The Digestive System

Digestive System is approximately 8 meters long

Human digestive system

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Digestion

• Processing of food• Types• Mechanical (physical)• Chew• Tear• Grind• Mash• Mix

• Chemical• Catabolic reactions• Enzymatic hydrolysis• Carbohydrate• Protein• Lipid

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Digestion• Phases• Ingestion• Movement• Digestion• Absorption• Further digestion

• After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested.

• Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours.

• In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

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Digestive System Organization

• Gastrointestinal (Gl) tract or alimentary canal• Tube within a tube• Direct link/path between organs• Structures

• Mouth• Oral Cavity• Pharynx• Esophagus• Stomach• Duedenum • Jejenum• kIleum• Cecum• Ascending colon• Transverse colon

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Digestive System Organization• Descending colon• Sigmoid colon• Rectum• Anus

• Accessory structures• Not in tube path• Organs

• Teeth• Tongue• Salivary glands• Liver• Gall bladder• Pancreas

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Anatomy of the Mouth and Throat

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Human Deciduous and Permanent Teeth

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Dorsal Surface of the Tongue

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The Major Salivary Glands

Alimentary canal

• Mouth- physical and chemical breakdown.• Mastication-the act of

chewing• Saliva contains enzyme

amylase to break down carbohydrates

• Pharynx- contains opening to trachea as well. Epiglottis covers opening of trachea.

• Esophagus-muscular tube dorsal to trachea• Relies on a rhythmic

wave-like motion called peristalsis

Peristalsis and Segmentationseries of involuntary wave-like muscle contractions which move food along the digestive tract

Anatomy of the Stomach

Stomach• 3 muscle layers• Oblique• Circular• Longitudinal

• Regions• Cardiac sphincter• Fundus• Antrum (pylorus)• Pyloric sphincter

• Vascular• Inner surface thrown into folds – Rugae• Contains enzymes that work best at pH 1-2

Stomach• Functions• Mix food• Reservoir• Start digestion of

• Protein• Nucleic acids• Fats

• Activates some enzymes• Destroy some bacteria• Makes intrinsic factor – B 12 absorption• Destroys some bacteria

• Absorbs• Alcohol• Water• Lipophilic acid• B 12

Small Intestine

• Extends from pyloric sphincter ileocecal valve• Regions• Duodenum• Jejenum• Ileum

• Movements• Segmentation• Peristalsis

Small intestine• The walls of the small

intestine are lined with millions of microvilli. This is the site of nutrient absorption.

• Small intestines also produce many digestive enzymes to break large polymers completely down into monomers.

• Chyme- the thin liquid mush that is your food being passed through the intestines

Small Intestine• Intestinal glands – Intestinal enzymes• Duodenal glands – Alkaline mucous• Paneth cells – Lysozyme• Microvilli• Lacteals• Plica circularis• Smooth muscle• Lymphatic tissue – GALT• Vascular

Small Intestine• Absorbs• 80% ingested water• Electrolytes• Vitamins• Minerals• Carbonates

• Active/facilitated transport• Monosaccharides

• Proteins• Di-/tripeptides• Amino acids

• Lipids• Monoglycerides• Fatty acids• Micelles• Chylomicrons

Structure of the Villi in the Small Intestine

Villi and microvilli

Small Intestine• Secretes digestive enzymes• Peptidases

• Amino-• Di-• Tri-

• Sucrases• Maltase• Lactase• Saccharidases

• Di-• Tri-

• Lipase• Nucleases

Small Intestine

• Requires pancreatic enzymes & bile to complete digestion

Liver• Location• R. Hypochondrium• Epigastric region

• 4 Lobes• Left • Quadrate• Caudate• Right

• Each lobe has lobules – Contains hepatocytes – Surround sinusoids – Feed into central vein

Liver• Functions• Makes bile

• Detergent – emulsifies fats• Release promoted by:• Vagus n.• CCK• Secretin

• Contains• Water• Bile salts• Bile pigments• Electrolytes• Cholesterol• Lecithin

Liver• Detoxifies/removes

• Drugs• Alcohol

• Stores• Gycolgen• Vitamins (A, D, E, K)• Fe and other minerals• Cholesterol

• Activates vitamin D• Fetal RBC production• Phagocytosis• Metabolizes absorbed food

molecules• Carbohydrates• Proteins• Lipids

Liver

• Dual blood supply• Hepatic portal vein

• Direct input from small intestine

• Hepatic artery/vein• Direct links to heart

• Gall bladder- stores and concentrates bile• Bile is a bitter, greenish-

yellow alkaline fluid, stored in the gallbladder between meals and upon eating is discharged into the duodenum where it aids the process of digestion.

• Pancreas- produces insulin, pancreatic juices amylase and lipase. • Pancreatic juice digests all

major nutrient types.

The Duodenum and Related Organs

The Organs and Positions in the Abdominal Cavity

Large Intestine

• Extends from ileocecal valve to anus• Regions• Cecum – Appendix• Colon• Ascending• Transverse• Descending

• Rectum• Anal canal

Anatomy of the Large Intestine

Large Intestine• Histology• No villi• No permanent circular folds• Smooth muscle

• Taeniae coli• Haustra

• Epiploic appendages• Otherwise like rest of Gl tract

Large Intestine• Functions• Mechanical digestion• Haustral churning• Peristalsis• Reflexes • Gastroileal• Gastrocolic

• Chemical digestion – Bacterial digestion• Ferment carbohydrates• Protein/amino acid

breakdown

– Absorbs• More water• Vitamins

– B– K

– Concentrate/eliminate wastes

You’ve got company!• Living in the large intestine is a community of helpful bacteria• Escherichia coli (E. coli)

• produce vitamins • vitamin K; B vitamins

• generate gases• by-product of bacterial metabolism • methane, hydrogen sulfide

Feces Formation and Defecation• Chyme dehydrated to form feces• Feces composition• Water• Inorganic salts• Epithelial cells• Bacteria• Byproducts of digestion

• Defecation• Peristalsis pushes feces into rectum• Rectal walls stretch

• Control• Parasympathetic• Voluntary

Structures of the Alimentary Canal

Digestion and pHLocation pH Enzymes Molecules digested

Mouth neutral Amylase Starch

Stomach acidic Pepsin (a peptidase) Initial protein digestion

Small intestine Basic to neutral Mixture of amylase, peptidases, lipases

Digestion of starches, final breakdown of proteins, digestion of lipids.

Hormones and digestion• Many hormones produced by the digestive system itself are

responsible for appetite and digestion.

• Gastrin in the stomach, produced when food is present, signals release of acid.

• Secretin signals the release of sodium bicarbonate by the pancreas.

• Ghrelin and Leptin, recently discovered, control appetite. Changes in these hormones cause overeating, because people with too little of these hormones don’t realize they’re full.

Does a colon need cleaned?• Products like these raise worry in

people who believe their colon is “dirty.” These are laxatives that are unnecessary, or even harmful.

• “Cleansing” products can disrupt bowel function and do not cure any real illnesses. A diet high in fiber is a healthier option.