Chapter 25 Digestive System Outline

5 stages of digestion

Ingestion: intake of food

Digestion: mechanical and chemical breakdown of food into usable form

Absorption: uptake of nutrients by digestive tract, then blood and lymph

Compaction: absorbing water and consolidating indigestible residue into feces

Defecation: elimination of feces

Mechanical digestion occurs…

in the mouth (physical breakdown of food into smaller particles)

teeth cut and grind while stomach and small intestines churn

Chemical digestion

HYDROLYSIS reactions that break dietary macromolecules into monomers

• by digestive enzymes from salivary glands, stomach, pancreas, and small intestine

Digestive tract or alimentary canal consists of

mouth, pharynx, esophagus, stomach, small & large intestines

Accessory organs consist of

teeth, tongue, salivary glands, liver, gallbladder, and pancreas

4 layers of the digestive tract wall inner to outer

mucosa, submucosa, muscularis externa, serosa

Layers of mucosa

stratified squamous epithelium

lamina propria

muscularis mucosae

Layers of muscularis externa

inner circular layer

outer longitudinal layer

Enteric nervous system

network in esophagus, stomach, and intestines

• senses tension in gut wall, conditions in lumen

• regulates motility, secretion, and blood flow

Mesenteries relationship to peritoneum

connective tissue, suspend stomach and intestines from abdominal wall

• hold viscera in proper place

• prevent intestinal twisting

• provide passage of blood vessels and nerves

• hold lymph nodes & lymphatic vessels

Mesocolon

anchors colon to posterior abdominal wall

Motility and secretion controlled by what 3 mechanisms?

neural, hormonal, and paracrine mechanisms

Neural control of motility and secretion (short vs long reflexes)

short: stretch or chemical stimulation acts through myenteric plexus

• stimulates peristalsis (wave-like muscle contractions)

long: parasympathetic stimulation of digestive motility and secretion

Hormonal control of motility and secretion

via blood, stimulate DISTANT parts of digestive tract

• gastrin & secretin

Paracrine secretions

diffuse through tissue fluids to stimulate NEARBY target cells

mouth=

buccal cavity

tongue functions

agile and sensitive organ

• manipulates food between teeth while avoids being bitten

• sensitive enough to feel stray hair in bite of food

• tastes

lingual papillae

tiny projections—sites of taste buds

vallate papillae

v-shaped row—boundary between body and root

terminal sulcus

groove behind vallate papillae

lingual frenulum

median fold— attaches body to floor

intrinsic muscles

contained entirely within tongue

• subtle movements of speech

32 adult teeth, 20 baby teeth from midline to rear type of teeth

2 incisors—chisel-like; cut/bite

1 canine—pointed; puncture & shred

2 premolars—broad surface; crush & grind

3 molars—even broader surface! crush & grind

crown, root, neck, and gingival sulcus of tooth

above gum

below gum

where crown, root, and gum meet

space between tooth and gum

Cement, Dentin, and Enamel of tooth

covers root—living

makes up most inside of tooth—living

covers crown and neck—non-living

Root canal and apical foramen of tooth

nerves and blood vessels

pore at basal end of canal

Mastication is first step in…

mechanical digestion (breaks down food into smaller particles)

temporalis, masseter, medial and lateral pterygoids are key muscles involved in mastication

Saliva functions

moisten mouth

– begin starch & fat digestion

– cleanse teeth

– inhibit bacterial growth

– dissolve molecules for taste

buds

– moistens food, binds into

bolus for swallowing

Saliva is hypotonic solution of 97-99.5% water plus…

salivary amylase: begins STARCH digestion in mouth

lingual lipase: activated by stomach acid; digests FAT after food swallowed

mucus: aids swallowing

lysozyme: kills bacteria

immunoglobulin A (IgA)

electrolytes: sodium, potassium, chloride, phosphate, bicarbonate

pH of saliva range

6.8-7

Bolus

mass of food swallowed

Salivation regulated by

facial and glossopharyngeal nerves to glands (parasympathetic)

Pharynx

connects oral cavity to esophagus

digestive and respiratory tracts intersect

Esophagus

straight muscular tube 25-30 cm long

• connects pharynx to cardiac orifice of stomach

• stratified squamous epithelium

lower esophageal sphincter prevents

stomach contents from regurgitating in esophagus

protects esophagus from stomach acid

• heartburn—burning sensation from acid reflux

2 phases of swallowing

Buccal phase (voluntary)

Pharyngoesophageal phase (involuntary)

Buccal phase of swallowing

voluntary

TONGUE presses bolus against palate, pushes posteriorly

epiglottis tips posteriorly, bolus slides by

bolus enters laryngopharynx; stimulates tactile receptors to activate next phase

Pharyngoesophageal phase of swallowing

involuntary

prevent food/drink going back to mouth

tongue root blocks oral cavity

– soft palate rises, blocks nasopharynx

– larynx moves up to meet epiglottis,

vestibular folds close airway

– epiglottis covers airway

• bolus driven downward by constriction

of pharyngeal constrictors

• bolus enters esophagus, stimulates

peristalsis

Esophageal phase of swallowing

peristalsis—wave of muscular contraction that pushes bolus ahead—involuntary

(liquid reaches stomach in 1-2 sec, bolus 4-8 sec)

at end of esophagus, lower esophageal sphincter relaxes—food enters stomach

What constricts and contracts during peristalsis?

constricting of inner circle and contracting of outer longitudinal

Primary function of stomach

food storage (upper left abdominal cavity)

1-1.5 L after typical meal

4 L extremely full

chyme

soupy/pasty mixture of semi-digested food in stomach

Stomach….breaks up food particles

MECHANICALLY, liquefies food, and begins chemical digestion of PROTEIN and fat

most digestion occurs after chyme passes on to small intestine

cardiac region of stomach

small area within 3 cm of cardiac orifice

Fundic region of stomach

dome-shaped superior portion

Body (corpus) of stomach

greatest part of stomach

3 parts of pyloric region of stomach

pyloric region is the inferior end

pyloric canal: terminates at pylorus

pylorus: narrow passage to duodenum

pyloric sphincter: regulates passage of chyme into duodenum

Rugae

longitudinal wrinkles inside stomach

parts of stomach mucosa, submucosa, and muscularis externa

simple columnar epithelium covers mucosa

mucosa and submucosa form rugae when empty, flatten when full

muscularis externa (has 3 layers NOT 2)—outer longitudinal, middle circular, inner oblique layers

gastric pits

depressions in gastric mucosa

• pyloric glands in pyloric region

• gastric glands in rest of stomach

Mucous cells secrete

(blue) mucus

includes mucous neck cells

Parietal cells secrete

(orange) HYDROCHLORIC ACID (HCl), intrinsic factor, and ghrelin

G cells secrete

(dark pink) gastrin

Chief cells secrete

(most numerous, light pink) gastric lipase and pepsinogen (pepsin—DIGESTS PROTEIN)

ONLY IN GASTRIC GLANDS

Gastric secretions

gastric juice— 2-3 L/day produced by gastric glands (water, HCl, and pepsin)

gastric lipase—produced by chief cells

gastric lipase and lingual lipase—minor role in fat digestion but remainder digested in small intestine

Cephalic phase

brain—vagus nerve—enteric NS—lots of secretion

food is coming GET READY

Gastric phase

food stretches stomach and stimulates secretion of ACh, histamine, gastrin prompts acid, pepsin, and mucus

Intestinal phase

enterogastric reflex

secretin and CCK from duodenum suppress gastric activity

vomiting

forceful ejection of stomach contents (chyme) from mouth

emetic center in medulla oblongata integrates multiple muscle actions

Projectile vomiting vs chronic vomiting

no prior nausea or retching vs causes dangerous fluid, electrolyte, and acid-base imbalances

Most digestion and nearly all absorption occur in

small intestine

gastric enzymes digest what?

mainly protein (and a little fat)

PROTEIN DIGESTION BEGINS IN STOMACH (PEPSINOGEN)

What are the 3 ways the stomach is protected?

mucous coat (thick, alkaline mucus resists action of acid/enzymes), tight junctions (between epithelial cells, prevent gastric juice from seeping through to underlying CT), epithelial cell replacement (stomach epithelial cells live only 3-6 days)

breakdown of the 3 ways stomach is protected can result in inflammation or…

peptic ulcer (mucous coat, tight junctions, epithelial cell replacement)

1.) What are two locations where mechanical digestion takes place? Compare and contrast mechanical and chemical digestion.

Mouth and stomach. Mechanical digestion involves physical breakdown of food into smaller pieces while chemical digestion involves molecular breakdown of food molecules.