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LECTURE - 8 DR. ZAHOOR ALI SHAIKH

SMALL INTESTINE

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SMALL INTESTINE ABSORPTION

Small intestine primary role is absorption. Its structure is well adapted for it.

Small Intestine StructureInner surface of small intestine is thrown in

folds, projecting from the folds there are finger like projections known as VILLI, which increase the surface area.

The surface of each villus is covered by epithelial cells.

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Small Intestine Structure

Small hair like projections – MicroVilli [brush border] arise from the epithelial cell of villi and increase the surface area further.

MicroVilli can be seen with electron microscope.

Small intestine enzyme function within the membrane of this brush border.

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APPLIED

MAL ABSORPTION [impairment of Absorption]

It can occur due to damage or decreased surface area of small intestine.

Most common cause is CELIAC disease, also called GLUTEN ENTROPATHY.

In this disease persons small intestine is abnormally sensitive to gluten, a protein present in wheat, barley, and rye.

These grain products are present in the food.

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APPLIED

CELIAC DISEASE [cont]They activate T-cell response and damage

intestinal villi.Due to loss of villi, surface area of absorption

is decreased and absorption of nutrients is impaired.

Treatment – Eliminate gluten from the diet.

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STRUCTURE OF VILLUS

Each villus has epithelial cells that cover the surface of the villus.

They have carriers for absorption and membrane bound digestive enzyme that complete carbohydrate and protein digestion.

Villus has connective tissue core – LAMINA PROPRIA.

Villus has capillary network.Villus has lymphatic vessel called Central

Lacteal which occupies the center of villus.

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STRUCTURE OF SMALL INTESTINE ABSORPTIVE SURFACE

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SMALL INTESTINE STRUCTURE

Between the villi are shallow invaginations known as Crypts Of Lieberkuhn – they secrete water and electrolyte.

There are mucus secreting cells on the surface of villus.

These secretions constitute the succus entricus.

Succus Entricus has no enzyme, but enzyme are present in brush border [MicroVilli].

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MUCOSAL LINING TURNOVER

Stem cell are present in crypts and replace the epithelial lining of small intestine in three days [more than 100 million intestinal cells are shed per minute].

In addition to stem cells, Paneth cells are present in crypts. They have defensive function due to lysozyme, an enzyme which kills bacteria.

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SMALL INTESTINE ABSORPTION

Sodium Absorption is active and passive in small intestine.

Carbohydrate [glucose and galactose] and protein are both absorbed by secondary active transport.

Fructose is absorbed by facilitated diffusion.

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ABSORPTION OF FAT

Digested fat [Monoglycerides and free fatty acids] are absorbed passively.

Monoglycerides and free fatty acids are resynthesized into triglycerides, they are coated with layer of endoplasmic reticulum of epithelial cells, this is called CHYLOMICRONS.

CHYLOMICRONS are sent out by exocytosis from the epithelial cells into the interstitial fluid of the villus.

CHYLOMICRONS go to lymph [lacteals].

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VITAMIN ABSORPTION

Vitamin absorption is largely passive.Water soluble vitamins are absorbed

passively.Fat soluble vitamins [A, D, E, K] are carried

with micelle [Monoglyceride, fatty acid, bile salt] and absorbed passively.

Vitamin B12 is absorbed with intrinsic factor in terminal ileum.

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IRON AND CALCIUM ABSORPTION

They are absorbed depending on the body needs of iron and calcium.

Iron AbsorptionIron is essential for hemoglobin synthesis.Normal iron intake is 15 – 20 milligram/day,

but man absorbs 0.5 to 1 milligram/day into the blood.

Woman take up 1 to 1.5 milligram/day. Woman need more iron because they lose iron in menstrual blood flow.

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IRON AND CALCIUM ABSORPTION

Iron AbsorptionWe take iron in ferric form [Fe3+] but iron is

absorbed in ferrous form [Fe2+].After taking iron in ferric form, it is reduced

to ferrous form by membrane bound enzyme at the luminal membrane of small intestine.

Iron is absorbed in the duodenum.

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CALCIUM ABSORPTION

Calcium is absorbed according to the needs of the body.

Calcium enters in the epithelial cells, lining the lumen through calcium channel.

Vitamin D helps in calcium absorption.Daily calcium intake about 1000 milligram – 60 - 70% is absorbed in the small intestine,

rest passes in the stools.

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ABSORPTION IN SMALL INTESTINE

Absorbed material [carbohydrate and protein] from the GIT, go to liver [portal system] for metabolism and then it goes to general circulation.

Fat is absorbed in the central lacteal and enter the lymphatic system. Lymph vessel form thoracic duct, empties into the venous system within the chest.

Fat in the systemic circulation is carried to the liver and other tissues of the body.

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ABSORPTION BY SMALL INTESTINE KEEPS PACE WITH SECRETION

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APPLIED‘DIARRHEA’

Diarrhea results in loss of fluid and electrolytes.

Diarrhea is characterized by passage of highly fluid fecal matter, with increased frequency of defecation.

It can lead to loss of fluids and acid base imbalance.

Increased loss of fluid causes Dehydration.

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Causes of Diarrhea

Bacterial or viral infection of small intestineLactase deficiency Emotional stressBacterium Vibrio Cholera – causes cholera in

which profuse diarrhea occurs.

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What You Should Know From This Lecture

Structure and Function of Villi and MicroVilliCeliac DiseaseAbsorption of Carbohydrate, Protein and FatVitamin AbsorptionIron and Calcium AbsorptionDiarrhea

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Thank you