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Small Intestine and Large Intestine Function & Motility

SMALL INTESTINE

Is part of GIT extending from pyloric sphincter of stomach to ileocecal valve.

Major site for digestion & absorption of carbohydrates, proteins & fats.

Length around 4-5 mts.3 parts – Duodenum, jejunum & ileum.Secretion from small intestine called as succus

entericus.

Mucous membrane of SI is covered by Intestinal villi of height 1 mm. The villi are lined by columnar cells called enterocytes.

Each enterocyte gives hair like projection called microvilli, increases surface area for absorption.

Simple tubular glands lined by columnar cells in intestine called Crypts of Lieberkuhn.

These columnar cells show active mitosis.Brunner’s glands seen majorly in duodenum

region.

Crypts of Lieberkuhn – secrete the intestinal juice

1. Enterocytes : digestive Enzymes2. Goblet cell : Mucus3. Paneth cell : Glycoproteins , Immunoglobulin,

Lysozymes (defensins)4. Enterochromaffin cells : Seretonin , 5HT

Endocrine cells: GI hormones, e.g. Secretin, CCK, VIP, GIP

Volume 1800ml/day Alkaline, 8.3 pHisotonic with plasma S.G : 1010composed of water (about 98%)Solids

inorganic salts Cations - K+, Na+, Ca++ Anions - Cl-, HCO3-

jejunal secretions : Cl-, K+ is ileal secretions : Na+, Ca++

SUCCUS ENTERICUS

Organic constituents: ProteinsMucinAlbuminImmunoglobulin A numerous shed epithelial cells of the intestinal

mucous membraneDigestive enzymes - present in the shed

epithelial cells & little role in digestion

Functions of succus entericus

1) Digestive functionBy Proteolytic enzymesBy Amylolytic enzymes

By Lipolytic enzymes

.

2). Protective function. Mucus protects lining of small intestine.Paneth cells of intestinal glands secrete defensins .

3) Enterokinase present in intestinal juice activates trypsinogen into trypsin

Intestinal Enzymes:

I. Activating Enzyme

Enteropeptidase or enterokinase is a brush border enzyme of the duodenum and jejunum.

released in response to secretin, CCK or bile acids

converts inactive trypsinogen into active trypsin.

II. Digestive Enzymes

(a) Proteolytic Enzymes. Break down polypeptide into aminoacids

CarboxypeptidasesEndopeptidasesDipeptidases

(b) Enzymes Acting on Nucleic AcidsNuclease. Nucleotidase Nucleosidase

Fat Splitting (Lipolytic) Enzyme: Intestinal lipase present in the brush border of epithelial cell

its action is much weaker than that of pancreatic lipase.

Enzymes Acting on Carbohydrates: important amylolytic enzymes are the

disaccharidases Maltase Sucrase Lactase. Isomaltase Trehlase Amylase

Regulation of Small Intestinal Secretion Local ; Neural ; Hormonal ;

Local Stimuli: Presence of chyme in the intestine increases

intestinal secretion. Distention of intestine stimulate secretion,

probably by local nervous reflexes.

2. Extrinsic Nerves- Vagal stimulation - increases secretion of

glands Sympathetic stimulation inhibits secretion.

Inhibition helps to prevent loss of fluid & electrolyte help absorptive process

3. Hormones:

GIT hormones: VIP, GIP & Gastrin – stimulates intestinal secretion

Acetylcholine – increases & noradrenaline inhibits secretion

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Small intestinal movements

FunctionMixing AbsorptionPropulsion

Types: BER MMC

Mixing - segmentation

- pendular movement

propulsive -peristalsis

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Types of movements

Segmentation contractionsRhythmic contractions Intestine is divided into segments of 1-2cmsCircular muscle contractionEach contraction lasts for 1 – 3 secondFunction

• mixing

• absorption

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Segmentation

CommonestAlternating

contractionMixing

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Pendular movementsSome times seen Simple constrictions of the intestinal wall

which moves up & down for short distancesCauses to & fro movements of chymeAids mixing & exposure to greater area of

mucosa

Tonic contractionsProlonged contraction of segments of

intestineOne segment – isolated from another

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Peristalsis

Distended bowel, causes contraction behind the stimulus (proximal) & relaxation in front (distal)

Velocity of peristaltic wave – 0.5 – 2.0 cms/secMovement of chyme – 1 cm/minDepends on the integrity of myentric plexus.

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Acetylcholine & substance P - cause contraction behind stimulus

VIP & Nitric oxide - cause relaxation in front of stimulus

LAW OF GUT.

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Peristalsis

Function Rapid propulsion

of intestinal content

‘clearing’

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Peristaltic rush – strong / rapid peristaltic waves –

travel long distance & quickly sweep the intestinal

content into colon e.g. intestinal irritation

(catharitics) or excess distension

Antiperistalsis - vomiting

Peristalsis

For peristalsis,intact nerve supply has to be there.

Do not occur in denervated intestine.The extrinsic hormones also influence. As parasympathetic system- stimulate &

sympathetic nerves- inhibit the peristalsis. Gastrin, CCK-PZ, serotonin, etc ↑, whereas

secretin,etc ↓ peristalsis. But the presence of food in intestine is an

important stimulus.

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Movement of intestinal villi

Fasting – villi ~ inactive & lie flatContact with food

To & fro Lashing or swaying movementsShorten & elongate alternatelyStirring of fluids – help in absorption, expulsion of

lymph from lactealsDependent on intact submucosal plexusStimulation

Vagal – increases movementsSympathetic – pale & motionless

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Regulation of intestinal motility

Coordinated & regulated by – Local , Neural & Hormonal mechanisms

Segmental & pendular movements – myogenic , occurs in denervated loop.

Peristalsis is an intrinsic neural reflex

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Extrinsic influence & regulation Parasympathetic - Vagus – increases – tone & intestinal

motilitySympathetic – inhibits

Humoral factors - Motility & Tone

Increased by Decreased byAcetylcholine NoradrenalinSerotonin SecretinMotilin Glucagon CCK VIPGastrin NOInsulinHistamineSubstance P

Applied physiology1) Malabsorption syndrome:

Causes- a} Resection of small intestine

b} Gastro-colic fistula

c} Sprue

d} Coeliac disease

Sprue : a intestinal disorder multiple causes leading to damaged intestinal mucosa.

3) Coeliac disease: characterised by congenital absence of enzyme gluten hydrolase in intestinal mucosal cells, results in formation of a toxic polypeptide ‘Gliadin’ from Gluten (protein in barley,rye,wheat,etc).

Gliadin causes intestinal T cells to produce an inflammatory allergic response that flattens & disrupts the formation of microvilli.

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clinical features : Indigestion, diarrhoea, anorexia & weight loss,

abdominal cramps Nutritional defieciancy ,macrocytic anemia.Vit B, Folic acid and Vit k defieciency. Osteomalcia ,

Steatorrhea .etc.

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Adynamic ileus: (Paralytic ileus)

Intestines are traumatised, smooth muscles are directly inhibited.→ motility↓

Peritoneal irritation.—reflex inhibition due to ↑ noradrenergic dischrage.

Intestines will be irregularly distended by pockets of gas and fluid.

Treatment :obstruction relieved by aspiration of gas and fluid by Ryle’s tube till peristalsis returns.

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Large Intestine

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Functions of large intestine

Secretions - Goblet cells secrete mucus – protective

lubricant layer. Helps in stool formation. (adherent medium for holding fecal matter together). Protects wall from bacterial activity.

- Secretion of bicarbonate to fight acidity caused in stool due to bacterial action.

- Secretion of potassium. Digestion none except by bacteria

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Functions of large intestine (contd.)Absorption water & electrolytes, sodium & chlorides are absorbed.

Bile salts, certain products of bacterial action (indole) also absorbed.

90% fluid removed , 1000-2000ml of chyme converted to 200-250ml of semisolid feaces

ExcretionHeavy metals, metabolites, drugs e.g.. Emetine.

Fluids for transfusion purposes & drugs (anesthetics) maybe given through large gut.

Also used to transplant ureters after removal of a pathological urinary bladder.

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Large Intestine Absorption

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Bacterial Flora

consist of:Bacteria surviving the small intestine that enter the cecum

and Those entering via the anus

(pathogens,symbionts,commensals )

These bacteria: Colonize the colonFerment indigestible carbohydratesRelease irritating acids and gases (flatus)Synthesize B complex vitamins ,vitamin K,

folic acid ,short chain fatty acid

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Movements of large intestineColonic transit time

Food enters caecum – 4 hrs after meal

Ileum empty – 8hrs

Caecum & ascending colon empty – 13 to 17 hrs

Distal colon - 18 hrs

Rectum – 24 hrs

Some remnants of meal are present in rectum ~ 72 hrs

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Types of movement

Similar to SI

Segmentation contraction – present in

proximal region of colon (i.e, ascending &

transverse region)

Haustral contractions or churning in which

colonic wall roll back & forth

Kneading movements - alternate contraction

& relaxation of large segments of colon

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Pendular type of movement – peristalsis cum

anti peristalsis causing mixing of colonic content

Peristalsis – wave of contraction sweeping down LI

Frequency less than SI

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Mass movement or mass peristalsis

1-3 times/day

Forceful contractions

Involve contraction of large segment of colon

Propel contents into rectum & induce desire

for defecation

Mass movement can occur after meal

• Gastro colic reflex

• Duodeno colic reflexMediated by ANSMediated by ANS

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Defecation

A spinal reflex under voluntary control.

Rectum usually (almost) empty (retrograde contractions return content to sigmoid, until there is too much of it)

Just before defecation mass movement fills rectum → ↑ pressure → reflex relaxation of inner sphincter (smooth muscle) via parasympathetic fibers in pelvic nerves & contraction of outer sph. (skeletal muscle controlled intentionally via pudendal nerves)

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Defecation

Pelvic Nerve

Pudendal Nerve

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DefecationStretch receptors in rectal wall can adapt - urge

to defecate can temporarily subside if suppressed

Reflex controlled from sacral spinal cord, modulated from higher levels

Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

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Applied aspectConstipation: commonest disorder of large bowel motility

Cause :

Irregular bowel habits

Hypothyroidism

Anal strictures

Colon cancer

Diarrhoea: increase in frequency of passage of stools, due to large bowel irritation

Cause:

Infection

Emotional tension (psychogenic diarrhoea)

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Adynamic or Paralytic ileus : trauma to intestine cause

inhibition of smooth muscle

Hirschsprung`s disease or aganglionic megacolon :

congenital absence of both myentric & submucous

plexus

Blind loop syndrome –excessive bacterial growth due

to stasis

Digestion of carbohydrates

The different carbohydrates in diet are Polysaccharides like glycogen,amylose,amylopectin,etc

The disaccharides in the diet are sucrose, lactose, starch,etc. , whereas monosaccharides are mostly glucose & fructose.

Other carbohydrates in diet include Alcohol, Lactic acid, pyruvic acid,pectins,dextrins and of course cellulose.

The end products of carbohydrates is monosaccharides.

I. Carbohydrate

Starch amylase Disaccharide (salivary, pancreatic, intestinal) (Maltose, Isomaltose, maltotriose)

+

dietary disaccharide

Sucrose + Lactose

Monosaccharide(Glucose, fructose, galactose)

Intestinal disaccharidases. Maltase, sucrase, lactase

Digestion

In the mouth: The salivary amylase present. In the stomach: The salivary amylase act here for a

longer time. As such less enzymes (weak amylase) present in stomach region.

In the intestine: Along with enzymes in stomach from pancreas like pancreatic amylase & succus entericus enzymes like maltase, sucrase, lactase,dextrinase, trehalase,etc act on the carbohydrates in food to break it down into m.s.

The pancreatic amylase can act both on boiled & unboiled starch but salivary amylase acts on boiled starch only.

Brokendown products (glucose) transported from lumen of S.I. into epithelial cells in the mucus membrane of S.I by means of sodium co-transport.

From epithelial cell, glucose is absorbed into portal vein by facilitated diffusion.

Utilization of carbohydrates occurs mainly by oxidative process in which the carbohydrates are burnt down slowly to release energy. This process is called catabolism.

The part of released energy is utilized by the tissues for the physiological actions & rest of the energy is stored as rich energy phosphate bonds and in the form of proteins, carbohydrates & lipids in the tissues. This process is called anabolism.

Carbohydrates: Hydrolyzed into Monosaccharides

Glucose is transported to cells requiring energy; insulin influences rate of transport

Absorption of CHO

DIGESTION, ABSORPTION AND METABOLISM OF PROTEINS.

Proteins: Food containing high protein are- meat,fish egg & milk and the various proteins in them are collagen, albumin,casein,lactalbumin, vitellin,etc.

Digestion of proteins: (by proteolytic enzymes)

A) Mouth- No protein digestion occurs.

B) Stomach- By pepsin & rennin

C) Small intestine- By pancreatic enzymes like trypsin, chymotrypsin in duodenum & jejunum as well as succus entericus which contains dipeptidases,tripeptidases & aminopeptidases.

The final products of protein digestion are amino acids absorbed from intestine.

II. Proteins & Nucleoproteins

Proteins

Proteases, peptones, large polypeptides

also digest collagen

Smaller peptides & some free a.a.

Amino acids (98% of dietary protein)

(polypeptide chains composed of a.a. bound by peptide linkage)STOMACH

Pepsin

DUODENUM

Pancreatic trypsin & chymotrypsin (endopeptidase),

Carboxypeptidase (exopeptidase)

Intestinal exopeptidase (aminopeptidase, dipeptidase, amino-tripeptidase)

Digestion

Absorption: in form of amino acids from small intestine. The levo a.a actively absorbed by sodium co-transport. Whereas

dextro a.a. by facilitated diffusion.

Area Juice Enzyme Substrate Endproduct

Mouth Saliva NO proteolytic enzyme present

Stomach Gastric juice Pepsin Proteins Proteoses,Peptones,etc

Small intestine

Pancreatic juice

Trypsin Proteosuspeptones

Amino acids

Chymotrypsin

Carboxy peptidases A&B

DipeptidasesTripeptidasesPolypeptides

Succus entericus

Dipeptidases DipeptidesAmino acids

Tripeptidases Tripeptides

Nucleoproteins

Nucleotides & di-tri polynucleotide

Nucleosides

Purine & pyrimidine bases

Pancreatic nuclease

Intestinal nuclease

nucleosidase

Digestion

Protein & Amino Acid Transport

Absorption of proteins

Sources of digested proteins50% - ingested food25% - digested juices25% - desquamated mucosal cells

7 different transport system transport a.a into enterocytes 5 – co-transport a.a & Na+ 2 – independent of Na+

protein absorption - jejunum & some in upper ileum Proteins with mol wt > 200 – 300 are poorly absorbedIntestine of newborn can absorb intact protein by

pinocytosis.

This enables absorption of antibodies from colostrum.minute amount of native proteins is absorbed in

M – cells overlying peyer’s patches is responsible for food allergies

DIGESTION OF LIPIDSLipids are consumed in form of neutral fats (triglycerides)The different types of fat available are Saturated,

monounsaturated,polyunsaturated fats, etc.The various sources are milk,cheese,butter,oils,fish,

meats,nuts,etc.

Digestion:

a) Mouth- By lingual lipase, But digestion does not occur.

b) Stomach- By Gastric lipase in gastric juice.

c) Intestine- By bile salts, pancreatic enzymes,intestinal lipase,etc.

FINAL PRODUCTS OF FAT DIGESTION are fatty acids, cholesterol & monoglycerides.

Absorption: Monoglycerides,cholesterol,etc form miscelles and enter the enterocytes by simple diffusion.

In mucosal cells, most of monoglycerides are converted into triglycerides. These are coated with a layer of protein,cholesterol & phospholipids to form particles called chylomicrons. These chylomicrons being larger in size cannot pass through membrane of blood capillaries.Hence these are transported through lymph vessels and finally into blood.

Storage: The lipids are stored in adipose tissue & liver. When the above chylomicrons are travelling through capillaries of adipose tissue or liver, enzyme called lipoprotein lipase present in capillary endothelium hydrolyses triglycerides of chylomicrons into free fatty acids (FFA) & glycerol.

FFA & glycerol enter fat cells of adipose tissue or liver cells (i.e. storage points). Here again the FFA & glycerol is converted into triglycerides and stored in these cells.

Other contents of chylomicrons such as cholesterol & phopholipids which are released into the blood combine with proteins to form lipoproteins.

When tissues of body need energy,the triglycerides stored in this adipose tissue is hydrolyzed into FFA & glycerol. The FFA are transported to the body tissues through blood.

These lipids (FFAs) are transported in blood in combination with albumin or in form of lipoproteins.

Transport of Lipids Across Intestinal Epithelium

Water & Vitamin absorption

Out of 10 liters, only 100-200 ml of water is excreted out.Osmotic difference is the driving force for water

absorption.Plenty of water is absorbed in large intestine.All vitamins except vitamin D which is mostly formed in

the skin are absorbed in diet. Fat soluble vitamins absorbed along with lipids require presence of bile salts.

Vitamin B12 is majorly absorbed from ileum. Other vitamins from jejunum..

Water and Ions

Water: can move in either direction across wall of small intestine depending on osmotic gradients

Ions: sodium, potassium, calcium, magnesium, phosphate are actively transported