THE DIGESTIVE SYSTEM

D. C. MikuleckyFaculty Mentoring ProgramVirginia Commonwealth University

FUNCTIONS OF THE DIGESTIVE SYSTEM

MotilitySecretionDigestionAbsorption

Motility

The digestive tract is surrounded by layers of smooth muscle

These muscles enable mixing and propulsive movement to be carried out by the digestive tract

Secretion

Digestion requires that enzymes be secreted by the pancreas and other organs

Mucous secretions protect the digestive tract

Acid is secreted in the stomachThere are other secretions of

importance

DIGESTION

Breaking down complex foodstuffs into absorbable units by enzymes produced in the digestive system

Involves the breakdown of carbohydrates, proteins fats, and other foods

Absorption

All other functions support this one

This is how we obtain the necessary fuel for our cells

THE DIGESTIVE TRACT

Mouth and pharynxEsophagusStomachSmall IntestineLarge IntestineAnus

THE DIGESTIVE TRACT

Lumen

Mucosa

SubmucosalPlexus

CircularMuscle

Myenteric Plexus

Logitudinal Muscle

ExtrinsicNerves

ACCESSORY DIGESTIVE ORGANS

Salivary GlandsExocrine PancreasBilliary System: (Liver and

Gallbladder)

CONTROL OF DIGESTIVE SYSTEM

ExternalInfluences

LocalInfluences

Receptors in Digestive Tract

IntrinsicNervePlexuses

Extrinsic Autonomic Nerves

GIHormones

Smooth muscleExocrine Gland CellsEndocrine Gland Cells

THE MOUTH

Ingestion of food

Chewing and swallowing

Salivary secretion

Chewing

Chewing is the first step in the digestive process

Food is mixed with saliva and broken up into smaller pieces

Taste buds are stimulated

Salivary secretion

Salivary amylase: digests carbohydrateMoistening and lubricating agentAntibacterial action: lysozyme and

rinsingSolvent for taste stimuliSpeech facilitationOral hygeneBicarbonate buffers neutralize acids

Swallowing A programmed all-or-none reflex Chewing and moving the bolus of food back is manly

voluntary (striated muscle) Pressure of bolus on pharynx triggers involuntary

reflex (smooth muscle) Tongue prevents food from moving back Uvula elevated, sealing nasal passage Larynx elevates and closure of glottis Respiration briefly inhibited Pharyngeal muscles force bolus back Peristaltic waves move bolus through esophagus

PERISTALSIS

Ringlikecontractionsweeps downthe esophagus

THE STOMACH

Esophagus

Body

Antrum

GastroesophagealSphincter

Pyloric

Sphincter

OxynticMucosa

PyloricGlandArea

Fundus

THE STOMACH

Mixing and storage organHas its own pacemaker: sets frequency

of rhythmic peristaltic contractionsPeristalsis becomes stronger in antrumMotility is highly controlledNo absorption of food, only aspirin and

alcohol

FACTORS INFLUENCING GASTRIC MOTILITY

Distension of stomach: increasesFeedback from the small intestine:

decreasesControl from CNSGastrin: increases

MIXING MOTIONS IN THE STOMACH

Pyloric SphincterClosed

PeristalsisBegins

MIXING MOTIONS IN THE STOMACH

Pyloric SphincterClosed

Peristaltic WaveTravels Downward

STOMACH EMPTYING

Pyloric SphincterOpens

Peristaltic WaveTravels Downward

ChymeSquirtedintoDuodenum

FACTORS CONTROLLING STOMAC EMPTYING

Gastric MotilityEnterogastric reflex: Via intrinsic and

autonomic nervesEnterogastrones: secretin,

cholecystokinin (CCK), gastric inhibitory peptide

FACTORS IN SMALL INTESTINE CONTROLLING STOMAC EMPTYING

FatAcidHypertonicityDistension

GASTRIC SECRETIONS

Mucous: Surface Cells Mucous: Mucous Neck

Cells Hydrochloric Acid: Parietal

cells Intrinsic Factor: Parietal

Cells Pepsinogen: Chief Cells Gastrin: Only in the Pyloric

Gland Area

Activation of Pepsin

ChiefCell

PEPSIN-OGEN

HCl

ParietalCell

PEPSIN

DigestsProtein

STIMULATION OF GASTRIC SECRETION: Cephalic Phase

Seeing,Smelling,TastingFood

Vagus Intrinsic Nerves

Pyloric Area

Parietal& ChiefCells

GastrinIncreasedGastricSecretion

STIMULATION OF GASTRIC SECRETION: Gastric Phase

Stimuliin Stomach:protein, distension,caffeine,alcohol

VagusIntrinsic Nerves

Pyloric Area

Parietal& ChiefCells

GastrinIncreasedGastricSecretion

THE GASTRIC MUCOSAL BARRIER

Protects the cells from contents of stomach

Luminal membranes of cells are impermeable to protons

Cells are tightly adjoinedRapid turnoverIf broken, peptic ulcer may result:

positive feedback involving histamine

Gastrin secretion inhibition

Acid in antrum

removal of protein as stomach empties