Digestive System (Day 1) Flashcards
Digestive System Overview
processing of ingested food and delivery of nutrients
largest immune organ
largest single habitat for microflora - bacteria, archaea, fungi: roughly 10^14 organisms (10X more than other human cells
Functions
- Motility
- Secretion
- Digestion
- Absorption
- Storage/Elimination
- Immune Barrier
Functions: Motility
Movement of food through the tract
- ingestion: taking food into mouth
- mastication: chewing, mixing food w/saliva
- deglutination: swallowing
- peristalsis: wave-like, one-way movement through tract
- segmentation: churning/mixing while moving forward
Functions: Secretion
- Exocrine: digestive enzymes, HCl, mucus, water, and bicarbonate
- ->open tube from one end to other –> to GI tube - Endocrine: hormones to regulate digestion
Functions: Digestion
break food down into smaller units via physical/chemical actions
Functions: Absorption
transport of digestion products (nutrients) into blood or lymph
Functions: Storage/Elimination
temp storage and subsequent elimination of undigested food molecules
functions: Immune Barrier
simple columnar epithelium w/tight junctions prevents swallowed pathogens from entering body
immune cells in CT of tract promote immune responses
Gastrointestinal Tract
mouth esophagus stomach small intestine large intestine rectum anus
Tract
open at both ends - therefore is continous w/environment
“outside” the body
undigested materials (ex. cellulose) never actually enter body
one-way transport allows specialization of function along tract
Accessory Organs
salivary glands
liver
gallbladder
pancreas
What are the GI Tract Layers?
- Mucosa
- Lamina Propria
- Muscularis Mucosa
- Submucosa
- Muscularis Propria
- Subserosa
- Serosa
Mucosa
single layer of epithelial cells: enterocytes, endocrine cells, goblet cells
major functions: secretion/absorption
Lamina Propria
hydrated, CT matrix - supports epithelium, has capillaries, is collection point for lymph, contains sensory nerves, and immune cells
Muscularis Mucosa
thin layer of smooth muscle - controls movement of villi, contains efferent nerve endings from submucosal plexus of ENS
Submucosa
CT layer containing larger blood vessels, lymph ducts, and the submucosal plexus (Meissner’s plexus) of nerves from the ENS
Muscularis Propria
inner layer of circular muscle - affects lumen diameter
outer layer of longitudinal muscle - affects length
Myenteric Plexus (Auerbach’s Plexus) - lies between muscle layers, controls gut motility via control of contraction/relaxation of the two muscle layers
Serosa
outer covering - CT membrane continuous with peritoneal membrane
Regulation of the GI Tract: Parasympathetic Division
a.k.a. Extrinsic Regulation
a. Stimulates esophagus, stomach, small intestine, pancreas, gallbladder,
and first part of large intestine via vagus nerve
b. Spinal nerves in sacral region stimulate lower large intestine.
c. Preganglionic neurons synapse on submucosal and myenteric
plexuses.
Regulation of GI Tract: Sympathetic Division
a. k.a. Extrinsic Regulation
a. inhibits peristalsis and secretion
b. stimulates contraction of sphincters
Regulation of GI Tract: Hormones
from brain or other digestive organs
Regulation of GI Tract: Intrinsic Regulation
- really where ENS comes into it’s own here
a. intrinsic sensory neurons in gut wall help in intrinsic regulation via separate enteric nervous system (ENS)
b. paracrine signals
Enteric Nervous System (ENS)
• Intrinsic nervous system in wall of digestive tract, able to generate reflexes independently of CNS input
—if cut neural connections to CNS, gut can still engage in regulated function
— gut is only organ with such a system
• However, CNS and gut do communicate via afferent/efferent connections to ENS + direct innervation to gut sensory/effector neurons
Mouth to Stomach: Mastication
Large pieces of food –> chewing –> smaller pieces & mixes it with saliva, which contains mucus, antimicrobial agents & SALIVARY AMYLASE to start digestion of starch.