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57 Cards in this Set

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Where does the vast majority of absorption occur?
small intestine.
What is the main purpose of the colon?
Removal of electrolyte and water (also some short chain FA absorption)
How much do circular folds increase surface area in small intestine?
3x
How much do villi increase surface area in small intestine?
10x
How much do microvilli increase surface area in small intestine?
20x (major specialization)
What do lacteals do?
They're an extensive lymph network that removes lipids rapidly to maintain gradient for lipid absorption
What is the significance of the <b>unstirred layer?</b>
It's adjacent to the brush border and consists of mucous, IgA, and brush border enzymes. It occurs because stuff in the center of moving fluid goes fastest. It tends to impede absorption, and can be very thick in malabsorption syndromes.
Difference between <b>paracellular and transcellular route</b> of absorption.
Paracellular: Between enterocytes (gut is "leakier" in some regions than others). Always passive.
Transcellular: across the enterocyte.
Is the transcellular route active or passive?
Both. Occurs by multiple mechanisms incuding channels, carrier molecules, exchangers, pumps in both sides of membrane.
Which part of the intestine is the "leakiest?"
The duodenum. The tight junctions get tighter as you move distally.
What type of membrane protein is used to maintain electroneutrality and is used for ions?
Exchangers. (eg., Na+/K+ exchanger)
At what level(s) does <b>carbohydrate</b> absorption occur?
1) Mouth (salivary amylase aka ptyalin)
2) Stomach
3) Small intestine (brush border, pancreatic amylase)
How is salivary amylase activity protected in the stomach?
Since its optimal pH is 7-10, it is protected from digestion in the stomach by buffers or in the center of a bolus wherever pH remains neutral.
What is responsible for final carbohydrate breakdown to monosaccharides?
Amylase breaks into 2 molecules; brush border enzymes responsible for final digestion to monosaccharides.
What is SGLT?
Sodium-Glucose Linked Transport Protein.

In the membrane of epithelial cells. A Na+-linked glucose carrier.

Once Na+ binds, glucose binds and both are transported into the cell.

Na+ then actively gets pumped out via Na+/K+/ATPase.

So SGLT is 2ndary active.

It's how glucose gets INTO the cell.
What is GLUT2?
It's how glucose gets OUT OF cell at basal membrane. Facilitated diffusion protein channel.
What are the steps of glucose absorption ?
1) SGLT1 binds Na+.
2) There is a conformation change.
3) SGLT1 binds glucose.
4) Glucose and Na+ move into cell to apical layer.
5) Glucose moves down its concentration gradient to basolateral portion of cell.
6) Glucose goes thru GLUT2 via facilitated diffusion
How are galactose and fructose absorption similar to and different from glucose absorption?
1) Galactose is EXACTLY the same. Uses SGLT1 and GLUT2.

2) Fructose uses GLUT5 (apical membrane, not coupled to Na+) and uses GLUT2 (same) to get out. Passive but facilitated.
Where is glucose absorption most efficient?
Duodenum and jejunum
Once glucose gets out of basolateral side of cell, where does it go?
To liver to get stored as glycogen
At what levels of the GI tract does <b>protein</b> absorption occur?
Stomach - pepsin
Small intestine - pancreatic enzymes, brush border enzymes
What is the major difference between carbohydrate and protein absorption?
We can absorb some di and tripeptides (not JUST amino acids), but we can only absorb monosaccharides (no di or trisaccharides)
What does GIP (glucose-dependent insulinotrophic factor) do?
It is released into the blood by presence of glucose in duodenal lumen. It acts on pancreatic endocrine cells to secrete insulin which then acts to increase uptake of nutrient glucose from the blood thereby blunting rise in glucose following a meal.
What is Enteropeptidase (also called enterokinase)
an enzyme involved in human digestion. It is produced by cells in the duodenum wall, and is secreted from duodenum's glands, the crypts of Lieberkühn, whenever ingested food enters the duodenum from the stomach. Enteropeptidase has the critical job of turning trypsinogen (a zymogen) to trypsin, indirectly activating a number of pancreatic digestive enzymes.
Acinar cells contain granules in (apical or basal) cytoplasm
Apical
What is purpose of gallbladder?
Stores and concentrates bile
What does bile do?
Acts as a detergent to make lipids soluble.
What is the arrangement of a (classical) liver lobule?
Hepatic artery/portal vein at periphery, central vein in middle
Where is the sphincter of oddi?
It's where the common bile duct empties into the duodenum. It's contracted at rest causing bile to be diverted into the gallbladder.
On ingestion of a meal, the gallbladder _____________ and the Sphincter of Oddi _____________. (relaxes or contracts)
contracts; relaxes
What mediates contraction of gallbladder/relaxation of sphincter of Oddi?
1) CCK
2) Neural reflexes
How is bile recycled? (enterohepatic circulation of bile)
1) After bile acts in the lumen, they are actively reabsorbed in the terminal ileum and proximal colon. (Na-linked carrier mechs).

2) They enter portal venous blood and go to liver.

3) They are removed by hepatocytes and resecreted into liver bile.

(about 20% is lost in feces daily)
What is the difference between primary and secondary bile acids?
Primary: synthesized by hepatocytes from cholesterol

Secondary: The primarily bile acids get modified by intestinal bacteria
How does the first bile of a meal compare with later bile (after enterohepatic circulation)?
First part of meal is high in primary bile acids. Later parts are higher in secondary (obviously, because they're going thru the intestine).
What does conjugation do to bile salts?
It connects them with amino acids to become more water soluble. The more water soluble, the better they are at keeping lipids in solution.
Where in GI Tract does lipid digestion take place?
Oral cavity: lingual lipase
Stomach: gastric lipase; emulsification
Intestine: lipase, colipase, bile salts
Most dietary lipid is ingested in form of _____________.
Triglycerides.
Are there any brush border enzymes for lipids?
NO.
How do lipids get into the cell?
Micelles form with help of bile. Then they move to gut wall where lipid digestion products diffuse across brush border and into epithelial cells.


PASSIVE DIFFUSION down concentration gradient.
How do lipids get OUT OF the cell and into circulation?
They are combined into chylomicrons with apoproteins (function similarly as bile). It's then exocytosed and goes thru lacteals. They enter the circulatory system at thoracic duct.


diffusion into cell --> chylomicrons with apoproteins --> lacteal --> thoracic duct
Where does most fat absorption occur?
jejunum
How does sodium get into cell in postprandial state??
Nutrient coupled electrogenic
How does sodium get into cell in interdigestive state??
Electroneutral NaCl absorption
How does water get into enterocytes?
Mainly follows Na+. Solute driven. Goes thru paracellular route.
Absorption of water is coupled to absorption of _____
Na+
Most water enters in the ____________. Why?
Duodenum. Because the major route is paracellularly, the duodenum has leakiest cell-cell junctions and therefore is easiest to get thru.
Can the "tightness" of the junction be modified? If so, how?
Yes, by hormones, neurotransmitters, nutrients. Glucose decreases tightness and therefore increases water absorption.
Is net water and sodium absorption active or passive?
Active. It's driven by Na+ movement into cell, which depends on the gradient being maintained by Na+/K+/ATPase.
How is potassium absorbed? (active or passive)?
Passive absorption in small intestine. This occurs after water leaves, the concentration increases. When lumenal concentrations are high, there is net absorption. Vice versa.
How is Cl- absorbed?
Differs depending on region of small intestine.

<b>Jejunum</b>: Net absorption
<b>Ileum and colon</b>: Net absorption


The enterocytes in Crypts of Lieberkuhn secrete Cl- via CFTR as a way to secrete water, nut we're concerned with NET.
How is bicarb absorbed?
Differs depending on region of intestine.

<b>Jejunum:</b> Net absorption
<b>Ileum and colon:</b> Net secretion
How is Ca2+ absorbed?
ACTIVELY absorbed throughout intestine.

Dietary Ca2+ binds to calbindin and is kept separate from signaling Ca2+.

It's pumped out of cell at basolateral membrane by active protein.
What increases Ca2+ absorption?
Vitamin D, Parathyroid hormone.
How is Iron absorbed?
Occurs in the unstirred layer.

Promoted by acidic pH and Vitamin C.

Transferrin binds it and the complex is absorbed by epithelial cell (receptor-mediated endocytosis).

Store in epithelial cells and released to blood (still bound to transferrin) when needed.

If not needed, when the cell dies, iron is lost.
What is mucosal block?
When cell dies, iron is lost. This prevents excess iron from entering blood and causing toxic effects.
How are vitamins absorbed?
All are passively absorbed except
Vitamin C, biotin, Vitamin B1, and Vitamin B12

Fat soluble vitamins follow same route as lipids.
Where does reabsorption of bile salts occur?
Terminal ileum, mainly