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The Digestive System

Chapter 24

Introduction •  Structure of the digestive system

–  A tube that extends from mouth to anus –  Accessory organs are attached

•  Functions include –  Ingestion –  Movement –  Digestion –  Absorption –  Defecation

Human Anatomy, 3rd edition Prentice Hall, © 2001

Overview of Digestive System

Human Anatomy, 3rd edition Prentice Hall, © 2001

Histological Organization

•  Same basic arrangement of tissues from esophagus to anal canal

•  Four layers (from innermost to outermost) –  Mucosa –  Submucosa –  Muscularis –  Serosa

Human Anatomy, 3rd edition Prentice Hall, © 2001

Movement and Mixing of Digestive Materials •  Peristalsis

–  Coordinated motion of two muscular layers

–  Circular muscles contract, then longitudinal muscles

•  Segmentation –  Mixing of food –  Circular muscles in

two areas contract –  Longitudinal muscles

alternately contract & relax

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Oral Cavity •  Structure

–  Lined with stratified squamous epithelium

–  Lips surround the opening

–  Roof is formed from the hard & soft palate

–  Tongue dominates the floor

•  Functions –  Take in food –  Prepare food for

digestion

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Human Anatomy, 3rd edition Prentice Hall, © 2001

The Tongue •  Structure

–  Skeletal muscle covered with mucosa

–  The lingual frenulum connects the tongue to the floor of the mouth

–  Surface •  Papillae

•  Functions –  Maneuvers food

Human Anatomy, 3rd edition Prentice Hall, © 2001

Salivary Glands •  Found outside mouth •  Ducts carry saliva to

mouth •  3 pairs

–  Parotid glands –  Submandibular glands –  Sublingual glands

•  Saliva –  Functions

•  Keeps mucous membranes moist

•  Lubricates food •  Dissolves food •  Begins carbohydrate

digestion

Human Anatomy, 3rd edition Prentice Hall, © 2001

Teeth •  2 sets

–  Deciduous (20) –  Permanent (32)

•  Held in sockets •  Gingiva = gums •  Structure

–  Crown –  Root –  Neck

•  Composition

–  Dentin –  Enamel –  Cementum

•  Dental caries •  Wisdom teeth Human Anatomy, 3rd edition

Prentice Hall, © 2001

The Pharynx and Esophagus •  Food enters the

esophagus from the pharynx

•  The esophagus is a muscular tube behind the trachea –  Food is moved by

peristalsis from the pharynx to the stomach

–  Cardiac sphincter separates esophagus from stomach

Human Anatomy, 3rd edition Prentice Hall, © 2001

External Anatomy of the Stomach

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Stomach •  Same 4 basic layers •  When the stomach is

empty, the mucosa lies in large folds –  Rugae

•  Pyloric sphincter separates stomach from small intestine

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Human Anatomy, 3rd edition Prentice Hall, © 2001

Histology of the Stomach

•  Mucosa is simple columnar epithelium with goblet cells

•  Mucosa is folded to form gastric pits –  Gastric glands

secrete gastric juice

Human Anatomy, 3rd edition Prentice Hall, © 2001

Gastric Gland

•  Several kinds of cells produce substances that form gastric juice –  Mucus cells –  Chief cells –  Parietal cells –  Enteroendocrine

cells

Functions of the Stomach •  Mechanical digestion

–  Food reaches pylorus •  Chemical digestion

–  Digestion of proteins •  Absorption

–  No food –  Water, electrolytes –  Some drugs –  Alcohol

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Small Intestine •  About 18 feet long •  The duodenum

–  About 8 inches long –  Common bile duct &

pancreatic duct empty here •  The jejunum

–  About 8 feet long –  Most digestion occurs here

•  The ileum –  About 9.5 feet long –  Most absorption occurs here –  Ends in the ileocecal valve

Human Anatomy, 3rd edition Prentice Hall, © 2001

Histology of the Small Intestine

•  The lining is folded into circular pleats –  Plicae circulares

•  The mucosal surface is folded into villi

•  The epithelial cell membranes are highly folded into microvilli

•  Intestinal glands are found in the crypts at the base of villi –  Secrete intestinal

juice Human Anatomy, 3rd edition Prentice Hall, © 2001

A Villus

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Functions of the Small Intestine

•  Chyme is further broken down –  Proteins –  Carbohydrates –  Fats

•  Most absorption is in the small intestine

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Large Intestine (Colon) •  About 4.5 feet long •  Mesocolon supports •  Begins with the cecum •  Appendix is attached •  Ascending colon •  Transverse colon •  Descending colon •  Sigmoid colon •  Colon connects to rectum •  Rectum connects to anal

canal •  Empties to the exterior

through the anus

Human Anatomy, 3rd edition Prentice Hall, © 2001

Histology and Functions of the Large Intestine

•  Mucosa - simple columnar epithelium –  Completion of

absorption –  Formation of feces

•  Lots of mucus glands •  Expulsion of feces

from the body

Human Anatomy, 3rd edition Prentice Hall, © 2001

Digestion in the Large Intestine •  Mechanical

–  Regulated by the ileocecal valve –  Mixing and peristalsis –  Mass peristalsis

•  Chemical –  Mucus secreted –  No enzymes –  Bacteria – prepare chyme for elimination

Human Anatomy, 3rd edition Prentice Hall, © 2001

Feces Formation & Defecation •  Chyme is now solid or semi-solid - feces •  Large intestine absorbs any more water and

electrolytes from feces •  Defecation

–  Mass peristalsis pushes fecal material into rectum –  Rectum stretches

•  Defecation reflex

Accessory Organs

•  Liver •  Pancreas •  Gall bladder

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Human Anatomy, 3rd edition Prentice Hall, © 2001

The Liver •  Performs many life-

sustaining functions •  Location – under the

diaphragm on the right –  Connected to the

diaphragm by the falciform ligament

•  Divided into lobes –  Right lobe –  Left lobe –  Caudate lobe –  Quadrate lobe

Human Anatomy, 3rd edition Prentice Hall, © 2001

Histology of the Liver

•  Outside is a capsule •  Composed of tiny

lobules •  Each lobule is

surrounded by liver cells and sinusoids –  Hepatocytes –  Kupffer cells

•  Bile ducts run between liver cells

Functions of the Liver •  Produces bile – the primary digestive function

– Composition • Water •  Bile salts •  Cholesterol •  Pigments

– Bilirubin – Digestive function

•  Emulsification of fats

Other Functions of the Liver

•  Absorbs and stores iron, vitamins A, D, E, B7, K •  Detoxifies toxins and hormones •  Metabolizes proteins, carbohydrates, and lipids •  Removes bacteria from the blood •  Produces plasma proteins •  Removes worn-out and damaged red blood cells

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Gallbladder

•  Location – underside of right lobe of liver

•  Function – concentrate and store bile –  Collected from liver

•  Hepatic ducts –  Adds bile to

duodenum •  Cystic duct •  Common bile

duct •  Gallstones

Human Anatomy, 3rd edition Prentice Hall, © 2001

The Pancreas •  Location – in the

curvature of the duodenum

•  Connected to the duodenum by the pancreatic duct

•  Produces pancreatic juice

•  Functions –  Exocrine - digestion

of all nutrient groups –  Endocrine – control

blood glucose level