Digestive System

GI tract

mouth, pharynx, esophagus, stomach, small intestine, large intestine

accessory digestive organs

salivary glands, liver, gallbladder, pancreas

mouth/oral cavity

lips, cheeks, palate, uvula, frenulum, tongue, papillae, lingual tonsils, teeth

lips

- orbicularis oris muscle
- connective tissue
- skin
- numerous sensory receptors and blood vessels (gives lips their red color)

cheeks

- lateral walls
- some facial muscles
- lined with stratified squamous epithelium

hard palate (bone)

- maxilla and palatine bones
- covered with mucous membrane

soft palate

- muscular arch: skeletal muscle
- covered with mucous membrane

uvula

cone shaped projection of soft palate

frenulum

fold of tissue under tongue, restricts movement

intrinsic muscles of tongue

interwoven skeletal muscle

extrinsic muscles of tongue

- attached to the base of the tongue
- originate on hyoid and other structures
- ex: hyoglossus muscle

papillae

- small raised areas
- taste buds
- structures sensitive to touch

lingual tonsils

lymphoid tissue on posterior portion of tongue

general structure of teeth

crown, neck, root, periodontal membrane, gingiva (gum), alveolus

internal structure of teeth

enamel, dentin, pulp, root canal

crown (teeth)

exposed portion

neck (teeth)

gums attach here

root (teeth)

anchors tooth firmly in jaw

periodontal membrane (teeth)

special type of connective tissue which lines socket

gingiva (gum) (teeth)

mucous membrane

alveolus (teeth)

socket (cavity) in bone

enamel (teeth)

mostly CaPO4

dentin (teeth)

bone-like material

pulp (teeth)

nerves and blood vessels

root canal (teeth)

canal in which pulp is found

salivary glands

parotid, submandibular, sublingual, buccal glands

parotid glands

- located inferior and anterior to ear
- drains into oral cavity near 2nd upper molar

submandibular glands

- located just inside the mandible
- empties into the floor of the mouth

sublingual glands

located under mucosa in floor of mouth

buccal glands

small glands located on the mucosal membrane of the mouth

components of saliva

water, enzymes, mucous, salts

water (saliva)

- 99%
- dissolves food and chemicals so they can be digested (and tasted)

lysozymes (saliva)

- destroy bacteria
- clean teeth

amylase (saliva)

begins digestion by breaking down starches

mucous (saliva)

lubrication

salts (saliva)

maintain proper pH for digestive enzymes

mumps

swollen salivary glands

structure of pharynx

- lined with stratified squamous epithelium
- mucous membrane (lubrication)
- skeletal muscle

notes about swallowing

- swallowing begins as voluntary action
- sets of reflexive opening/closing of openings (ex: nasal cavity)
- once past mid-esophagus, it is completely involuntary

structural layers of the GI tract

mucosa (where the food touches), submucosa, muscularis, serosa

epithelium

- stratified squamous at beginning and end of GI tract (mouth, pharynx, esophagus, anal canal)
- simple columnar in middle of tract (stomach, small & large intestine)
- glands

lamina propria

basement membrane under mucosa

submucosa

- rich blood supply nourishes mucosa and absorbs nutrients
- elastic and collagenous fibers allow GI tract to expand
- nerve plexuses innervate glands
- lacteals (lymphatic vessels that absorb fat)

muscularis

smooth muscle
- inner circular layer mixes food
- outer longitudinal layer propels food
- stomach has innermost 3rd oblique layer (only one with an extra 3rd layer)
- innervated by ANS

serosa

outer covering composed of fibrous CT

location of esophagus

- posterior to trachea
- begins at larynx
- passes through hiatus in the diaphragm to join the stomach

structure of esophagus

- mucosal (stratified squamous)
- submucosa (blood vessels and nerves)
- muscularis (upper 1/3 = skeletal, middle 1/3 = combo of skeletal & smooth, lower 1/3 = smooth muscle)
- serosa (CT)

gastroesophageal sphincter

- prevents stomach contents from entering esophagus
- smooth muscle

histal hernia

stomach bulges/herniates through diaphragm

innervation of the GI tract

done via plexi

celiac, superior/inferior mesenteric plexuses

- located around arteries of the same names
- sympathetics synapse here
- branches of CN X provide parasympathetics and relay sensory

Meissner's plexus

- innervates smooth muscle layer that squeezes mucosa
- aids in gland secretion

myenteric plexus

- major nerve supply to smooth muscle of tract
- peristalsis

blood supply to GI tract of unpaired arteries

celiac trunk, superior mesenteric artery, inferior mesenteric artery

celiac trunk

supplies esophagus, stomach, spleen, liver, gallbladder

superior mesenteric artery

supplies duodenum, pancreas, jejunum, ileum, cecum, ascending colon, transverse colon

inferior mesenteric artery

supplies transverse colon, descending colon, sigmoid colon, rectum

peritoneum

serous membrane which lines the abdominopelvic cavity and covers and supports the organs

parietal peritoneum

lines body cavity

visceral peritoneum

covers outer surfaces of organs

peritoneal cavity

space between the two which contains a small amount of peritoneal fluid

special peritoneal membrane

folds of peritoneum that isolate organs, protect organs, support organs and blood vessels, attach organs to the body wall, store fat

lesser omentum

special peritoneal membrane that attaches the lesser curvature of the stomach to the liver

greater omentum

special peritoneal membrane that suspends from the inferior curvature of the stomach

falciform ligament

special peritoneal membrane that attaches liver to inferior diaphragm anterior body wall

round ligament

special peritoneal membrane which is remnant of umbilical vein (carried O2 rich blood from placenta)

mesentery

special peritoneal membrane that attaches the small intestine to posterior abdominal wall

mesocolon

special peritoneal membrane that attaches the large intestine to posterior abdominal wall

retroperitoneal organs

"behind the peritoneum"
- pancreas
- kidneys
- duodenum
- ascending & descending colon

stomach

mechanical and chemical digestion

location of stomach

- upper left quadrant
- between esophagus and small intestine

regions of stomach

cardiac, fundus, body, pyloric

borders of stomach

greater and lesser curvature

layers of stomach wall

mucosa, submucosa, muscularis, serosa

mucosa (stomach)

simple columnar epithelium

submucosa (stomach)

nerves and blood vessels

muscularis (stomach)

three layers of smooth muscle
1. inner oblique
2. middle circular
3. outer longitudinal

serosa (stomach)

connective tissue

rugae

- folds of mucosal layer
- permit stomach to distend
- increase surface area for secretion and digestion
- disappear as stomach expands

chief (zymogenic) cells

- produce inactive pepsinogen
- converted to active enzyme pepsin for protein digestion

parietal cells

- produce HCl which decreases the pH of stomach contents
- produce intrinsic factor required for B12 absorption

mucous cells

protective mucous

enteroendocrine cells

- hormone-producing cells
- ex: gastrin: increase GI activity

functions of stomach

- mechanical mixing of food and production of chyme
- initiates major protein digestion
- storage of chyme until it passes into the duodenum
- minimal absorption (ex: some drugs, alcohol)

location of small intestine

- extends from pyloric sphincter to large intestine
- occupies central and lower portion of abdominal cavity

ulcers

overproduction of HCl by the stomach due to stress, infection, etc.

chyme

acidic paste in stomach

regions of the small intestine

duodenum, jejunum, ileum

duodenum

- C-shaped region
- first ten inches of small intestine
- retroperitoneal
- duodenal papilla

duodenal papilla

- opening in duodenum
- bile from common bile duct
- enzymes from pancreatic duct

Brunner's glands

- in duodenum
- secrete alkaline solution
- protect intestinal lining from acidic chyme

jejunum

- middle section
- slightly larger lumen

ileum

- third region of small intestine
- contains Peyer's patches

Peyer's patches

clusters of lymphatic tissue

functions of small intestine

- major site of chemical digestion
- mechanical mixing
- major site for absorption of nutrients
- propels undigested nutrients or materials to large intestine

pilcae circulares

- deep folds of mucosa within wall of intestine
- increase surface area for digestion and absorption
- do NOT disappear as intestine expands (unlike rugae)

villi

- small finger-like projections on wall of intestine
- increase surface area
- capillary network
- lacteals

microvilli

same thing as villi but even more absorptive surface area

intestinal glands

- found on surface of villi and in crypts at base of villi
- digestive and antibacterial enzymes
- hormones affect GI function

location of large intestine

- begins at end of ileum in lower right quadrant
- extends superiorly to liver
- passes to the left to spleen
- descends on left to pelvis
- terminates at anus