Digestion Lecture 26

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Mouth

Mechanical and chemical breakdown, deglutition (swallowing)

Pharynx and esophagus

swallowing and avoiding airway

stomach

mechanical and chemical breakdown with some absorption

small intestine

chemical and mechanical breakdown with major absorption

large intestine

absoption of electrolytes and some vitamins

rectum and anus

defecation

Epithelium

Mucosa
-Stratified squamous in mouth, espohagus and anus
-Simple columnar in the rest of the GI tract
-Secretes enzymes and absorbs nutrients
-specialized cells (goblet) secrete mucous onto cell surfaces
-enteroendocrine cells secrete hormones controlling organ function

lamina propria

Mucsoa
-thin layer of loose connective tissue
-contains blood vessels and lymphatic tissue

Muscularis mucosae

Mucosa
-thin layer of smooth muscle causes folds to form in mucosal layer
-drives local movements to increase absorption by exposing ingested material to absorptive surfaces

Loose connective tissue

Submucosa
-containing blood vessels, glands and lymphatic tissues

Submucosal plexus

Submucosa (Meissner's plexus)
-part of the enteric nervous system
-receives input from sympathetic (thoracic and lumbar spinal cord) and parasympathetic neurons (vagus nerve and sacral spinal cord)
-Regulates blood vessel diameter, secretion from glands and neurosecretory neurons, and local motility caused by muscularis mucosae

Skeletal muscle

Muscularis Externa
--- in mouth, pharynx, upper esophagus and anus under voluntary control
-control over deglutition and defecation

Smooth muscle

Muscularis Externa
--- under involuntary control
-inner circular fibers and outer longitudinal fibers
-allows motility for mixing and propulsion

Myenteric Plexus

Muscularis Externa (auerbach's plexus)
-part of the enteric nervous system with some functions shared with submucosal plexus
-provides parasympathetic & sympathetic innervation of circular and longitudinal smooth muscle layers

Serosa

for stomach and intestines
-a serous membrane also called the visceral peritoneum
-secretes serous fluid
-consists of areolar connective tissue covered with simple squamous epithelium

Adventitia

for esophagus
-consists of areolar connective tissue without the epithelium

Peritoneum

-visceral --- covers organs
-parietal --- lines the walls of body cavity

Peritoneal cavity

-potential space containing serous fluid

Mechanical Digestion

(mastication or chewing)
-breaks food into pieces
-mixes with saliva so that it forms a moist bolus

salivary amylase

Chemical digestion
begins starch digestion in the mouth (pH 6.5-7), but when the bolus hits the acidic gastric juices (pH 2.5) digestion stops

Lingual lipase

Chemical digestion
is secreted by glands in tongue and begins breakdown of triglycerides (lipids) into fatty acids and glycerol

Salivary glands

-Major glands: parotid, submandibular and sublingual glands
-All have ducts that empty into the oral cavity
-Sight, smell, sounds, memory of food, and tongue stimulation increase salivation
-Fear and anxiety decrease salivation -> dry mouth

Saliva

-Water, HCO3-, enzymes
-Moistens food (swallow)
-Dissolves food (taste)
-HCO3- buffers acidic foods
-Protects mouth from infection with rinsing action
-Lysozyme helps destroy bacteria

Voluntary Phase

Phases of swallowing 1
-Oral cavity to oropharynx

Involuntary Phase

Phases of swallowing 2
-Pharygneal stage - pharynx to espohagus
-Esophageal stage - esophagus to stomach

Voluntary phase

Oral-pharyngeal stage
Deglutition (swallowing) starts with a ---: food bolus pushed by the tongue into the oropharynx

Involuntary phase

Oral-pharyngeal stage
--- begins when sensory nerves in the pharynx signal the deglutition center in brainstem
-breathing stops
-soft palate & uvula rise closing off nasopharynx
-vocal cords adduct
-larynx rises
-epiglottis covers the larynx

Peristalsis

Esophageal phase
-Upper esophageal sphincter relaxes
- --- pushes food down
-circular fibers contract behind bolus
-longitudinal fibers contract in front of bolus to shorten the distance of travel: solid (4-8sec) liquids (1sec)
Lower espohageal sphincter relaxes as food aporoaches

Gastro-esophageal reflux disease

GERD
-If lower sphincter fails to open: distention of esophagus feels like chest pain or heart attack
-If lower esophageal sphincter fails to close:
-stomach acids enter esophagus & cause heartburn (GERD)
-worsened by a weak sphincter
-particularly bad when laying down after a large meal
-smoking and alcohol make the sphincter relax worsening the situation

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