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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 alsoabsorbed 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 intestineB- 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.

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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 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 especially from Crypts of Lieberkühn ,lie between the intestinal villi. 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.

Pancreas 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 two major types of tissues:- (1) the acini, which secrete digestive juices (pancreatic juice) into the duodenum , through the papilla of Vater, surrounded by the sphincter of Oddi. (2) the islets of Langerhans ,found around small capillaries Scattered among the acini, play an important role regulation of glucose, lipid, and protein metabolism and blood glucose homeostasis ,which contain three major types of cells, alpha, beta, and delta cells ,The beta cells, about 60 % of all the cells of the islets, lie in the middle of each islet and secrete insulin.The alpha cells, about 25 % of the total, secrete glucagon.

colon

ileum

And the delta cells, about 10 %of the total, secrete somatostatin. These cells are close interrelations allow cell-to-cell communication and direct control of secretion of some of the hormones by the other hormones. For example insulin inhibits glucagon secretion, , and somatostatin inhibits the secretion of both insulin and glucagon.

pancreatic 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 about I.5 litter I day of pancreatic secretion

contains :-1-an aqueous component high in bicarbonate HCO3¯( to neutralize the H⁺ from stomach)Na⁺, K⁺, CL ⁻,water ,its responsible for washing out the acini and duct to pare enzymes secretion into the duodenum.2-an enzymatic component to digest carbohydrates, proteins and lipid.

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).

Pancreatic Digestive Enzymes:

1- The proteolytic enzymes for proteins digestion Are trypsin, chymotrypsin, split digested proteins into peptides ,and carboxypolypeptidase does split some peptides into individual amino acid.

2 -The digestive enzyme for carbohydrates

Is pancreatic amylase which hydrolyzes starches, glycogen and most other carbohydrates (except cellulose) to form disaccharides and a few trisaccharides ..

3- The main enzyme for fat digestion is(a) pancreatic lipase which is capable of hydrolyzing neutral fat into fatty acids (b) cholesterol esterase, which causes hydrolysis of cholesterol esters(c) phospholipase, which splits fatty acids from Phospholipids.

Pancreas protects itself from auto digestion by proteolytic enzymes-:

by the following ways:

A- When proteolytic enzymes synthesized in the pancreatic cells, they are in the inactive forms, These become activated only after they are secreted into the intestinal tract, they are

activated by an enzymes are secreted by the intestinal mucous called enterokinase .

B- The same cells that secrete the proteolytic enzymes into the acini of the pancreas secrete another substance called trypsin inhibitor. This substance is stored in the cytoplasm of the glandular cells surrounding the enzyme granules and it prevents activation of trypsin inside the secretary cells and in( the acini and ducts) of the pancreas.

Regulation of Pancreatic SecretionThree basic stimuli are important in causing pancreatic secretion:

1 .Acetylcholine, which is released from the parasympathetic vagus nerve endings and from other cholinergic nerves in the enteric nervous system

2 .Cholecystokinin, which is secreted by the duodenal and upper jejunal mucosa when food enters the small intestine

3 .Secretin, which is also secreted by the duodenal and jejunal mucosa when highly acid food enters the small intestine

The first two of these stimuli, acetylcholine and cholecystokinin, stimulate the acinar cells of the pancreas, causing production of large quantities of pancreatic digestive enzymes but relatively small quantities of water and electrolytes to go with the enzymes, while secretin, in contrast to the first two basic stimuli, stimulates secretion of large quantities of water solution of sodium bicarbonate by the pancreatic ductal epithelium. pancreatic secretion normally results from the combined effects of the multiple basic stimuli, not from one alone.