Digestive System
Digestive system
• Organs of the digestive system are put
into 2 groups
– Alimentary canal (aka GI tract)
• Ingests, digests, absorbs and defecates
– Accessory organs
• Accessory organs
– Primarily glands, that secrete fluids into tract
– Mouth or oral cavity with salivary glands and
tonsils
– Pharynx (throat) with tubular mucous glands
– Esophagus with tubular mucous glands
– Stomach with many different kinds of glands that are
tubular
– Small intestine (duodenum, ileum, jejunum) with
liver, gallbladder and pancreas as major accessory
organs
– Large intestine including cecum, colon, rectum and
anal canal with mucous glands
– Anus
1. Ingestion: introduction of food
into stomach
2. Mastication: chewing.
Chemical digestion requires
large surface area so breaking
down large particles
mechanically facilitates
chemical digestion.
3. Propulsion
– Deglutition: swallowing
– Peristalsis: moves
material through digestive
tract . A wave of circular
smooth muscle relaxation
moves ahead of the bolus of
food or chyme allowing the
digestive tract to expand.
Functions (8 we will look
at, please read through
these)
Functions, cont.
5. Secretion: lubricate, liquefy, digest
Mucus: secreted along entire digestive tract,
lubricates food and lining, coats lining and protects
from mechanical digestion, from acid and from
digestive enzymes. (NSAID’s)
Water: liquefaction makes food easier to digest and
absorb
Bile: emulsifies fats
Enzymes: chemical digestion
6. Digestion: Mechanical and chemical
7. Absorption: Movement from tract into circulation or
lymph
8. Elimination: Waste products removed from body;
feces. Defecation
The digestive tract & tunics
• The alimentary canal wall contains 2
important intrinsic nerve plexuses
– Submucosal nn plexus
– Myenteric nn plexus
• These are part of the ANS
• They regulate the mobility and secretory
activity of the GI tract
• The 4 tunics (or coverings or layers) of the
stomach
– Mucosa (inner most layer, stratified squamous
epithelium)
– Submucosa contains the nerves, blood
vessels, lymph vessels
– Muscularis (smooth mm, inner layer and outer
layer or circular and longitudinal mm)
– Serosa (outer most layer)
The tunics and nerve plexuses
• Nerve regulation of the digestive system
– Enteric (bowels) nerve plexus
• They detect the change in chemical composition
(food)
• stimulate smooth mm contraction
– CNS
• Reflex centers in the medulla (vagus nn)
• Food, smell, sight all can stimulate
• SymNS vs ParasymNS
• Stomach
–
–
–
–
C-shaped
On the left side of the abdominal cavity
Hidden by the liver and diaphragm
Different anatomical parts
• The stomach is a temporary storage tank for
food
• Also a site for food breakdown
• Mm help move food through the stomach as well
as churn, mix and pummel food, physically
breaking it down into smaller fragments
• Chemical breakdown of protein starts in the
stomach
• Most of the digestion occurs in the pyloric
region of the stomach
• After food has been processed in the
stomach it is now chyme
• Chyme enters the small intestine through
the pyloric sphincter
• Small intestine
– Is the bodies major digestive organ
– Goes from the pyloric sphincter to the
ileocecal valve
– Longest part of the alimentary canal
– Avg length of 8-18 feet
• 3 subdivisions of the small intestine
– Duodenum (12 finger widths long) 5%
– Jejunum (empty) 40%
– Ileum (twisted intestine) 60%
• Ileum joins the large intestine at the ICV
• SI can only digest a small amount of food at a
time
• So the pyloric sphincter (gatekeeper)
– Slowly releases food into the SI
• The pancreas
– Produces enzymes that enter the duodenum through
ducts
• Bile enters the duodenum from the gallbladder
through ducts
• As food passes through the SI, nutrients are
absorbed through the microvilli the capillary
system to nourish the body
• Chime is “waved”
toward the pyloric
sphincter
• The more liquid part
is moved to the
pyloric s. where as
the thicker is moved
back to the body
• Finally released into
the duodenum
• Large intestine
– About 5 feet
– Ileocecal valve to anus
– Fxn: to dry out undigested food
• Absorbing water
• Expelling waste