Human Anatomy, 3rd edition Prentice Hall, © 2001 The Digestive System Chapter 25.

Human Anatomy, 3rd editionPrentice Hall, © 2001

The Digestive System

Chapter 25


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


Overview of Digestive System


Histological Organization– The wall of the digestive tract has the same

basic arrangement of tissues from the esophagus to the anal canal

– Four layers (from innermost to outermost)• Mucosa

• Submucosa

• Muscularis

• Serosa


Layers of the Digestive Tract


Movement & Mixing of Digestive Materials– Peristalsis

• Coordinated motion of the two muscular layers

• Circular muscles contract, then longitudinal muscles

– Segmentation• Mixing of food

• Circular muscles in two areas contract

• Longitudinal muscles alternately contract & relax


Peristalsis & Segmentation


The Oral Cavity– Functions

• Take in food

• Prepare food for digestion

– Structure• Lined with stratified squamous epithelium

• Lips surround the opening

• Roof is formed from the hard & soft palate

• Tongue dominates the floor


Oral Cavity


The Tongue– Functions

• Maneuvers food

– Structure • Skeletal muscle covered with mucosa

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

• Surface– Papillae


The Salivary Glands– Found outside the mouth– Ducts carry saliva to the mouth– Saliva

• Functions– Keeps mucous membranes moist– Lubricates food– Dissolves food– Begins carbohydrate digestion

– 3 pairs• Parotid glands• Submandibular glands• Sublingual glands


Salivary Glands


Teeth– 2 sets

• Deciduous (20)• Permanent (32)

– Held in sockets– Gingiva surrounds the base of the teeth– Structure

• Crown• Root• Neck

– Composition • Dentin – primary substance in tooth• Enamel – covers crown• Cementum – covers root

– Tooth decay– Wisdom teeth – 3rd molars


A Tooth


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


The Esophagus


The Stomach– Has the same 4 basic layers– When the stomach is empty, the mucosa lies in

large folds• Rugae

– Pyloric sphincter separates stomach from small intestine


External Anatomy of the Stomach


Anatomy of the Stomach


Histology of the Stomach– Mucosa is simple columnar epithelium with

goblet cells– Mucosa is folded to form gastric pits

• Gastric glands secrete gastric juice– Several kinds of cells in each gland produce substances

that form the gastric juice

» Mucus cells

» Chief cells

» Parietal cells

» Enteroendocrine cells


The Stomach Wall


Gastric Gland


Functions of the Stomach– Mechanical digestion

• Food reaches pylorus

– Chemical digestion• Digestion of proteins

– Absorption • No food

• Water, electrolytes

• Some drugs

• Alcohol


The Small Intestine– About 18 feet long– 3 regions

• The duodenum (= “12 fingers’ length”)– About 8 inches long

– Common bile duct & pancreatic duct empty here

• The jejunum (= “empty”)– About 8 feet long

– Most digestion occurs here

• The ileum– About 9.5 feet long

– Most absorption occurs here

– Ends in the ileocecal valve


Regions of the Small Intestine


Histology of the Small Intestine– The lining is folded into circular pleats called

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

(“hidden place” – like gastric pits) at the base of villi

• Secrete intestinal juice


Histology of the Small Intestine


A Villus


Functions of the Small Intestine– Chyme is further broken down

• Proteins

• Carbohydrates

• Fats

– Most absorption is in the small intestine


The Large Intestine– AKA 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 the rectum

– Rectum connects to the anal canal which empties to the exterior through the anus


The Large Intestine


Histology & Functions of the Large Intestine– Mucosa - simple columnar epithelium (thin)

• Completion of absorption

• Formation of feces

– Lots of mucus glands– Expulsion of feces from the body


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


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


Histology of the Large Intestine


Accessory Organs– Liver– Pancreas– Gall bladder


The Liver– Performs many life-sustaining functions,

digestive and other.– Location – under the diaphragm on the left

• Connected to the diaphragm by the falciform ligament

– Divided into lobes• Right lobe

• Left lobe

• Caudate lobe

• Quadrate lobe


The Liver


Histology of the Liver

- Outside is a capsule

- Composed of tiny lobules

– Each lobule is surrounded by liver cells and sinusoids

– Bile ducts run between liver cells


Histology and Blood Supply of the Liver


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

• Kupffer cells

– Produces plasma proteins– Removes worn-out and damaged red blood

cells


The Gallbladder– Location – underside of the left lobe of the liver– Function – to concentrate and store bile– Gallstones

• Cholesterol in bile crystalizes, crystals fuse.


The Gallbladder


The Pancreas

- Location – in the curvature of the duodenum– Connected to the duodenum by the pancreatic

duct– Produces pancreatic juic– Functions

• Exocrine - digestion of all the nutrient groups

• Endocrine – production of insulin & glucagon– Control level of blood glucose


The Pancreas

Human Anatomy, 3rd edition Prentice Hall, © 2001 The Digestive System Chapter 25.

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Transcript of Human Anatomy, 3rd edition Prentice Hall, © 2001 The Digestive System Chapter 25.