Download Movement of the small intestine

GIT physiology
‫المحاضرة الخامسة‬
‫كلية الطب‬
‫المرحلة الثانية‬
3- Small intestine
The intestinal walls are made up of smooth muscle that contract and relax moving the
food or chyme forward then slightly backward. This gives the intestine additional time for
absorption.
It is major site of digestion and absorption of carbohydrates, proteins, and fat on GIT. The
small intestine has three parts: the duodenum, the jejunum, and the ileum It is 5 m long, it has
an absorption area of over 250 m2 .
The entire mucosal surface is lined by microvilli, protrude from the surface of intestinal
cells, give intestinal mucous its characteristics brush border appearance .The villi contain
capillary and lymphatic which transmit the absorbed substance into the portal circulation.
Brush border of a gastrointestinal epithelial cell, showing also
absorbed pinocytic vesicles, mitochondria, and endoplasmic
reticulum lying immediately beneath the brush border
Movement of the small intestine:
The smooth muscles of small intestine is myogenic property(spontaneous contraction )
serves two major functions:
1- Mixing the chyme with digestive juices and bile to facilitate digestion and absorption.
2- Propelling the chyme from the duodenum to the colon.
Types of movements:
A- Segmentation(mixing contraction):
It is most common type of intestinal contraction. in which the Chyme cause distention of
intestine wall lead to streach reflex and this cause concentric contraction of 1cm interval then
few seconds later the contraction of the previous segment will disappear and another one will
occur at the mid portion of relaxed segment, forcing the chyme back toward the stomach and
toward the colon. when muscle relaxes, the chyme return to the area from which it is displaced,
it is occur in more frequency in duodenum than the ileum. The segmentation contraction are
weak when enteric plexus is blocked by atropine.
Segmentation(mixing movements) of the small intestine
B- Peristalsis ( Propulsive movement)
Chyme is propelled through the small intestine by peristaltic waves occur in any part
of the small intestine, and they move toward the anus. Contraction of small section of
proximal muscles is followed by relaxation of muscles just distal to it. Resulting wave like
motion moves food along the GIT from proximal to distal direction as well spread out the
chyme along the intestinal mucosa. movement of the chyme is very slow only 1 cm/min.
This means that 3 to 5 hours are required for passage of chyme from the pylorus to the
ileocecal valve.
Peristalsis ( Propulsive movement)
It is greatly increased after meal. This is caused by enter of the chyme into duodenum by
gastroenteric reflex. In sever infection diarrhea can cause powerful and rapid peristalsis
called peristaltic rush. This is initiated by vagovagul nervous reflex to brain stem and back
to GIT.
C- Migrating motor complex contractions
During fasting between periods of digestion, the pattern of electrical and motor
activity in gastrointestinal smooth muscle becomes modified so that cycles of motor activity
migrate from the stomach to the distal ileum. These cycle called migrating motor complex
(MMC).
Gastric secretion, bile flow, and pancreatic secretion increase during each MMC. They are
immediately stopped by ingestion of food . It is peristaltic wave that remove food left in
stomach and intestine. They likely serve to clear the stomach and small intestine of luminal
contents in preparation for the next meal. It begin within esophagus and travel through
entire GIT and occur even 60-90 minutes.
The hormone motilin which is released from the epithelium of small intestine increase
the strength of contraction MMC ,(migrating motor complex) is a set of strong contractions
that lasts a few minutes in one section of the digestive tract, then moves aborally.
Their roles 1) flush remaining food and bacteria into the large intestine 2)tell you that
you’re hungry.
Intestinal secretion:
1- Mucus: Serves as a protective role, preventing HCL and chyme from damaging the intestinal wall. Mucus is
secreted by Brunner's glands which are located within duodenum, and by goblet cells which are located along the
length of intestinal epithelium. It is secreted
in response to tactile or irritating stimuli, to vagal stimulation, and to
GIT hormones secretion, Brunner's glands, Inhibited by sympathetic stimulation which may lead
to peptic ulceration.
2- Enzymes: Epithelial cell of the mucosa covering the villi contain digestive enzymes.
They capable of breaking down a small peptide by peptidase to amino acids ,disaccharide
to monosaccharides by sucrase ,maltase and lactase and lipase for fats into glycerol and
fatty acids .They are not secreted into intestine, they able to digest them during absorptive
process.
3- Water and electrolytes:
They are secreted by all the epithelial cells of the intestine. The water secretion provides
a solvent into which the products of digestion are dissolved.
Function of the ileocecal valve:
A principal function of the ileocecal valve is to prevent back flow of fecal contents
from the colon into the small intestine. the ileocecal valve itself protrudes into the lumen of
the cecum and therefore is forcefully closed when excess pressure builds up in the cecum
and tries to push cecal contents backward against the valve lips. The valve usually can resist
reverse pressure of at least 50 to 60 centimeters of water.
The wall of the ileum for several centimeters from the ileocecal valve has a thickened
circular muscle called the ileocecal sphincter. This sphincter normally remains mildly
constricted and slows emptying of ileal contents into the cecum. after a meal, a gastroileal
reflex intensifies peristalsis in the ileum, and emptying of ileal contents into the cecum
.Resistance to emptying at the ileocecal valve prolongs the stay of chyme in the ileum and
thereby facilitates absorption. Normally, only 1500 to 2000 milliliters of chyme empty into
the cecum each day.
colon
ileum
Pancreas
Anatomy:The pancreas, which lies parallel to and beneath the stomach is a large
compound gland with most of its internal structure similar to that of the salivary glands that
contains acini and salivary ducts.
The pancreas is composed of both exocrine cells and endocrine cells. The exocrine cells
are located within the acini. These cells are responsible for the digestive enzymes produced
by the pancreas, and large volumes of sodium bicarbonate solution are secreted by the
small ductules and larger ducts leading from the acini.
The combined product flows through a long pancreatic duct that normally joins the hepatic
duct immediately before it empties into the duodenum through the papilla of Vater,
surrounded by the sphincter of Oddi.Scattered among the acini are the pancreatic islets.
Within the islets are located endocrine cells which produce insulin and glucagon. These
hormones play an important role in blood glucose homeostasis.
Anatomy of pancreas
pancreas acini, is the blind end of a branching duct system is lined with aciner cells, that
secrete the enzymatic portion of pancreatic secretion. The ducts are lined with ductal cells,
ductal epithelial cells extend into the acinus' into a special region of centroacinar cells,which
secrete aqueousHC03.
pancreatic acini
Pancreatic secretion:
The exocrine part of pancreas secretes pancreatic juice about I.5 litter I day of an
aqueous component high in HCO3¯( to neutralize the H+ from stomach) and an enzymatic
component to digest carbohydrates, proteins and lipid. aqueous secretion rich in bicarbonate
,Na+, K+, CL -,water its responsible for washing out the acini and duct to pare enzymes secretion
into the duodenum.
pancreatic secretion
The pH is alkaline 6-7 which helps to neutralize the acid chyme and also this range of pH is
essential for pancreatic enzymes activity.Pancreatic juice is secreted most abundantly in
response to the presence of chyme in the upper portions of the small intestine, and the
characteristics of the pancreatic juice are determined to some extent by the types of food in the
chyme.
Exocrine pancreas is innervated by both parasympathetic and sympathetic nervous
system, Parasympathetic activity stimulates pancreatic secretion, and sympathetic activity
inhibits pancreatic secretion ( in contrast with the salivary gland, in which both sympathetic and
parasympathetic activity are stimulatory).