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What are the 2 types of intestinal motility of the small intestine?

- segmentation and the migrating motility complex



- segments alternate between contraction and relaxation to mix the chyme and slowly push it along the tract
- these contractions are NOT peristaltic


How can the contractions involved in segmentation propel the chyme forward?

- because the rate of segmentation is faster in the beginning of the tract and slows down as we progress down it


What is segmentation regulated by? Which segment of the small intestine does each regulating agent primarily act on?

- distension, gastrin, and extrinsic nerve activity regulate segmentation
- distension primarily affects the duodenum, while gastrin primarily affects the ileum (the gastroileal reflex)


What is the Gastroileal Reflex?

- it refers to the increase in segmentation in the ileum in response the chyme in the stomach (the chyme in the stomach results in an increase in gastrin, which increases segmentation in the ileum)


What is the role of the Migrating Motility Complex? What is it regulated by?

- the MMC sweeps the small intestine clean b/w meals (the "intestinal housekeeper")
- it is up-regulated by the hormone motilin, which is secreted in the unfed state and is inhibited by feeding


What is the role of the ileocecal sphincter? When does it contract? When does it relax?

- the ileocecal sphincter prevents colonic bacteria from entering the small intestine
- therefore; it contracts w/ increasing colonic pressure and relaxes w/ increasing ileum pressure (to allow the chyme to pass into the colon)


What does the small intestine secrete? How does this assist with digestion?

- succus entericus: an aqueous salt and mucus solution that is secreted in the presence of chyme
- it serves a protective/lubricative function
- the small intestine does NOT secrete digestive enzymes, but the H2O of the succus entericus provides water for hydrolysis (for the pancreatic enzymes)


Pancreatic digestive enzymes complete the digestion of which nutrients? What completes the digestion of the remaining nutrients?

- pancreatic lipase completes the digestion of fat
- pancreatic (and salivary) amylase incompletely digest carbohydrates; disaccharidases of the brush border are needed to finish the job
- pancreatic and gastric proteases incompletely digest proteins; aminopeptidases of the brush border are needed to finish the job


What are the absorbable units of fats, proteins, and carbohydrates?

- fat: monoglycerides and free fatty acids (in a micelle)
- protein: amino acids (and some small peptide fragments)
- carbohydrates: monosaccharides


Which proteins do the microvili of the brush border contain?

- enterokinase: activates pancreatic trypsinogen
- disaccharidases: hydrolyze disaccharides into monosaccharides
- aminopeptidases: hydrolyze peptide fragments into amino acids


The absorption of which nutrients are regulated to meet the body's needs?

- iron and calcium
- (all other nutrients are essentially absorbed indiscriminately)


The surface area of the small intestine is 600x greater than it would be without which 3 features?

- circular folds of the surface increase surface area by 3x
- villi protruding from these surfaces increase it by another 10x
- microvilli (the brush border) of each epithelial cell making up the villi increase it by another 20x


You can lose up to 50% of your small intestine before any loss of absorptive function occurs due to the massive reserve capacity of this organ - where is the exception? Why?

- the terminal ileum because this is the only place where vitamin B12 and bile salts are absorbed


What are the 4 major components of each villus?

- 1) the epithelial cells covering the surface w/ a brush border
- 2) a connective tissue core (the lamina propria)
- 3) a capillary network
- 4) a terminal lymphatic vessel at the center (the central lacteal)


What are the spaces between villi called? What do they contain?

- the crypts of Lieberkuhn
- they contain stem cells with very large mitotic activity (new cells "crawl" up the villus and replace the tips every 3 days) and protective paneth cells (these secrete lysozyme and defensins, which are anti-bacterial)


Na+ is actively transported INTO/OUT OF the cell at the LUMINAL/BASOLATERAL membrane.

- sodium is actively transported out of the cell at the basolateral membrane
- it enters the luminal membrane into the cell passively


Name the 3 disaccharides and the monosaccharides each is broken down into.

- lactose --> glucose + galactose
- maltose --> glucose + glucose
- sucrose --> glucose + fructose


Explain how monosaccharides are absorbed.

- both glucose and galactose enter the cell via secondary active transport through SGLT (sodium-glucose-co-transporter)
- fructose enters the cell via passive diffusion through GLUT-5 (glucose transporter-5)
- all three monosaccharides leave the cell via passive diffusion through GLUT-2


Which nutrients are absorbed into the blood? Which enter the lymph?

- proteins and carbohydrates are absorbed into the blood
- fats are absorbed into the lymph


Explain how proteins are absorbed.

- amino acids enter the cell via secondary active transport through Na+-AA symport
- some small peptides enter the cell via tertiary active transport through H+-peptide symport (and are then hydrolyzed into amino acids)
- amino acids passively diffuse out of the cell


Explain tertiary active transport in relation to small peptide fragments.

- small peptide fragments enter the cell passively with H+
- H+ passively enters the cell because it is less concentrated in the cell due to passive Na+-H+ antiporters, which bring Na+ in and H+ out
- Na+ is passively brought in because it is actively pumped out by Na+-K+-ATPase pumps
- thus, the peptide fragments rely on the active transport of Na+ out of the cell in a tertiary manner


Explain how fats are absorbed.

- the monoglycerides and free fatty acids in the micelle passively diffuse out of the micelle and into the cell once it reaches the membrane
- inside the cell, these components are re-synthesized into triglycerides which aggregate and get coated with lipoproteins, forming chylomicrons which exocytose out of the cell and into the central lacteal (lymph)


What's the carrier protein for vitamin B12? How is the carrier-vitamin complex absorbed?

- intrinsic factor
- (in the stomach, before intrinsic factor binds to vit B12, haptocorrin is the carrier protein)
- vitamin B12 + intrinsic factor is absorbed via specific cubulin receptors/channels


Which two electrolytes have their absorption regulated to meet the body's needs?

- iron and calcium


How much iron is ingested in a normal diet? Of this amount, how much actually absorbed into the blood?

- normal diet: 15 - 20 mg/day
- males absorb 0.5 - 1.0 mg/day
- females absorb 1.0 - 1.5 mg/day


Explain how the two types of iron enter the epithelial cell.

- dietary heme iron (from meat) is readily taken up by the cell via heme carrier protein 1
- dietary non-heme/inorganic iron (from plants) must first be reduced from Fe3+ to Fe2+ before being taken up by the cell via divalent metal transporter 1 (DMV1)


What happens to iron once it's inside the epithelial cell.

- iron that is immediately needed for Hb production is absorbed into the blood; it leaves the cell via ferroportin and is carried in the blood by transferrin
- iron that is not immediately needed is irreversibly stored in the cell in its granular form called ferritin


How is iron absorption regulated?

- the epithelial cells absorb the majority of iron we ingest; however, only small amounts are actually fully absorbed into the blood, so regulation occurs at the ferroportin channels at the basolateral membrane
- when blood transferrin levels are too high, the liver releases the hormone hepcidin, which binds to and degrades ferroportin


Why does vitamin C increase iron absorption? Which two compounds decrease iron absorption?

- vitamin C reduces Fe3+ to Fe2+ so that the brush border doesn't have to; less time spent reducing = more time spend absorbing
- phosphate and oxalate decrease iron absorption by forming insoluble iron salts with it