Digestive System

GI Tract Names

digestive tract and alimentary canal

GI Tract extension

continuous tube including oral cavity, pharynx, esophagus, stomach, SI, and LI

Where does the GI Tract end?

at the anus

7 Components of GI Tract

1) oral cavity

2) pharynx

3) esophagus

4) stomach

5) SI

6) LI

7) anal canal

GI Tract Length

30ft in cadaver, shorter in living individual due to smooth muscle tone

GI Tract Lining

lined by mucous membrane

Lumen of GI Tract Function

ingested food broken down to smaller components for absorption

6 Accessory Digestive Organs

1) teeth

2) tongue

3) salivary glands

4) liver

5) gallbladder

6) pancreas

Accessory Digestive Organs

connected to the GI tract and outgrowths from the tract

3 Accessory Digestive Glands

1) salivary glands

2) liver

3) pancreas

Accessory Digestive Glands Function

produce secretions that empty into lumen of GI tract

Teeth and Tongue Function

chewing and swallowing of food

Gallbladder Function

concentrates and stores the bile produced by the liver

6 Functions of Digestive System

1) ingestion

2) motility

3) secretion

4) digestion

5) absorption

6) elimination

Ingestion

introduction of solid and liquid nutrients to oral cavity, 1st step in digestion and absorption

Motility

voluntary muscular contraction (skeletal) and involuntary contractions (smooth muscle) for mixing and moving materials through GI tract

Secretion

producing and releasing substances to facilitate digestion and moving contents through GI tract

What produces secretions in GI tract?

produced by accessory digestive glands (pancreas, liver, salivary glands) and GI tract wall

Digestion

breakdown of ingested food to smaller components to be absorbed by GI tract

2 Types of Digestion

1) mechanical digestion

2) chemical digestion

Mechanical Digestion

breaking of ingested material to smaller pieces without changing chemical structure, no enzymes

Chemical Digestion

activity of specific enzymes to break complex molecules to smaller molecules for absorption

Absorption

membrane transport of digested molecules, minerals, vitamins, and water across epithelial lining of GI tract into blood/lymph

Primary Site of Absorption

small intestine

Elimination

expulsion of indigestible components through anal canal

4 Tunics of GI Tract, inner to outermost

mucosa, submucosa, muscularis, adventitia/serosa

Mucosa Composition

inner-lining mucous membrane, epithelium and underlying lamina propria, thin layer of muscularis mucosae

Epithelium of Mucosa

in contact with the contents of lumen, simple columnar epithelium in SI, LI, and stomach for secretion and absorption, nonkeratinized strat squamous epithelium in esophagus to withstand abrasion

Underlying Lamina Propria of GI Tract

areolar CT, blood and lymph vessels, small branches of nerves

When does absorption occur?

when substances are moved through simple columnar epithelial cells lining GI tract and absorbed into blood or lymph capillaries in lamina propria

Muscularis Mucosae

thin layer of smooth muscle which contracts

Smooth Muscle Contraction in Muscularis Mucosae

facilitates release of secretions from mucosa into lumen and increases contact of materials in lumen with epithelial layer of mucosa for more efficient absorption

Submucosa Components

areolar and denser irregular CT, contains many BVs, lymph vessels, nerves, and glands

Submucosal Nerve Plexus

branches of nerves in submucosa extend to mucosa along with their autonomic ganglia

Submucosal Nerve Plexus Innervation

innervate smooth muscle and glands of mucosa and submucosa

MALT Housing

housed by lamina propria of mucosa and submucosa

GALT

gut-associated lymphoid tissue, MALT in the GI tract

MALT in Submucosa

larger aggregates of lymphoid nodules, called aggregated lymphoid nodules of the ileum (Peyer Patches)

MALT Function

protection from harmful agents by preventing ingested microbes from crossing GI tract wall and entering body

Muscularis Composition and Arrangement

composed of smooth muscle tissue, arranges into inner circular and outer longitudinal layer

Inner Circular Layer of Muscularis Composition

contains muscle cells oriented circumferentially in GI tract wall

Outer Longitudinal Layer Composition

muscle cells oriented lengthwise in GI tract wall

Myenteric Nerve Plexus

branches of nerves and their autonomic ganglia located between the inner circular and outer longitudinal layers

Myenteric Nerve Plexus Function

motor neurons in the plexus control contractions of the muscularis

Muscularis Function

motility: mixing and propulsion

Contractions of Circular Layer

constrict the lumen of the GI tract tube

Contractions of Longitudinal Layer

shorten the GI tract tube

Mixing

blends secretions with ingested materials in GI tract, no directional movement of lumen contents, includes mixing waves by stomach and segmentation by SI

Propulsion

directional movement of materials through GI tract by muscularis by peristalsis

Peristalsis

contraction of muscularis within the GI tract wall, moves like a wave within different regions of GI tract, one way movement of lumen contents from esophagus to anus

Sphincter

thickened inner circular muscle layer at diff locations along GI tract

Sphincter Function

rings of smooth muscle relax (open) and contract (close) to control movement of materials into next section of GI tract and prevent their backflow

Pyloric Sphincter

regulates movement of materials from stomach into the SI

Adventitia Composition

areolar CT with dispersed collagen and elastic fibers

Serosa

adventitia and an outer covering of serous membrane called visceral peritoneum

Which organs have a serosa as their outermost tunic?

only the digestive organs that are intraperitoneal

Exceptions to typical Tunic Pattern

esophagus with nonkeratinized strat squamous epithelium in mucosa and 3 layers of smooth muscle in the stomach muscularis for mechanical digestion

Digestive Processes are Regulated by…?

the nervous and endocrine systems

GI Tract Walls Innervated by,,,?

enteric and autonomic nervous system

Enteric Nervous System

sensory and motor neurons extending from the esophagus to anus, forms the submucosal and myenteric nerve plexus

Enteric Nervous System Innervation and Function

innervates smooth muscle and glands of GI tract, mediates the reflexes for mixing and propulsion of materials through GI tract

Autonomic Nervous System and GI tract wall

parasympathetic and sympathetic divisions of autonomic nervous system innervate GI tract wall

ANS Synapse in GI Tract

parasympathetic and sympathetic axons synapse with smooth muscles and glands in GI tract wall and with neurons in the ENS

Parasympathetic Innervation of GI Tract Function

promotes GI tract activity, stimulates motility and release of secretions, relaxes GI tract sphincters

Sympathetic Innervation of GI tract Function

opposes GI tract activity, inhibits GI tract motility and release of secretions, contract GI tract sphincters and vasoconstricts BVs in GI tract walls

Nerve Reflex in GI Tract Initiation

starts with detection of clanging stimuli by stretch and chemoreceptors

Stretch Receptors

detect stretch of GI tract wall

Chemoreceptors

monitor chemical content of material in lumen, presence of protein and acid

Short reflex

local reflex involving only ENS, no CNS, sensory input detected by stretch or chemoreceptors and relayed to neurons in ENS to alter smooth muscle contraction and gland secretion of GI tract wall, coordinate small segments of GI tract

Long Reflex

sensory input relayed to CNS, autonomic motor output relayed to alter smooth muscle contraction and gland secretion of GI tract wall, includes accessory digestive organs, contractions and secretory activity of many different components of digestive system

Circulating Hormones

released into blood (gastrin, secretin, cholecystokinin (CKK)) to regulate process of digestion

Local Hormones

influence adjacent cells (histamine)

2 Stomach Hormones

1) gastrin

2) somatostatin

Gastrin

stimulated by bolus in stomach, targets the stomach, stimulates stomach motility and release of gastric secretions from parietal and chief cells

Somatostatin Stimulation

stimulated by increased stomach acidity

Somatostatin Targets

GI tract and pancreas

Somatostatin Function

regulates function of nearby enteroendocrine cells (inhibits gastrin) and exocrine (inhibit parietal cell secretion), inhibit pancreatic juice secretion by acinar cells

SI Hormones

1) CKK

2) secretin

3) glucose-dependent insulinotropic peptide (GIP)

4) motilin

CCK Stimulation

stimulated by chyme containing fatty acids entering duodenum

CCK Target

stomach, gallbladder, pancreas, hepatopancreatic sphincter, and ileocecal valve

CCK Effects

inhibit stomach motility and gastric secretions of stomach, stimulates release of bile in gallbladder, stimulates release of pancreatic juice, causes relaxation in hepatopancreatic sphincter and ileocecal valve

Secretin Stimulus

chyme entering duodenum, especially if acidic

Secretin Targets

stomach, liver, pancreas

Secretin Effects

inhibit stomach motility and gastric secretion, stimulate secretion of HCO3- (alkaline) in liver and pancreas

GIP Stimulus

increase of glucose in SI

GIP Targets

pancreas

GIP Effects

regulates release of insulin from beta cells in pancreas

Motilin Stimulus

increased amounts released in intestinal phase

Motilin Targets

SI

Motilin Function

regulate migrating motility complex to move contents in lumen from SI to LI

Liver Hormone

hepcidin

Hepcidin Stimulus

increased iron content in chyme

Hepcidin Target

SI

Hepcidin Effect