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1

gastroenterology

the study of the digestive tract and the diagnosis and treatment of its disorders

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digestive system

the organ system that processes food, extracts nutrients from it, and eliminates the residue

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five stages of digestion

ingestion
digestion
absorption-
compaction
defecation

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ingestion

selective intake of food

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digestion

mechanical and chemical breakdown of food into a form usable by the body

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absorption-

uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph

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compaction

absorbing water and consolidating the indigestible residue into feces

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defecation

elimination of feces

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mechanical digestion

the physical breakdown of food into smaller particles
–cutting and grinding action of the teeth
–churning action of stomach and small intestines
–exposes more food surface to the action of digestive enzymes

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chemical digestion

aseries of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
–carried out by digestive enzymes produced by salivary glands, stomach, pancreas and small intestine

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results:

•polysaccharides into monosaccharides
•proteins into amino acids
•fats into monoglycerides and fatty acids
•nucleic acids into nucleotides

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some nutrients are present in a usable form in ingested food

–absorbed without being digested
–vitamins, free amino acids, minerals, cholesterol, and water

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digestive system has two anatomical subdivisions

digestive tract
gastrointestinal

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digestive tract

(alimentary canal)
–30 foot long muscular tube extending from mouth to anus
–mouth, pharynx, esophagus, stomach, small intestine, and large intestine

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gastrointestinal (GI) tract

is the stomach and intestines

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accessory organs

teeth, tongue, salivary glands, liver, gallbladder, and pancreas

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General Anatomy

•digestive tract is open to the environment at both ends
•most material in it has not entered the body tissues
–is considered to be external to the body until it is absorbed by the epithelial cells of the alimentary canal
•in a strict sense, defecated food residue was never in the body

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basis structural plan

most of the digestive tract follows the basis structural plan with digestive tract wall consisting of the following tissue layers, in order from inner to outer surface

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basis structural plan

order

•mucosa
–epithelium
–lamina propria
–muscularis mucosae
•submucosa
•muscularis externa
–inner circular layer
–outer longitudinal layer
•serosa
–areolar tissue
–mesothelium

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mucosa

(mucous membrane) –lines the lumen and consists of:
–inner epithelium
–lamina propria –
–muscularis mucosa
–mucosa-associated lymphatic tissue (MALT) –

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inner epithelium

•simple columnar in most of digestive tract
•stratified squamous from mouth through esophagus, and in lower anal canal

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lamina propria

loose connective tissue layer

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muscularis mucosa

thin layer of smooth muscle
•tenses mucosa creating grooves and ridges that enhance surface area and contact with food
•improves efficiency of digestion and nutrient absorption

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mucosa-associated lymphatic tissue (MALT)

the mucosa exhibits an abundance of lymphocytes and lymphatic nodules

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submucosa

thicker layer of loose connective tissue
–contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus secreting glands that dump lubricating mucus into the lumen
–MALT extends into the submucosa in some parts of the GI tract

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muscularis externa

consists of usually two layers of muscle near the outer surface

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inner circular layer

•in some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract

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outer longitudinal layer

•responsible for the motility that propels food and residue through the tract

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serosa

composed of a thin layer of areolar tissue topped by simple squamous mesothelium
–begins in the lower 3 to 4 cm of the esophagus
–ends just before the rectum

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adventitia

a fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into the adjacent connective tissue of other organs