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Lecture 22

Digestive System II

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Small Intestine

• Site of greatest amount of digestion and absorption

• Divisions – Duodenum (“twelve finger widths”)– Jejunum (“empty”)– Ileum (“twisted”)

Fig. 26.14

Duodenum

Jejunum

Ileocecal valve

Ileum

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Small Intestine Secretions

• Mucus – Protects against digestive enzymes and stomach

acids

• Digestive enzymes– Disaccharidases: Break down disaccharides to

monosaccharides– Peptidases: Hydrolyze peptide bonds– Nucleases: Break down nucleic acids

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Duodenum and Pancreas

Duodenum• 25cm in adult• Accessory glands empty secretions into duodenum

Fig. 26.20

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Histology of Small Intestine

• Circular folds, villi and microvilli increase surface area• Epithelial cells produced by intestinal glands

Fig. 26.15

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Liver

• 4 Lobes• Ducts

– Cystic duct joins with common hepatic duct to form common bile duct

– Common bile duct joins pancreatic duct

Fig. 26.18

Right lobe

Inferior vena cava

Left lobe

Gallbladder(a) Anterior view

Left lobe

Hepaticportal vein

Commonhepatic duct

Quadrate lobe

Inferior vena cava

Right lobe

Cystic duct

Gallbladder

Caudate lobe

Posterior

Anterior(b) Posteroinferior view

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Functions of the Liver• Bile production

– Salts emulsify fats, neutralizes stomach acid– Stored in gall bladder– Gallstones can form as precipitate of cholesterol

• Storage– Glycogen, fat, vitamins, copper and iron

• Nutrient interconversion• Detoxification

– Removal of ammonia and conversion to urea (eliminated by kidneys in urine)

• Phagocytosis– Removal of worn-out and dying red and white blood cells, some

bacteria

• Synthesis– Blood proteins

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Pancreas• Anatomy

– Endocrine• Insulin

– Exocrine• Pancreatic juice

– Digestive enzymes

– Released in inactive form

– Become active in duodenum

– Digest protein, fats, carbohydrates, and nucleic acids

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Review Question

Digestion of proteins is started in the ______________ and completed in the ________________.

(a) Oral cavity, stomach

(b) Stomach, small intestine

(c) Esophagus, small intestine

(d) Oral cavity, small intestine

(e) Esophagus, stomach

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Large Intestine

• Extends from ileocecal valve to anus

• Consists of cecum, colon, rectum, anal canal

• Movements sluggish (18-24 hours)

Fig. 26.16

Transverse colon

Ascending colon

Descending colon

Cecum

Vermiformappendix

Sigmoid colon

Ileocecal valve

CecumVermiform appendix

Rectum

Ileum

Anal canal

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Movement in Large Intestine

• Mass movements– Common after meals

• Defecation reflex– Distension of the rectal wall by

feces

• Defecation– Usually accompanied by voluntary

movements to expel feces through abdominal cavity pressure caused by inspiration (breathing in) and contraction of abdominal wall muscles

Rectal valve

Anal canal

Veins

Internal anal sphincter

External anal sphincter

Anus

Rectum

Fig. 26.16

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Points to RememberPoints to Remember• Stomach

– Mixes food– Protein digestion– Limited absorption (aspirin)

• Small intestine– Receives secretions of liver and pancreas– Chemical and mechanical digestion– Transports undigested material

• Large intestine– Absorb water– Form, store and expel feces

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Questions?