Chapter 14: The Digestive System

Functions• Ingestion—taking in food• Digestion—breaking food down

both physically and chemically• Absorption—movement of

nutrients into the bloodstream• Defecation—rids the body of

indigestible waste

OrgansTwo main groups:• Alimentary canal (gastrointestinal

or GI tract)– continuous coiled hollow tube–Mouth, pharynx, esophagus,

stomach, small intestine, large intestine, anus

• Accessory digestive organs– Teeth, salivary glands, pancreas,

liver, gallbladder

Mouth

Anatomy:• Lips (labia)• Cheeks• Hard palate• Soft palate• Uvula– Helps with speech

and preventing food/liquid from entering nasal cavity when swallowing

• Vestibule– space between lips

externally and teeth and gums internally

• Oral cavity proper– area contained by the

teeth• Tongue– attached at hyoid

bone and styloid processes of the skull, and by the lingual frenulum to the floor of the mouth

• Tonsils– Palatine & Lingual

Function:• Mastication (= chewing) of food• Digestion by saliva• Swallowing by the tongue• Sense of taste– Gustatory receptors

Pharynx• Only oropharynx and

laryngopharynx are part of digestive system

• Passageway for air and food

• 2 muscle layers alternate contractions to push food down to esophagus (=peristalsis)– Longitudinal inner layer– Circular outer layer

Figure 14.2a

Esophagus• ~10 inches long– Runs from pharynx to stomach through

the diaphragm• Passageway for food only• moves food through slow rhythmic

squeezing (= peristalsis)

Layers of Alimentary Canal Past the Esophagus

4 layers from inside to outside:• Mucosa – Mostly moist epithelial tissue

• Submucosa– Connective tissue with blood vessels, nerve

endings, and lymph vessels• Muscularis externa– Muscle tissue made of inner circular layer and

outer longitudinal layer• Serosa– Fluid producing cells make up visceral

peritoneum layer that runs into parietal peritoneum (= lining of abdominopelvic cavity)

Figure 14.3

Nervous System’s Role• Alimentary canal controlled by

autonomic nervous system• 2 networks:–Myenteric nerve plexus: between circular

and longitudinal muscles– Submucosal nerve plexus: in submucosa

layer• Function is to regulate mobility and

secretory activity

Stomach• Located on the left side

of the abdominal cavity• Food enters at the

cardioesophageal sphincter (sphincter = valve)

• Food empties into the small intestine at the pyloric sphincter

Regions:• Cardiac region -

near the heart• Fundus –

rounded part next to the cardiac region

• Body - middle• Pylorus -

funnel-shaped end

• Rugae = internal folds of the mucosa

• External regions– Lesser

curvature—concave medial surface

– Greater curvature—convex lateral surface

Layers of peritoneum attached to the stomach

• Lesser omentum- attaches the liver to the lesser curvature

• Greater omentum- attaches the greater curvature to the posterior body wall– Sticky wall that

collects fat to insulate, cushion, and protect abdominal organs

Physiology• Temporary storage tank for food• Site of food breakdown– The enzyme pepsinogen and

hydrochloric acid break down proteins– Gastrin is hormone that stimulates HCl

secrection– Alkaline mucus lines the inside of the

stomach to protect it from HCl• Delivers chyme (processed food) to

the small intestine

Structure of the Stomach Mucosa

Figure 14.4c

Small Intestine• Muscular tube extending from the

pyloric sphincter to the ileocecal valve

• Suspended from the posterior abdominal wall by the mesentery

• The body’s major digestive organ• Function: nutrient absorption into the

blood

Divisions:• Duodenum– Attached to the

stomach– Curves around the

head of the pancreas

• Jejunum–Middle

• Ileum– From jejunum to

large intestine

Chemical digestion begins in the small intestine• Enzymes are produced by

– Intestinal cells– Pancreas

• Pancreatic ducts carry enzymes to the small intestine

• Bile, made by the liver, enters through bile duct

3 structural modifications that increase surface area (fom largest to smallest):

• Circular folds (plicae circulares)• Villi – fingerlike projections on circular

folds• Microvilli – smaller fingerlike projections on

villi that absorb; make the brush border

Figure 14.7a

Large Intestine• Larger in diameter, but shorter in

length, than the small intestine• Frames the internal abdomen• Function: absorbs water & processes

waste material

Parts:• Cecum- saclike first part of the large

intestine• Appendix– Vestigial structure – Accumulation of lymphatic tissue that hangs

from the cecum• Colon– Ascending—travels up right side of abdomen– Transverse—travels horizontally– Descending—travels down the left side– Sigmoid—enters the pelvis

• Rectum and anal canal– External anal sphincter (skeletal muscle) and

internal involuntary sphincter (smooth muscle)

Large Intestine

Figure 14.8

• No villi present• Goblet cells produce alkaline mucus

which lubricates the passage of feces• Muscularis externa layer is reduced

to three bands of muscle called teniae coli– Causes the wall to pucker into pocket

like sacs called haustra

Teeth• Function is to

masticate (chew) food

• Humans have two sets of teeth:– 20 deciduous

(baby) teeth by age 2

– 32 permanent teeth by age 12

Classification of teeth:

• Incisors—cutting• Canines—tearing

or piercing• Premolars—

grinding• Molars—grinding

Salivary Glands• Three pairs of salivary glands empty

secretions into the mouth– Parotid glands– Submandibular glands – Sublingual glands

Saliva• 98% water, 2% electrolytes, mucus,

and enzymes• Dissolves chemicals so they can be

tasted• Helps to form a food bolus• Your body produces 1-2 liters per day

Pancreas • Located deep to stomach• Produces enzymes that

break down carbohydrates

• Enzymes secreted into the duodenum

• Alkaline fluid mixed with enzymes neutralizes acidic chyme coming from stomach

• Produces hormones that regulate blood sugar:– Insulin (takes sugar from

blood to tissues)– Glucagon (takes sugar

from tissues to blood)

Figure 14.6

Pancreas

Figure 14.1

Liver• Largest gland in the body• Located on the right side of the

body under the diaphragm• 4 lobes• Connected to the gallbladder via

the common hepatic duct• Functions:

– makes bile to break down fat– makes cholesterol to transport fats– makes proteins for blood plasma – detoxifies blood from drugs and

alcohol– Stores glycogen, vitamins, and

minerals

Liver

Figure 14.1

Bile• Yellowish-green fluid produced in the liver• Made of:– Pigments, mostly bilirubin

• Made when the liver breaks down old RBCs• Causes yellow coloring of jaundice when there is

liver disease– Salts– Cholesterol– Phospholipids– Electrolytes

• Function: breaks large fat globules into smaller ones

Gallbladder• Sac found in hollow space of liver• When no digestion is occurring, bile

backs up the cystic duct for storage in the gallbladder

• When digestion of fatty food is occurring, bile is released into the duodenum from the gallbladder

• Gallstones are crystallized cholesterol which can cause blockages

Figure 14.1