The Digestive System - Mrs. Titus-Costello's Science...

Human Anatomy, 3rd edition

Prentice 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

– Excretion

– Two types of digestionMechanical and Chemical

• Occur at different points in the digestive tract, sometimes

both types at once!



Overview of Digestive System



Histological Organization

• Same basic arrangement

of tissues from

esophagus to anal canal

• Four layers (from

innermost to outermost)

– Mucosa

– Submucosa

– Muscularis

– Serosa



Movement and Mixing of Digestive Materials

• Peristalsis

– Coordinated motion of two muscular layers

– Circular musclescontract, then longitudinal muscles

• Segmentation

– Mixing of food

– Circular muscles in two areas contract

– Longitudinal muscles alternately contract & relax



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



The Tongue• Structure

– Skeletal muscle

covered with mucosa

– Surface

• Papillae/ taste

buds

• Functions

– Maneuvers food

– Forms bolus



Salivary Glands

• Found outside mouth

• Ducts carry saliva to mouth

• 3 pairs– Parotid glands

– Submandibular glands

– Sublingual glands

• Saliva (salivary amylase)– Functions

• Keeps mucous membranes moist

• Lubricates food

• Dissolves food

• Begins carbohydrate digestion



Teeth• 2 sets

– Deciduous (20)

– Permanent (32)

• Held in sockets

• Gingiva = gums

• Structure– Crown

– Root

– Neck

• Composition– Dentin

– Enamel

– Cementum



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 sphincterseparates esophagus from stomach



External Anatomy of the Stomach



The Stomach

• Same 4 basic layers

• When the stomach is

empty, the mucosa lies

in large folds

– Rugae- folds

• Pyloric sphincter

separates stomach

from small intestine



Histology of the Stomach

• Mucosa is simple

columnar epithelium with

goblet cells

• Mucosa is folded to form

gastric pits

– Gastric glands secrete

gastric juice



Gastric Glands• Several kinds of cells

produce substances that form gastric juice

– Mucous cells- protects stomach from acid

– pepsinogen- breaks down proteins

– intrinsic factor helps with absorption of vit. B12

– HCl ph of 2 to help break down food and kill bacteria



Functions of the Stomach

• Mechanical digestion

– Food reaches pylorus

• Chemical digestion

– Digestion of proteins- pepsin

– Digestion of carbohydrates- salivary amylase

• Absorption

– No food – too much mucous



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 chemical digestion occurs here

• The ileum

– About 9.5 feet long

– Most absorption occurs here

– Ends in the ileocecal valve



Histology of the Small Intestine

• The lining is folded into circular pleats

• The mucosal surface is folded into villi

• The epithelial cell membranes are highly folded into microvilli

• Intestinal glands

– Secrete intestinal juice



A Villus



Functions of the Small Intestine

• Intestinal juice- buffers acid, moistens contents, breaks down digestive enzymes

• Chyme is further broken down

– Proteins

– Carbohydrates

– Fats

• Most absorption is in the small intestine

• Hormones:

• Secretin increase secretion of buffers and bile

• Cholecystokinin secretion of enzymes and bile

• Gastric inhibitory peptide slows down gastric activity



The Large Intestine (Colon)

• About 4.5 feet long

• Begins with the cecum

• Appendix is attached

1. Ascending colon

2. Transverse colon

3. Descending colon

• Sigmoid colon

• Colon connects to rectum

• Rectum connects to anal canal

• Empties to the exterior through the anus



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



Functions of the large intestine

• Movement:

– Regulated by the ileocecal valve

– Mixing and peristalsis

– Mass peristalsis

• Chemicals:

– Mucus secreted

– No enzymes

– Bacteria – prepare chyme for elimination

• Absorption:

WATER

Bile salts



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



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



Histology of the Liver

• Outside is a capsule

• Composed of tiny lobules

• Each lobule is surrounded

by liver cells and

sinusoids capillaries

that run through liver

• Hepatocytes release

bile breakdown fats

• 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



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– salts

precipitate and form salt

‘chunks’



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

The Digestive System - Mrs. Titus-Costello's Science...

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