GI: Absorption/Excretion: Carbohydrates, Proteins, Lipids in Sm/Lg Int. -Mahmoud

Define absorption

movement of nutrients, electrolytes, and H2O out of the lumen of GI across the epithelial cell and into the blood or lymph

What prevents the passage of molecules except H2O between the enterocytes

tight junctions

What facilitates the movement of nutrients across the apical and basolateral membranes of enterocytes

Specific nutrient transporters

Simple passive diffusion

Occurs directly across membranes without the assistance of transporters and ATP.

What does simple diffusion depend on

Electrochemical gradients Lumen-->enterocyte-->blood

What is facilitated diffusion

Diffusion down electrochemical gradients through nutrient specific transporters without the use of ATP

What are two key examples of nutrients that use facilitated diffusion

Fructose (Glut5) and Amino Acids

What is primary active transport

Use of specific transporters that utilize ATP to move nutrients against their electrochemical gradient

What is a key example of primary active transport

Na+/K+ ATPase, Moves Na+ into the lumen against its gradient. Uses to drive other transport processes

What is secondary active transport

indirectly uses the energy used by Na+/K+ ATPase to transport nutrients into the lumen down the Na+ gradient

What are some examples of nutrients that utilize secondary active transport

Glucose/Galactose and some amino acids

What is special about the proximity of the villus and its blood supply in relation to absorption

It allows for complete and rapid absorption of nutrients

What is the pathway for blood in an enterocyte

A central artery supplies each villi with villus capillaries beneath the enterocyte membrane that lead into a peripheral villus vein

How does the blood flow in a villus, and where do the nutrients enter

The blood ascends centrally and descends peripherally and absorbed nutrients enter the descending network.

What do the villus capillaries drain into

The portal vein and eventually into the systemic circulation

What is the central lymphatic vessel and where is it located

Central lacteal in each villus

Where are fats absorbed and why

absorbed into the central lacteal-->lymphatics-->systemic. because they are too large to enter the villus capillaries

Glucose and galactose absorption; transporter, mechanism and what supplies the energy for transport

Both absorbed through SGLT1 via secondary active transport. Sugar/Na+ transport using the gradient set up by the Na+/K+ ATPase

What is an important nutritional concept about glucose/galactose and their transporter. whats the outcome?

They compete for the same transporter SGLT1, and excess of one inhibits the other. Usually glucose is predominantly absorbed.

Fructose absorption. Transporter, meachanism and location

Absorbed through GLUT5 via facilitated diffusion on the apical membrane, NOT ATP dependent

How do Glucose, Galactose and Fructose exit the enterocyte and enter the blood

Leave from the basolateral membrane through GLUT2 transporter via facilitated diffusion

Why cant monosaccharides be absorbed through passive diffusion

They are too large to enter through membrane pores or cross tight junctions

What part of the intestine are protein products absorbed in

Jejunum and Ileum

What comprised most of the protein products that are abosrbed by the small intestine

Free amino acids, di and tri peptides

What mechanism do amino acids use to enter the blood across enterocytes hydrophillic vs hydrophobic (2)

1)hydrophilic AAs: secondary active transport and Na+ dependent transporter
2)hydrophobic AAs: passive or facilitated diffusion

What is important about free AA/di/tri peptide absorbtion

they are absorbed through distinct independent transport mechanisms

Wha is true about the absorption of Free AA/di/tri-peptide mixtures

They are absorbed faster than either form alone

how does the amount of fat absorbed vs the amount excreted compare

150-200g fat absorbed vs 3-5 gm excreted in stool from unabsorbed fat and colonic bacteria

Glycerol absorption mechanism and related structure

Glycerol diffuses through the membrane because it is small and water soluble

Free Fatty acid and Monoglyceride absorption mechanism

From micelles that cross the brush border membrane though passive diffusion due to lipid solubility

What happens to the FFA and MG after they enter the cytplasm of the enterocyte

Resynthesis occurs forming triglyceride and lecithin

What are the two pathways through which resynthesis of FFAs and MG occurs in the enterocyte cytoplasm

Monoglyceride acetylation and phosphatidic acid pathways

Final product that is formed in the cytoplasm of enterocytes after resynthesis of FFAs and MG

Chylomicrons, after TGs are combined with proteins, phospholipids and cholesterol

after packaging in the golgi where do chylomicrons exit the enterocyte and finally end up

Through the basal membrane and enter the central lacteal because they are too large to enter villus capillaries

What is are the two ends (sphincters) of the large intestine (colon)

proximal end: ileocecal sphincter
distal end: external anal sphincter

Orad to Caudad, what is the anatomy of the large intestine (3)

1)Cecum; Ascending colon, transverse colon, descending colon, and Sigmoid Colon.
2)Rectum
3)Anal Canal

What separates the rectum from the anal canal

Cholorectal sphincter

What does the large intestine depend on for motility of its contrents

Actions of the thick smooth muscle layers

Tenai Coli

outer longitudinal layer organized into three distinct flat bands

Internal anal sphincter formation

The continuation of the inner circular muscle from the large intestine that thickens

external anal sphincter is formed from

striated muscle bundles

haustrations appearance and formation

give colon segmented appearance; formed by the structural smooth muscle thickening and functional colonic contractions

How is the external anal sphincter controlled

Through voluntary actions

What is the structure and function of the ileocecal sphincter, why is it important

acts as a one way valve; prevents fecal reflux from cecum to ileum, so bacteria stays in the colon

What is the primary control function of the ileocecal sphincter. what is its tone

ileal emptying rate into the colon; remains mildly constricted to slow ileal emptying because its the point of no return

What maintains the basal contrictive tone of the ileocecal sphincter

intrinsic myogenic control

How does ileal distension modulate ileocecal sphincter tone, how does this affect ileal emptying rate

Decreases sphincter tone to allow faster emptying of the ileal content, prevents buildup

How does colon distension modulate sphincter tone, how does this affect ileal emptying rate

Increases sphincter tone to slow down emptying of the ileal content, prevents excessive filling of the colon

How much H2O is absorbed by the large intestine

about 600ml of the total 7-10L that enters GI tract.

What is the absorptive reserve capacity of the large intestine and why is it significant

Can absorb 5-7 L of H2O per day as a reserve capacity, when intestinal absorption is impaired

Why is mixing important in the large intestine

To absorb whatever remains to be absorbed by the mucosal membrane

Where does propulsion happen in the large intestine

From the ileum -->rectum-->anal canal

Haustrations provide what type of motility in the large intestine

Segrmenting contractions: act to mix the contents with little propulsion

How are segmenting contractions of the large intestine different than those of the small intestine

They are stronger and last 2-4x longer (16-20 seconds)

What are mass movements of the large intestine

Peristaltic movements to propel contents towards the rectum

How are peristaltic movements of the large intestine different than those of the small intestine

The are less frequent (1-3x/day) and longer lasting 10-30 mins) and propel content a greater distance (20cm)

Gastrocolic reflex (3)

1)Stomach Distension (e.g. After a Meal)
2)Colon Mass Movement
3)Coordinated w/ Gastroileal Reflex

Dudenalcolic reflex (4)

1)Duodenal Distension (e.g. After Meal)
2)Colon Mass Movement
3)Augments Gastrocolic & Gastroileal
4)Defecation Sensation After Eating

What is the water vs solid composition of feces

Water 75% total weight
Solids 25% total weight

Why does feces have a low composition variability despite significant variability in diet composition

>50% of fecal components is of non-dietary origin

What is special about fecal production during starvation

Continues to be produced from waste of non dietary origin