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.
Mouth to Stomach: Deglutition
Oral: voluntary, muslces of mouth/tongue mix food w/saliva to form a bolus
Pharyngeal: involuntary, initiated by receptors in the posterior oral cavity and oropharynx
-uvula (soft palate) lifts to cover nasopharynx, and epiglottis covers vocal cords
Esophageal: automatic, controlled by swallowing center of brain stem
-bolus –> esophagus –> stomach (peristalsis)
Mouth to Stomach
- mouth, pharynx, and upper esophagus: skeletal muscles innervated by somatic motor neurons
- lower esophagus: smooth muscle controlled by ANS
- 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 = chyme)
- begins protein digestion
- kills bacteria in food (acid)
- moves chyme into SI
Stomach: Cardiac Region
food delivered here directly from the esophagus
Stomach: Fundus
storage
expands to accommodate load
receptive relaxation
limited motility
Stomach: Body
high motility - breaks large particles into small particles
mixers food w/secreted acid, enzymes and fluid –> forms chyme
continues the limited digestion begun in the mouth
Stomach: Gastric Rugae
at the base they contain gastric glands/pits
Stomach: Pylorus
limits amount emptied
restricts size of emptied particles in chyme
Parts of the Stomach to Know
- Cardiac Region
- Fundus
- Body
- Gastric Rugae
- Pylorus
Gastric Pits and Gastric Glands
at base of folds lead to gastric glands that contain several types of secretory cells:
- Mucous Cell
- Parietal Cell
- Chief Cell
Mucous Cell
secrete mucus to help protect stomach lining from acid
Parietal Cell
secrete HCl and IF - required for absorption of Vit B12
decreased B12 absorption –> anemia
Chief Cell
secrete pepsinogen (inactive form of proteolytic enzyme)
Enterochromaffin-like cells
histamine
serotonin
endocrine cells (“g-cells”)
somatostatin (“d-cells”)
Gastrin
?
Stimulation of HCl Secretion
*neurohormonal//paracrine
• Gastrin from G cells –> parietal cells via blood; Also –> ECL cells –> secrete histamine
• Histamine from ECL cells –> parietal cells via paracrine –> H2 histamine receptors –> stimulate H+ secretion [ECL = Enterochromaffin-like]
1) Examples: Tagamet and Zantac block H2 receptors.
• Parasympathetic neurons and Ach via vagus: stimulate G, parietal, & ECL cells
Functions of HCl
- creates acid environment in stomach –> pH 1 to 2
- ingested proteins are denatured (allows enzymes access)
- pepsinogen is converted toa ctive pepsin (digests proteins)
- serves as the optimal pH for pepsin activity
Function of Pepsin
catalyzes hydrolysis of peptide bonds in ingested proteins
Stomach Defenses
- -> why the stomach doesn’t digest itself
- adherent layer of mucus w/alkaline bicarbonate
- tight junctions between epithelial cells
- rapid epithelial mitosis (replaces epithelium every three days –> continued renewal)
Digestion and Absorption in the Stomach
Proteins: begin digestion in the stomach
Starches: begin digestion in the mouth, but salivary amylase is not active at pH 2, so this activity stops in the stomach
ETOH and NSAIDs (aspirin): only common substances absorbed in the stomach (due to high lipid solubility)
Small Intestine
starts at pyloric sphincter —- ends at ileocecal valve
- duodenum: 1st 10 in
- jejunum: middle 2/5
- ileum: last 3/5
Most digestion occurs in SI:
- carried out by pancreatic, intestinal enzymes, aided by other factors from liver via the gallbladder
- all exocrine secretions enter at duodenum
Folds of the SI
Mucosa folded into villi
Epithelial plasma membranes folded into microvilli –> ↑↑ surface area for absorption of nutrients
Intestinal Epithelium is continually being renewed
• Stem cell niche resides 4-5 cells from
bottom of crypt
• Two types of stem cells:
1) slowly dividing (stem cell renewal)
2) rapidly dividing (epithelial replacement)
• Continuous proliferation pushes cells up
the villus
• As they migrate, they differentiate into
enterocytes, enteroendocrine cells, goblet
cells; transit time to villus tip = 3-5 days
- On reaching tip, cells are shed, die (anoikis - specialized type of apoptosis), and are replaced
- Another population of proliferating cells moves downward from the stem cell niche to bottom of crypt –> paneth cells (live for ~ 20 days, then phagocytosed and replaced)
Small Intestine Functions
- Complete digestion of CHO, PRO, FAT
- Absorption of Nutrients
- duodenum and jejunum: sugars, lipids, AAs, Ca, and Fe
- Ileum: bile salts, vit B12, water, and electrolytes
- ->very rapid b/c of villi and microvilli
Villi
a. Capillaries absorb monosaccharides and amino acids; lacteals
absorb fats.
b. Intestinal crypts (Crypts of Lieberkuhn) with Paneth cells (secrete antibacterial molecules of lysozyme and defensin) and mitotic stem cells (divide by mitosis to replenish intestinal cells every 4 to 5 days)
- -> constantly renewing everything
Microvilli
“brush border”
folding of the apical surface of each epithelial cell
site of “brush border enzymes” - stay attached to plasma membrane w/active site exposed to chyme
-hydrolyze disaccharides, peptides, and other substrate to simple nutrient molecules
Intestinal Contractions/Motility: Role
1) moves chyme aborally from mouth –> anus
2) mixes chyme with digestive secretions
3) breaks chyme into small particles, ↑ SA
- -> smooth muscle contractions occur automatically due to endogenous pacemaker activity (within ENS)
Migrating Myoelectrical Motor Complex
MMMC: occurs in between meals (postabsorptive state)
slow wave of peristaltic activity occurring in the postabsorptive state (ex. overnight fast)
sweets gut clear of residue
ceases upon onset of eating
starts in stomach –> goes to anus
Peristalsis
net aboral (forward) movement by sequential muscular contraction and relaxation behind and ahead of bolus
Two Types:
- Primary: initiated in esophagus by swallowing
- Secondary: initiated by distension (local ENS reflexes)
Segmentaion
mixing by simultaneous contractions both behind and ahead of bolus
–> “mix” not “move”
Large Intestine Mucosa
columnar epithelial cells w/goblet cells, crypts, lymphatic nodules, but NO villi
Large Intestine Functions
- absorbs water, electrolytes, vit K, and some B vits
- production of Vit K and B vitamins via microbial organims
- habitat for microflora
- storage/processing of feces
Digestion/Absorption: overall strategy
From food, humans must get basic organic molecules to make ATP, build tissues, and serve as cofactors and coenzymes
- digestion breaks polymers (CHO/PRO/FAT) into monomer building blocks via hydrolysis reactions
- absorption takes monomers into bloodstream to be used by the cells
–> major nutrients are enzymatically split into their component molecules by hydrolysis (cleavage of bonds by H2O)
Digestion/Absorption of CHO
Most CHO ingested as starch/sugars (ex. sucrose, lactose)
- Starch digestion begins in mouth (salivary amylase: polysaccharides –> shorter chain “dextrins”)
- NO digestion in stomach, too acidic
- continues in intestines w/pancreateic amylase: short chains –> disaccharides and maltriose
- brush border enzymes finish breaking down disaccharides (maltose, sucrose, lactose) to simple sugars (mainly glucose)
Glucose absorbed via secondary active transport
- secondary active transport with Na (Cl follows)
- facilitated diffusion through GLUT carries into interstitial fluid and then to capillary blood of the villus
- Water follows NaCl through paracellular route and is absorbed w/glucose and NaCl
Digestion/Absorption of PRO
- begins in stomach w/pepsin and HCl to produce short-chain polypeptides
- finishes in duodenum and jejunum w/pancreatic trypsin, chymotrypsin, elastase, and carboxypeptidase, and the brush border enzyme aminopeptidase
- final products: AAs, some dipeptides and tripeptides
- free AA cotransported w/Na
- Dipeptides and tripeptides cross via secondary active transport using a H gradient
- -> hydrolyzed into free AA within the cytoplasm of the epithelial cells
-free AAs move by facilitated diffusion into interstitial fluid, then to the blood capillaries of the villi
