Gastrointestinal System

function of digestive system

convert ingested foods into molecular forms, transferred with salts and water into internal environment

two parts of GI system

1. GI tract/alimentary canal

2. accessory organs

parts of alimentary canal

mouth, pharynx, esophagus, stomach, small intestine, large intestine

accessory organs

salivary glands, liver, gallbladder, exocrine pancreas

lumen definition

inner open space/cavity of tubular organ, continuous with external environment

microbiome

GI tract colonized by billions of bacteria, 57% cell count

what helps with one-way traffic

sphincters: smooth muscle rings that surround tube

4 functions of digestive system

digestion, absorption, secretion, motility

digestion definition

dissolving and breaking food into small molecules

absorption definition

digestion products, water, vitamins, minerals from lumen into epithelial cells into circulation

secretion definition

bile from liver and digestive enzymes from exocrine glands to GI

motility definition

smooth muscle contractions to mix and move luminal contents from mouth to anus

two minor GI functions

elimination and immune function

feces made of

bacteria and non-digested/absorbed material

where are antibodies

lymphatic nodules with immune cells in small intestine

4 layers of GI tract wall

mucosa, submucosa, musclularis externa, serosa

enteric nervous system

neural GI regulation, 500 million neurons

innervated with sympathetic and parasympathetic NS

mucosa includes

epithelium, lamina propria, muscularis mucosa

epithelium in mucosa

directly faces lumen and nutrients

lamina propria function

layer of connective tissue innervated with small nerve fibers and blood + lymphatic vessels

submucosa contains

connective tissue and submucosal plexus, major blood and lymph vessels

submucosal plexus function

neuron network that projects to luminal surface and myenteric plexus

muscularis externa contains

smooth muscles that contract to provide mixing and moving forces

circular and longitudal muscle, myenteric plexus

thick circular muscle function

narrow lumen

thin longitudinal muscle function

shorten tube

myenteric plexus function

second neuronal network, separate muscle layers

innervated by ANS, project to submucosal plexus

serosa function

thin layer of connective tissue connected to abdominal cavity to support GI

luminal epithelial cell layer invagination

exocrine glands: secrete acid, water, ions, mucus

endocrine cells: secrete hormones into blood

3 types of small intestine folding

circular folds, villi, microvilli

circular folds

mucosa and submucosa folding into lumen

villi

fingerlike protrusions for absorption and increase surface area

villi contain

goblet cells (mucus secretion), capillaries, lacteals, enteroendocrine cells (hormones), epithelia cells with microvilli, Paneth cell (antimicrobial)

lacteal

blind-ended lymphatic vessel in center of intestinal villus (lacteal), empties through lymphatic system into thoracic duct

how do nutrients diffuse through capillaries

hepatic portal vein to liver

microvilli

epithelia cell protrusions for cell surface folding

what is the function of highly convoluted luminal side

increase absorption surface area

after stomach, how many epithelial layers

single

how is barrier formed

tight junctions between epithelia

mouth 3 functions

chewing, salivary glands produce saliva, swallowing reflex

saliva 4 functions

1. dissolves food molecules for taste

2. moistens and lubricates food

3. amylase to digest polysaccharides

4. antimicrobial

esophagus 2 functions

food from pharynx to stomach in peristaltic waves

mucus to lubricate

stomach

saclike organ between esophagus and small intestine

stomach 3 functions

1. stores, dissolves, partially digests macromolecules

2. regulates food emptying into small intestine

3. HCl to kill bacteria, dissolve food, pepsinogen into pepsin, pepsinogen

pepsin function

breaks down protein

3 layers of stomach

longitudinal, circular, oblique

chyme made of

protein, carb, fat, water, salt fragments

small intestine dimensions

1.5 inch diameter, 9 feet length

duodenum, jejunum, ileum

3 functions of small intestine

1. digestion: enzymes break partially digested

2. mixing: muscularis mucosa move villi to increase chyme contact and move to large intestine

3. absorption: digestion products, water, salt, vitamins (most in D and J)

duodenum located

at intersection of GI tract and secretory organs (common bile duct)

three duodenum function

juice secretion, digestion, nutrient absorption

3 large intestine actions

1. temporarily stores undigested material

2. concentrates: absorbs salt and water

3. rectum contracts to expel feces

4 large intestine functions

fermenting, drying, packing, egesting

4 parts of colon

ascending, transverse, descending, sigmoid

pancreas 2 functions

1, secretes digestive enzymes

2. secretes bicarbonate-rich fluid to neutralize stomach acid

liver 2 functions

1. synthesizes and secretes bile salts for fat digestion

2. secretes bicarbonate

gallbladder 2 functions

1. stores and concentrates bile from liver in between meals

2. contracts and releases bile into small intestine

villi increase surface area by how much

600 x

epithelium replacement

every 5 days, base of villi up

single-unit muscle characteristics

contracts as single unit, myogenic, activated by stretch, no motor input needed, gap junctions

in hollow organs

multiunit smooth muscle characteristics

independent contraction, neurogenic, not activated by stretch, no gap junction

4 parts of enteric nervous system

program library, feedback control, reflexes, information processing

cajal cell function

pacemaker: stimulation to reach threshold, Ca driven

input from myenteric plexus

basal electrical rhythm

chemical digestion

macromolecule breakdown with water and digestive enzymes

macromolecule and bond

protein: peptide

carb: glycosidic

fat: ester

trypsinogen converted to trypsin by

enterokinase

chymotrypsinogen, procarboxypeptidase, proelastase converted to chymotrypsin, carboxypeptidase, elastase by

trypsin

what is a masking sequence

additional AAs that cover active site

three digestive tools

crushing in mouth and stomach, HCl in stomach, enzymes in mouth, stomach, small intestine

enzymes + what they break down

amylase: polysaccharides

pepsin, trypsin, chymotrypsin, carboxypeptidase: protein

lipase: fat

daily carb intake

250-300 g/day, 2/3 starch, rest sucrose and lactose

protein daily intake vs needed

60-90 vs 40=50

fat daily intake

70-100 g, mostly TAGs

fluid ingested

1200 mL/day

fluid secreted

7000 mL/day

fluid in feces

100 mL/day

what fiber is undigestable

cellulose: beta linkages

disaccharides

sucrose: glucose + fructose

lactose: glucose + galactose

maltose: glucose (glycogen digestion)

polysaccharides

cellulose, amylose, amylopectin (plant), glycogen (animal)

brush-border enzymes

catalyze oligo/disaccharide breakdown

sucrase, maltase, isomaltase, glucoamylase, lactase

how does fructose enter epithelial cells

GLUT facilitated diffusion

how do glucose and galactose enter epithelial cells

secondary active transport coupled to Na: SGLUT

stomach enzymes

proteinases, HCl, pepsinogen

pancreas enzymes

trypsin, elastase, chymotrypsin (zymogens)

intestine enzymes

carboxy+aminopeptidases, dipeptidase

proteins converted to peptides by

pepsin, trypsin, chymotrypsin

peptides converted to free AAs by

carboxypeptidase and aminopeptidase

free amino acids moved into epithelial cells by

specific AA transporters coupled with Na transport

di and tripeptides moved into epithelial cells by

secondary active transport with H+ gradient

free AAs exit epithelial cells basolaterally by

facilitated diffusion with Na gradient maintenance

infant protein absorption > adult because

they need antibodies from mother’s milk

major fat digestive enzyme

pancreatic lipase

why is fat digestion only at surface

insoluble

emulsification

large fat droplets become smaller

emulsifying agent

prevents re-aggregation

phospholipids + bile salts (amphipathic)

fats in solution

micelles

what happens to fatty acids in smooth ER epithelial cells

resynthesis into TAGs

aggregate and coated with amphipathic molecule

vesicles exocytose into ISF as chylomicrons

chylomicrons circulate in

lymph: too big for blood basement membrnae

fat soluble vitamin absorption

ADEK: same as fat

water-soluble vitamin absorption

(B + C): diffusion and mediated transport

B12 absorption

binds to parietal cell intrinsic factor

endocytosis into ileum