GI electrolytes

Question 1

How much fluid is presented to the digestive system daily and from where?

Answer 1

8.5 L total; 1.5 L from saliva, 2L from stomach, 1/2 L in bile, pancreas 1.5 L, 1 L from intestine, 2 L by intake

Question 2

Where is fluid reabsorbed and how much is absorbed in each site?

Answer 2

Small intestine reabsorbs 6.5 L and colon absorbs 1.9 L

Question 3

What is the net movement of ions in the small intestine?

Answer 3

Net absorption: Na, Cl, H20

Net secretion: HCO3-

Question 4

What is the net movement of ions in the large intestine?

Answer 4

Net absorption: Na, Cl, H2O

Net secretion: HCO3, K

Question 5

What is the difference between a paracellular pathway and a transcellular pathway and where is the former more likely to appear?

Answer 5

Paracellular travel around tight junctions, more common in proximal gut; transcellular often require active transport somewhere

Question 6

What is the driving force of sodium absorption in the intestines?

Answer 6

Na/K ATPase establishes gradients

Question 7

What is the most common method of sodium uptake in feeding conditions and where does it occur in the gut?

Answer 7

Na/Glucose or Na/Amino acid cotransporters; in the villi of the jejunum

Question 8

Which mechanism is triggered in an alkaline environment and where does it occur in the gut?

Answer 8

Na/H exchanger; in the proximal gut

Question 9

Which mechanism predominates in fasting conditions and what cellular messengers promote or inhibit it? Where does it predominantly occur?

Answer 9

Parallel Na/H and Cl/HCO3 mechanism (electroneutral); decreased by cAMP, cGMP, and low Ca; promoted by Ca and angiotensin; happens in the ileum and proximal colon

Question 10

Which mechanism occurs in the colon and how is it modulated?

Answer 10

Aldosterone allows uptake of sodium via electrogenic sodium channels (ENaC). It opens existing channels, transports vesicles to the surface, and produces new Na channels

Question 11

What are the three mechanisms by which chloride is absorbed?

Answer 11

1) voltage dependent [Cl uptake linked with Na], can be paracellular or transcellular
2) Cl/HCO3 exchanger in ileum and colon
3) Na/H//Cl/HCO3 parallel exchanger

Question 12

How can chloride be excreted?

Answer 12

Secretagogue promotes a channel to reach the apical surface of a cell in the crypts of the intestines; Cl will flow into the lumen and Na and H20 follow

Question 13

How is potassium absorbed in the small intestine?

Answer 13

Usually passively via solvent drag; normally does not involve active transport to get into the body

Question 14

How would the colon absorb potassium?

Answer 14

In low-potassium circumstances, the colon can send H/K ATPases to the apical surface to draw K in

Question 15

How does the colon normally excrete K?

Answer 15

Na absorption creates an electrical gradient that promotes K secretion; can be induced by cAMP or aldosterone

Question 16

What are the main secretagogues, where do they act, and what consequence do they all have in common.

Answer 16

Acetylcholine, laxatives [increase Ca], prostaglandins [Ca] and histamines from immune cells, angiotensin and aldosterone (humoral) [cAMP], bacterial toxins [cAMP, c GMP, or Ca]. All activate Cl- secretion

Question 17

What are the main absorptagogues?

Answer 17

Aldosterone, Cortisol, Somatostatin—all promote Na absorption

Question 18

What are the main types of starch and what are their chemical bonds?

Answer 18

Amylose–alpha 1-4 glycosidic bonds
Amylopectin–alpha 1-4 with alpha 1-6 branchpoints
Glycogen–animal starches with extensive branching

Question 19

How do amylases break starches into smaller components? Where can they not work?

Answer 19

They cleave internal glycosidic 1-4 bonds; but cannot access terminal bonds or those near 1-6 bonds.

Question 20

What are the products of amylase cleavages?

Answer 20

Maltose, maltotriose, alpha-limit dextrins (glucose never results!)

Question 21

Which glycolytic enzymes are on the surface of enterocytes and what cleavages do they promote?

Answer 21

1) maltase–residual alpha 1-4 bonds
2) sucrase–cleaves sucrose
3) isomaltase–residual alpha1-6 bonds
4) lactase–cleaves lactose

Question 22

How are the three main monosaccharides absorbed?

Answer 22

Glucose–facilitated diffusion via GLUT2 or Na-coupled
Fructose–GLUT5
Galactose–GLUT2

Question 23

From where does the body get much of its protein load?

Answer 23

Recycled from endogenous proteins in saliva, secretions, and epithelial cells

Question 24

What are the main proteases and in how much of protein digestion they function?

Answer 24

Pepsin–10-15% of total digestion
Endopeptidases (trypsin, chymotrypsin, elastase)–cleave short peptides, 70% of pancreatic digestion
Exopeptidases (carboxypeptidases)–cleave AA one at a time, 30% of pancreatic digestion

Question 25

What enteric enzymes are involved in protein digestion and absorption?

Answer 25

Brush border peptidases, endopeptidases and exopeptidases; most peptides are absorbed in series of 2-4 AA via PepT1 transporter with Na; individual AA have special channels

Question 26

How does fat emulsification begin?

Answer 26

Chewing and gastric churning start process, addition of bile prevents recombination

Question 27

What enzymes begin fat digestion and how do they work?

Answer 27

Lingual and gastric lipases start functioning at low pHs and conduct 10-30% of digestion; they often cleave the first fatty acid off a TAG

Question 28

What are the roles of pancreatic lipases?

Answer 28

They will remove fatty acids from the 1 and 3 positions of a TAG; colipases anchor lipase to micelles to help absorption

Question 29

How are short and medium chain fatty acids absorbed?

Answer 29

Since they are smaller, they are more readily soluble and will be absorbed directly by the mucosa

Question 30

How do enterocytes package fats for the rest of the body?

Answer 30

They put triacylglycerides into chylomicrons with proteins, phospholipids, and lecithin and enter lacteals.

Question 31

How is folate absorbed?

Answer 31

1) Proximal enterocytes link glutamate residues which are cleaved down to one 2) folate-glutamate exchanged for a hydroxyl group 3) Hepatocytes and (minorly) enterocytes conjugate it into THF

Question 32

How is Vitamin B12 absorbed?

Answer 32

1) Pepsin releases it from animal products 2) Initially binds to salivary haptocorrin 3) Parietal cells make intrinsic factor to bind to cobalamin 4) Enterocytes in the terminal ileum take up the complex via endocytosis

Question 33

What class of antacids block B12 absorption?

Answer 33

Anti-histamine agents like ranitidine; proton pump inhibitors do not

Question 34

How does calcium absorption occur?

Answer 34

Vitamin D promotes opening of Ca channels in the duodenum; calbindin takes up Ca ions to prevent action and to maintain a strong gradient in favor of entry; Ca shunted out of the cell via Ca-ATPase or Ca/Na antiporter