Lecture 8: Small Intestine (digestion and absorption of carbohydrates, protein, fat), Large Intestine

enzymes from pancreatic acini won’t work when

in an acidic environment

carbohydrates digestion and absorption

1. salivary amylase secreted in mouth

2. salivary amylase de-activated by stomach acid

3. pancreatic amylase secreted in small intestine (breaks down glucose → maltose + limit dextrins)

4. digestion by brush border enzymes

   1. dextrinase, glucoamylase: break down limit dextrins into disaccharides (maltose, sucrose, lactose)

   2. maltase: breaks down maltose into 2 glucose

   3. sucrase: breaks down sucrose to glucose + fructose

   4. lactase: breaks down lactose to glucose + galactose

5. absorption transporters

   1. glucose and galactose cross apical membrane by secondary active transport (Na+ coupled transport via SGLT-1)

   2. fructose crosses apical membrane by facilitated diffusion (GLUT 5)

   3. glucose, galactose, and fructose all cross basolateral membrane via facilitated diffusion transporters (GLUT 2)

   4. Na/K ATPase on basolateral membrane that generates Na+ gradient

      

activation of brush border (BB) and pancreatic enzymes (PE) for protein digestion in intestine

1. enterokinase activates trypsinogen → trypsin

2. trypsin activates chymotrypsinogen→ chymotrypsin

3. trypsin activates procarboxypeptidase → carboxypeptidase

4. trypsin and chymotrypsin break down proteins into peptides

5. carboxypeptidase break down amino acids off from the carboxyl end

6. aminopeptidases break down amino acids off from the amino end

protein digestion and absorption transporters

• apical membrane

  • H+/peptide cotransporter

  • Na+/H+ exchanger

  • Na+/AA cotransporter (at least 7 AA transporters w/ different specificities)

• basolateral membrane

  • Na/K ATPase

  • Amino acid transporter

substances from small and large intestine that move into circulatory route

amino acids, carbohydrates, NO FAT SOLUBLE SUBSTANCES

bile secretion and bile salt recycling mechanism

1. CCK → gallbladder contraction and relaxation of Sphincter of Oddi

2. bile salts move down bile duct and travel from duodenum to ileum

3. bile salts recycled back into liver via hepatic portal vein

4. bile secretion is increased by increased hepatic bile salt concentration

digestion and absorption of fat steps

1. emulsification: breaking up large fat globules into smaller ones w/ the help of bile salt and phospholipid

2. digestion by lipase (+ colipase) from the pancreas → triglycerides broken down to free fatty acids (FFA) and monoglycerides

3. formation of micelles from FFA, monoglyceride, bile salts

   1. equilibrium between free product and those in micelles

4. absorption: FFA & monoglycerides diffuse into epithelial cell

5. fatty acids and monoglycerides are resynthesized into triglycerides and packaged into chylomicrons in the ER and Golgi

6. exocytosis of chylomicrons

7. uptake of chylomicrone by lacteal (lymphatic capillary) which takes chylomicrons to the general circulation

how is water absorbed in the small intestine

• 80% of water is absorbed due to active absorption of solutes (particularly Na+)

how are vitamins absorbed in the small intestine

• fat soluble (A, D, E, K) follow pathway for fat absorption

• water-soluble: B12 requires intrinsic factors that are secreted from the stomach

ileocecal sphincter

between ileum and cecum

• contractions of the ileum open the sphincter

• contractions of the large intestine close the sphincter

diarrhea can be caused by

bacterial/viral toxins or entry of atypical amounts on non-absorbable products into large intestine → stimulate secretion and motility → diarrhea

why metabolic acidosis with diarrhea?

high flow rate and stimulation of secretion leads to significant loss of HCO3- in the feces and gain of H+ in the blood

• also lose a lot of K+ → hypokalemia can develp

• also lose a lot of H2o → dehydration can develop

mass movement

wave of intense contraction in large intestine (also called gastrocolic reflex). initiated by stimuli in the upper GI tract and moves content toward rectum for defecation

reflexes in the colon steps

1. the presence of food in the stomach and chyme in the duodenum stimulate mass movement in the colon

2. mass movements are integrated by the enteric plexus

3. they propel the contents toward the rectum

4. presence of feces in the rectum stimulates parasympathetic and local reflexes that result in defecation