Introduction to the Digestive System

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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 layers

1. 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 Folds

1. 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