Mouth to Stomach

Motility

move food through system

Secretion

endocrine and exocrine secretions to aid in digestion

Digestion

hydrolyze food into molecule monomers

Absorption

absorb monomers and water

Storage and elimination

temporary storage and elimination of indigestible food

Immunity

physical barrier against pathogens
- location of a large number of immune cells

Gi tract

extends from mouth to anus

length of GI tract in an adult

9 m

the Gi tract lumen is considered the

outside of the body

structures in the GI tract

Oral cavity
pharynx
Esophagus
stomach
Small intestines
Large intestines

Accessory Digestive Organs

required for completes digestion
- not part of main tube

Structures of Accessory Digestive Organs

teeth
tonque
salivary glands
liver
gallbladder
pancreas

tunics of GI tracts

Serosa
Muscularis
Submucosa
Mucosa

Serosa

outer most layer
connective tissue and epithelium
continuous with mesentery and peritoneum

Muscularis

muscular layer responsible for peristalsis and segmentation

layers of muscularis

inner circular layer
outer longitudinal layer

Muscularis innervated by

myenteric plexus

Mucosa

innermost layer

motility

movement of food through GI tract

Ingestion

putting food in the mouth

Matication

chewing food and mixing with saliva

Deglutiton

Swallowing

Peristalsis

wave-like contractions that propel food

Segmentation contractions

Contractions within a segment to churn food
- no propulsion!

GI tract innervation

ANS

Gi trat innervation parasympathetic

stimulates motility of tract and secretions into tract
- synpases at ganglia in submucosal and myenteric plexuses

Nerve innervating the GI tract

Vagus nerve and Pelvic splanchnic nerve

Vagus nerve

stimulates from pharynx to upper portion of large intestine

Pelvic Splanchnic nerves

Stimulates lower portion of large intestine

Sympathetic innervation of Gi tract

inhibits parasympathetic stimulation to slow motility and secretion

Enteric Nervous system

local (intrinsic) nervous regulation of digestive tract
- cell bodies and integration of signals occur outside of CNS

Mouth responsible for

ingestion and mastication

ingestion and mastication

grind food into smaller pieces

food is mixed with saliva by

secretions from 3 pairs of salivary glands

Saliva contains

mucus, antimicrobial agents and salivary amylase

Deglutition

swalling

oral phase of deglutition

voluntary control
- food mixes with saliva to form bolus
bolus moved to back of oral cavity

involuntary control phases of deglutition

Pharyngeal phase and Esophageal phase

Pharyngeal phase

involuntary
soft palate blocks off nasopharynx
larynx moves away from laryngopharynx

during deglutition the laryngopharynx is closed off by the

epiglottis

last step of pharyngeal phase

upper esophageal sphincter releases

Esophageal phase

involuntary
- peristalsis moved bolus down esophagus
passes through lower esophageal sphincter
\-- Empties into Cardiac region of the stomach

Peristalsis in pharynx and esophagus

propels bolus 2-4 cm/sec forward
-circular muscle contracts behind bolus
\--- relaxes in front of bolus
longitudinal muscle shortens tube

Stomach functions

store food
- initiate digestion of proteins
kill bacteria by high acidity
move chyme into small intetine

Stomach regions

cardia, fundus, body, and pylorus

a bolus of food enter stomach in

cardiac region

chyme exits the stomach at the

pyloric sphincter

stomach lined by

long muscular folds called rugae

Muscularis contains three layers of muscle

longitudinal circular and oblique

muscularis layer aid in

churning and thorough mixing of chyme

gastric mucosa

microscopic folds in mucosa called gastric pits

gastric pits

contain cells of the gastic glands

mucous. neck cells

secrete mucus

Chief cells

secrete pepsinogen

Enterochromaffin-like (ECL) cells

secrete histamine and serotonin

Parietal cells

secrete HCl
probably secrete intrinsic factor
\- - probably required for absorption of vitamin B12 and RBC production

G cells

secrete gastrin into the blood

D cells

secrete somatostatin

Exocrine

gastric juice composed of mucus, HCL, pepsinogen, and water
Highly acidic

exocrine / gastric juice produced per day

2-3 liters

Endocrine

histamine
serotonin
gastrin
somatostatin
intrinsic factor
Ghrelin

Ghrelin

may be hunger/ emptiness signal to brain
- stimulates food intake, fat deposition, and growth hormone release

HCL production

H+/ K+. ATPase pumps localized to apical membrane of parietal celss

large number of H+/ K+. ATPase pumps on

microvilli

H+/ K+. ATPase pumps H+

pump H+ into stomach lumen against gradients

H+/ K+. ATPase pumps K+

pumped into the cell

K+ leaks back into the lumen via

passive transport through K+ channels on apical membrane
aka K+ recycling

K+ recycling and Cl- secretion required for

ATPase pump function

HCL production transport of Cl-

secondary active transport coupled with HCO3- transport into capillaries

HCo3- transport

down the concentration gradient and produced fro CO2 in the cell

Cl- pumped from capillaries into cell

aginst concentration gradient

Cl- diffuses into lumen via passive transport

through CL- channels on apical membrane

H+/K+ pumps and Cl- transport combine to

lower the gastric pH to less than 2.0
- can be as low as 0.8

Parietal cell regulation

G cells secrete gastrin into the blood

gastrin secrete may bind

directly to receptors on parietal cells

gastrin binds to receptors on parietal cells

stimulates HCl production

Usually stimulates HCl production indirectly

stimulates ECL cells to secrete histamine
- histamie binds H2 receptors on parietal cells to stimulate HCl

Parasympathetic stimulation

stimulates ECL and parietal cells

Highest ECL stimulation occurs

at night while sleeping

HCl allows for activation of

pepsinogen into pepsin

Pepsin begins

digesting of protein

Very low pH and active enzyme should

degrade epithelial lining

mucosal lining protective mechanisms

adherent layer of mucus that protects the epithelium
- tight junctions in epithelial membrane
\-- rapid proliferation of epithelial cells in mucosa

adherent layer of mucus protects epithelium

binds HCO3- to neutralize HCl near mucosa surface

pepsin is less active at a

neutral pH

Tight junctions in epithelial membrane

contains HCl and pepsin to lumen of stomach

Rapid proliferation of epithelial cells in the mucosa

entirely replaced every 3 days

if protective mechanism are absent,

peptic ulcers develop

HCl and pepsin erode the

mucosa and submucosa

3 phases of gastric regulation

-Cephalic phase
-Gastric phase
- intestinal phase

Cephalic phase stimulated by

thinking about food through first 30 minutes of a meal

Cephalic phase controlled by

CNS through vagus nerve

CNS and vagus nerce stimulates

chief cells to secrete pepsinogen
parietal cells to secrete pepsinogen - direct or indirect

Gastric phase stimulated by

stomach distension
composition of the chyme

Trp and Phe stimulate

chief and G cells

amino acids buffer

gastric juice pH

Positive feedback of gastric phase

more HCl and pepsin produces more amino acid
- stimulates more HCl and pepsin

Negative feedback

more HCl lowers the pH to 1.0

D cells produce somatostatin which

inhibits G cells

Negative feedback stops

Gastrin production
and ends HCl stimulation