestinal secretions (endogenous secretions)
such as salivary glands, stomach, bile, pancreatic juice, and small
intestine. About 7 L. is absorbed by small intestine, only 1-2 L.
passes to the colon.0nly 100ml is excreted in the feces.
Composition of small intestinal secretion: about 1 – 2 liters of intestinal
juice is secreted daily in response to distension of intestinal mucosa or
irritation by acids or hypertonic chime. It's slightly alkaline with pH of
7.4 -7.8
1- Mucus secreted by Brunner’s gland and crypts of Lieberkühn.
these glands secrete alkaline mucus. The function of mucus is to
protect the duodenal from digestion by the gastric juice.
2- Digestive enzymes: Most of these enzymes in brush border of
enterocytes and secreted by crypts of Lieberkühn.
a. Peptidase for splitting peptides into amino acids
b. Four enzymes for splitting disaccharides into monosaccharide:
sucarse, maltase, lactase and isomaltase.
c. Intestinal lipase – for splitting neutral fats into glycerol and
fatty acids.
3. Water secreted by crypts of Lieberkühn.
Regulation of small intestinal secretion
1. Neural control: by far the most important means for regulating
small intestine secretion are: a. local enteric nervous reflexes
(submucosal or Meissner's plexus), especially reflexes initiated
by tactile or irritative stimuli from the chyme in the intestines.
Vagal stimulation increases the intestinal secretion. The greater
amount of chyme in the small intestine, the greater the secretion.
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2. Hormonal regulation: especially hormone secretin which
enhances small intestinal secretions.
Intestinal Motility
The movement of small intestine can be divided:
1) Mixing contractions (segmentation contractions) are localized
concentric contractions spaced at intervals along the intestine
dividing the intestine into spaced segments. When a portion of the
small intestine become distended with chyme to intestinal wall
elicits mixing contractions these mixing contractions is determined
by BER which is about 12/min in the duodenum and 8 /min in
distal ileum Mixing contractions – mix chyme with intestinal
secretion. Mixing contractions depend on myenteric plexus which
inhibited by atropine. In the duodenum, the intestinal contents are
mixed with pancreatic juice (from pancreas) and bile (from
gallbladder) that delivered to it via common bile duct through
sphincter of Oddi, and with mucosal secretion.
2) Peristalsis – propulsive movements: It propels the intestinal
contact toward the large intestine. Are much faster in the proximal
intestine and much slower in the terminal intestine (ileum). Are
very weak, so that the movement of the chyme is also very poor
normally about 3-4 hours are required for passage of chyme from
pylorus to the ileocecal valve.
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Regulation of intestinal motility and emptying
Ileocecal valve is present between ileum and cecum and a principal
function of the ileocecal valve is to prevent backflow of fecal material
from colon into the small intestine. This sphincter normally remains
mildly constricted and slows emptying of ileal contents into the cecum.
A. Factors that promote emptying
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1. Intense peristaltic contraction of distal part of ileum
2. Relaxation of ileocecal sphincter.
After a meal there is a reflex called gastroileal reflex (that is
initiated by distension of the stomach and conducted through
myenteric plexus from stomach down along the wall of the small
intestine) which intensified the peristalsis in the ileum. Also the
hormone gastrin which liberated from the stomach, has a relaxant
effect on the ileocecal sphincter, thus allowing increased emptying
B. Factors that inhibit emptying
1. Inhibition of peristaltic contractions in the distal ileum.
2. Increase in the tone(contraction) of ileocecal sphincter
The degree of contraction of ileocecal sphincter, and the
intensity of peristalsis in the terminal ileum, is also controlled
by reflexes from the cecum. For example colonileal reflex:
Distention of the cecum increase contraction of ileocecal
sphincter and cause inhibition of ileal peristalsis and
emptying, this greatly delays emptying of chyme from ileum.
Also Inflammation of the appendix (appendicitis) cause
spasm of the ileocecal sphincter
Peristaltic rushes: are abnormal very intense, powerful, rapid
peristaltic waves in small intestine caused by intense irritation of
the intestinal mucosa as occurs in some severe cases of infections
diarrhea can cause peristaltic rush.
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