Ch. 18 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 (about 10 times more than all the human cells in our body)
What are the 6 functions of the digestive system?
- Motility
- Secretion
- Digestion
- Absorption
- Storage and elimination
- Immune barrier
Motility
Movement of food through the tract
a) Ingestion: taking food into mouth
b) Mastication: chewing and mixing food w/ saliva
c) Deglutination: swallowing
d) Peristalsis: wave-like, one-way movement through tract
e) Segmentation: churning and mixing while moving forward
Secretion
Exocrine: digestive enzymes, HCl acid, mucus, water and bicarbonate
–open tube from one end to the other, secretions put into GI tube
Endocrine: hormones to regulate digestion; secreted into blood stream
Digestion
Break down food into smaller units via both PHYSICAL and CHEMICAL actions
–so nutrients can be liberated and taken up by body
Absorption
Transport of digestion products (nutrients) into blood or lymph
Storage and Elimination
Temporary storage and subsequent elimination of undigested food molecules
Immune Barrier
a) Simple columnar epithelium with tight junctions prevents swallowed pathogens from entering body
b) Immune cells in CT (connective tissue) of tract promote immune responses
What organs are included in the GI tract? (NOT ACCESSORY ORGANS)
Mouth Esophagus Stomach Small intestine Large intestine Rectum, anus
Define Tract
Open at both ends; therefore is continuous w/ environment
- -“outside” the bod
- -undigested materials (e.g. cellulose) never actually enter the body
____-____ transport allows specialization of function along [GI] tract.
One-way
What are the accessory organs of the GI tract?
Salivary glands
Liver/gallbladder
Pancreas
What are the layers of the GI tract (from superficial to deep)?
- Serosa
- Subserosa
- Muscular propria
- Submucosa
- Muscularis Mucosa
- Lamina propria
- Mucosa
What are the 4 MAIN LAYERS of the GI tract (from superficial to deep)?
- Serosa
- Muscularis propria
- Submusoca
- Mucosa
Serosa
Most superficial layer
Outer covering - CT membrane continuous w/ peritoneal membrane
Muscularis Propria
2nd layer (below Serosa)
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 2 muscle layers
Submucosa
3rd layer (below Muscularis Propria)
CT layer containing larger blood vessels, lymph ducts, and the Submucosal Plexus (Meissner’s Plexus)
Mucosa
Deepest layer; closest to food
Single layer of epithelial cells - enterocytes, endocrine cells, goblet cells
Major functions: secretion, absorption
Muscularis Mucosa
Thin layer of smooth muscle - controls movement of villi, contains efferent nerve endings form submucosal plexus of ENS
Between Submucosa and Mucosa, superficial to Lamina Propria
Lamina Propria
Hydrated, CT matrix - supports epithelium, has capillaries, is collection point for lymph, contains sensory nerves and immune cells
Between Submucosa and Mucosa, deep to Muscularis Mucosa
Regulation of GI Tract
Parasympathetic Division (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 intestine
- c) preganglionic neurons synapse on submucosal and myenteric plexuses
Sympathetic Division (extrinsic regulation)
- a) inhibits peristalsis and secretion
- b) stimulates contraction of sphincters
Hormones - from brain or other digestive organs
Intrinsic regulation
- -> where ENS really comes into its own here
- a) intrinsic sensory neurons in gut wall help in intrinsic regulation via separate Enteric Nervous System (ENS)
- b) paracrine signals
What is meant by “extrinsic regulation”?
Regulation from outside the gut in this case (from brain)
Enteric Nervous System (ENS)
“Second Brain”
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 W/ SUCH A SYSTEM
However, CNS and gut do communicate via afferent/efferent connections to ENS + direction innervation to gut sensory/effector neurons
Mouth to Stomach
Mastication: large pieces of food –> chewing –> smaller pieces and mixes it w/ saliva, which contains mucus, antimicrobial agents, and salivary amylase to start digestion of starch
Deglutition (swallowing):
- 1) Oral: voluntary; muscles of mouth and tongue to mix food w/ saliva to form Bolus
- 2) Pharyngeal: involuntary; initiated by receptors in posterior oral cavity and oropharynx
- ->a) Uvula (soft palate) lifts to cover nasopharynx, and epiglottis covers vocal cords
- ->b) Upper esophageal sphincter (UES) relaxes
- 3) Esophageal: automatic; controlled by swallowing center of brain stem; bolus –> esophagus –> stomach (peristalsis)
Mouth and pharynx and upper esophagus: skeletal muscles innervated by somatic motor neurons
Lower esophagus: smooth muscle controlled by ANS
Lower esophageal sphincter (LES) opens to allow passage into stomach; otherwise stays closed to prevent regurgitation
Stomach
Stores food, breaks it into smaller pieces
Churns food to mix w/ gastric secretions - mixture is now CHYME
Begins protein digestion
Kills bacteria in food
Moves chyme into small intestine
What starts in mouth stops in stomach, but stomach picks it up w/ its own materials
–e.g. secretions in mouth inactivated by stomach acid, secretions in stomach activated by stomach acid
How many layers of muscle does the stomach have? How many layers of muscle do the intestines have?
Stomach has 3 layers, which allows it to churn and mix everything up
Intestines have 2 layers
