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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



1. Motility
2. Secretion
3. Digestion
4. Absorption
5. Storage/Elimination
6. Immune Barrier


Functions: Motility

Movement of food through the tract

1. ingestion: taking food into mouth
2. mastication: chewing, mixing food w/saliva
3. deglutination: swallowing
4. peristalsis: wave-like, one-way movement through tract
5. segmentation: churning/mixing while moving forward


Functions: Secretion

1. Exocrine: digestive enzymes, HCl, mucus, water, and bicarbonate
-->open tube from one end to other --> to GI tube

2. 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

small intestine
large intestine



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


What are the GI Tract Layers?

1. Mucosa
2. Lamina Propria
3. Muscularis Mucosa
4. Submucosa
5. Muscularis Propria
6. Subserosa
7. Serosa



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



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



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


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



-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

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

1. Cardiac Region
2. Fundus
3. Body
4. Gastric Rugae
5. Pylorus


Gastric Pits and Gastric Glands

at base of folds lead to gastric glands that contain several types of secretory cells:
1. Mucous Cell
2. Parietal Cell
3. 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

endocrine cells ("g-cells")
somatostatin ("d-cells")





Stimulation of HCl Secretion


• 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

1. duodenum: 1st 10 in
2. jejunum: middle 2/5
3. 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

1. Complete digestion of CHO, PRO, FAT

2. 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



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



"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



net aboral (forward) movement by sequential muscular contraction and relaxation behind and ahead of bolus

Two Types:
1. Primary: initiated in esophagus by swallowing
2. Secondary: initiated by distension (local ENS reflexes)



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