Introduction to the Digestive System

Introduction to the Digestive System

For student copy

Digestive system & homeostasis

• Digestive system contributes to homeostasis of body by breaking down food into forms that can be absorbed & used by body cells.

• also: – absorbs : • water, vitamins, minerals

– eliminates:• wastes

2 groups of organs:

GI Tract Accessory Organs

• aka Alimentary Canal

• continuous tube mouth anus

• mouth• most of pharynx• esophagus• stomach• small & large

intestine

• teeth• tongue• salivary glands• liver• gallbladder• pancreas

vocabulary• Ingestion: eating• Secretion: ~7 liters water, acids,

buffers, & enzymes into lumen of GI tract

• Motility: contraction/relaxation of smooth muscle in wall of GI tract mix & propel food & secretions anus

• Mechanical Digestion: physically breaking down food

• Chemical Digestion:• Absorption• Defecation: wastes leave body/ material

defecated called feces/ gas called flatus

Layers of the gi tract(inner outer)1. mucosa2. submucosa3. muscularis4. serosa

Mucosa: 3 layers1. epithelium– mouth esophagus: stratified sq. epith for

protection– stomach intestines: simple columnar– cells slough off q 5-7 d– exocrine mucus glands (mucus & water) &

several types endocrine glands called enteroendocrine cells interspersed

Mucosa: 2nd layer:lamina propia

• areolar CT• rich in blood &

lymph vessels• contains most of

MALT (mucosa-associated lymphatic tissue)

Mucosa: 3rd layer:muscularis mucosa

• thin layer of smooth muscle

• creates small folds in epithelium increases surface area for digestion & absorption

submucosa• areolar CT that binds mucosa to

muscularis• rich blood & lymphatics, glands• submucosal plexus: extensive

network of neurons

Muscularis• skeletal muscle in mouth, pharynx,

superior & middle parts of esophagus, external anal sphincter– voluntary swallowing & defecation

• 2 sheets smooth muscle in rest of GI tract with myenteric plexus between them

1. outer longitudinal2. inner circular

Serosa• parts of GI tract suspended in

abdominopelvic cavity have this superficial layer = visceral peritoneum

• esophagus lacks a serosa; has adventitia a single layer of areolar CT

Innervation of thegi tract

Enteric Nervous System(ENS)

• 100 million neurons that extends from esophagus anus

• 2 plexuses:1. myenteric plexus (plexus of Auerbach)– motor neurons of longitudinal & circular

muscle 2. submucosal plexus (plexus of

Meissner)– w/in submucosa supplying the secretory

cells

ANS• parasympathetic fibers via X to most of GI

tract (not to 2nd ½ large intestine: gets it from sacral spinal cord)– increase secretions & motility

• sympathetic fibers from thoracic & upper lumbar spinal nerves– decrease secretions & motility

Peritoneum • largest serous membrane of body• simple squamous epithelium with

underlying areolar CT• divided:1. parietal peritoneum2. visceral peritoneum

Peritoneal Cavity• space between parietal & visceral

peritoneums– contains serous fluid: decreases friction– Ascites: excess serous fluid ass’c w/ some

diseases

Retroperitoneal• “behind peritoneum”• kidneys & most of pancreas, end of

sigmoid colon under parietal peritoneum

5 Major Peritoneal Folds1. greater omentum2. falciform ligament3. lesser omentum4. mesentery5. mesocolon

Greater Omentum• largest peritoneal fold• “fatty apron”• drapes over transverse colon & coils of sm

intestine• extends downward anterior to sm

intestine• contains adipose cells, macrophages,

plasma cells;• function to combat & contain infections

Greater omentum

Falciform Ligament• attaches liver

to anterior abdominal wall & diaphragm– liver only

organ in abdominopelvic cavity attached to anterior abd wall

Lesser Omentum• arises as 2 folds in serosa of stomach &

duodenum

Mesentery• fold of peritoneum

that attaches small intestine to posterior abdominal wall– starts @ posterior

wall wraps around sm intestine reflects back to posterior abd wall

– route of bld supply to sm intestine

Mesocolon• binds large

intestine to posterior abd wall

• route of blood supply, lymphatic drainage

Peritonitis• acute inflammation of the

peritoneum: contamination by infectious microbes– from punctures of abdominal organs• trauma• surgical wounds