GIT Lec 6: Small Intestine Flashcards
small intestine is between
stomach + large intestine
size of small intestine
- tube of 2.4 cm diameter
- 3m length (doubles during autopsy–> loss of tone)
small intestine sections
duodenum- 30 cm - close to stomach
jejunum - 1 m
ileum - 1.7 m - close to large intestine
small intestine functions
-digestion + absorption of protein, fat, carbohydrates, electrolytes, H2O, minerals, vitamins
functions of duodenum
- mixing pancreatic digestive enzymes + bile with food
- absorption of nutrients, iron, calcium
- release of endocrine hormones–> secretin, CCK
functions of jejunum
- digestion + absorption
- most chyme digested + absorbed in first 25% of small intestine
functions of ileum
-digestion + absorption (bile acids, vitamin B12)
large SA is important because
absorption, exposure to enzymes needs to be increased
folds of small intestine increases
SA (epithelial layer)
folds of small intestine (largest to smallest)
folds of kerckring –> villi + crypts —> microvilli
type of cells in villus
- stem cells in crypts
- paneth cells in crypts
- enterocyte (absorptive) cells -villus stalk
- goblet cells -villus stalk
- enteroendocrine cells -villus stalk
paneth cells
secrete antibacterial proteins
enterocyte (absorptive) cells
absorption , brush-border enzymes
goblet cells
secretion of mucus
enteroendocrine cells
release of hormones
brush border
-small projections (microvilli) at epithelial cells –> major absorptive surface
brush border enzyme
- enzyme anchored to brush border, catalytic activity in lumen
- breakdown of carbohydrates, pep–> sugars, aa before transport into enterocyte
starch (amylose +amylopectin) products by salivary + pancreatic amylase
maltose, maltotriose, a-limit dextrins
enzymes that turn maltose, maltotriose to glucose
maltase
sucrase
a-dextrinase
enzymes that turn a-limit dextrins to glucose
a-dextrinase
sucrose enzyme + products
- sucrase
- glucose + fructose
lactose enzyme + products
- lactase
- glucose + galactose
SGLT
sodium -glucose linked transporter
GLUT2/5
glucose transporter 2/5
intestinal absorption of glucose/galactose
apical: glucose/galactose and 2Na+ from lumen into cytosol through SGLT
basolateral: glucose/galactose from cytosol into blood through GLUT2
Na+/K+ ATPase– 3 Na+ out/ 2 K+ in
intestinal absorption of fructose
apical: fructose from lumen into cytosol through GLUT5
basolateral: fructose from cytosol into blood through GLUT2
lactose intolerance
- all mammals lose some lactase expression
- can’t completely digest lactose -
- decreased water absorption
- more water in gut (osmotic gradient into lumen)
- bacteria in large intestine digest lactose
consequence: pain, diarrhea
major pancreatic proteases
trypsin + chymotrypsin
further protein digestion (breakdown)
- carboxypeptidase (pancreatic protease)
- aminopeptidase (brush border)
protein absorption (apical/basolateral)
apical: free aa coupled with Na+ absorbed (2 active transport)
small peptides absorbed by 2 active transport coupled with H+ then peptidases in cytosol: peptides to a.a
basolateral: Na+/K+ ATPase, a.a facilitated diffusion across basolateral surface
fat absorption
- fatty acids+ monoglycerides processed by ER to make triglycerides= maintains diffusion gradient
- aggregate into lipid droplets coated by amphipathic proteins
chylomicrons
- extracellular fat droplets
- triglycerides, phospholipids, fat soluble vitamins, cholesterol
- larger ones enter lymphatic system through lacteals (leakier)- enter systematic circulation through thoraic duct
lipoprotein lipase on endothelial cells (blood vessels)
break down of chylomicrons for uptake
absorption of iron for the intestine
- Fe2+ actively transported into enterocyte and stored in ferritin protein
- not stored: transported into blood + stored in transferrin (plasma protein)
- increase iron in body stores –> increase ferritin expression –> decrease absorption of iron
- decrease iron i body stores –> decrease ferritin expression –> increase absorption of iron
no iron excretion
- iron is toxic
- increase skin pigmentation/heart failure
iron deficiency anemia
-decrease number/size in RBC
symptoms-tiredness, light-headness, headaches
cause: decrease iron /blood loss, poor iron abs. , intestinal disease
- Water amount absorbed by small intestine vs feces?
- why water in intestine?
- 8-9 L –> majority absorbed by small intestine – 100 ml in feces
- water in intestine – permits contact food + enzymes, diffusion of digested nutrients to absorption site, prevents damage to epithelial layer
water absorption
determined by Na+ gradients out of cell, apical side
-at villi
water secretion
depends on Cl- gradients by 2 active Na+/K+/2Cl- transporter (NKCC1)- basolateral side into cell
Cl- (+cAMP) out of cell - apical side
increasing cAMP, increasing Cl-
Cholera
- ingesting food+water vibrio cholerae bacteria – produces toxin that increase cAMP, Cl- channel secretion of Cl- into small intestine, increase water in lumen
- 20 L of stool causes: vomiting + excessive diarrhea
treatment: clean water +electrolytes, intravenous fluids
motility (small intestine)- during digestion
- “segmentation” - mechanical breakdown of food, digestive enzymes mixes
contractions per min (duodenum)
12
contractions per min (ileum)
9
motility (small intestine)- after digestion
- segmentation stops, MMC (migrating myoelectric complex) starts
- starts lower stomach then 2 feet in small intestine (overlapping waves starts further down - 2hrs then repeat)
purpose of MMC
- pushes any undigested material from small to large intestine
- removal of bacteria in small intestine
Regulation of MMC
- motilin hormone secretion by cells of small intestine – initiate MMC
- feeding inhibits motilin
