Exam #5: Fluid & Electrolytes

Question 1

Where is Na+ absorbed?

Answer 1

Generally, Na+ is absorbed in BOTH the small & large intestine

Question 2

What are the mechanisms of Na+ absorption? Where are these transporters located? When are they primarily functioning?

Answer 2

There are four key transport proteins involved in the uptake of Na+:
Jejunum= following a meal
1) SGTL1 (glucose)
2) Amino Acid Transporter B

Jejunum, Ileum, & Proximal Colon= interdigestive period
3) Na+/H+ Exchanger

Colon
4) Na+ channel

*****Note that the basolateral membrane has Na+/K+ ATPase, which keeps Na+ flowing DOWN its concentration gradient

Question 3

Where is Cl- absorbed?

Answer 3

Cl- is absorbed in BOTH the small & large intestine

Question 4

What are the mechanisms of Cl- absorption?

Answer 4

There are two mechanisms for Cl- absorption:

1) Passive diffusion of Cl- following Na+ & water via a paracellular pathway
2) Cl-/bicarbonate Exchanger i.e. Cl- IN & HCO3- OUT

Question 5

Where is Cl- secreted?

Answer 5

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Question 6

What is the mechanism of Cl- secretion?

Answer 6

CFTR that is located in the apical membrane of the enterocyte.

*****Note that Na+ follows and causes water to stay in the lumen

Question 7

What regulates the CFTR?

Answer 7

Intracellular signaling & accumulation of 2nd messengers result in secretion of Cl- (Na+ follows & water stays in the lumen). Specifically,

1) cAMP opens the CFTR
2) Ca++ opens the CFTR

Question 8

Where is K+ absorbed? Where is K+ secreted?

Answer 8

Small intestine= absorption

Large intestine= secretion

Question 9

What are the mechanisms of K+ absorption? What is the mechanism for K+ secretion?

Answer 9

Small intestine= “solvent drag”

• NaCl are absorbed from the gut lumen
• Water follows & DRAGS K+ with it

Colon= lumen is negative

• K+ is attracted to the negative lumen & goes the opposite direction
• result is net secretion

Question 10

What are the signals that regulate electrolyte absorption & secretion?

Answer 10

Intracellular signaling & 2nd messenger accumulation i.e. cAMP & Ca++

• Secretion of Cl- via CFTR opening
• Inhibit Na+ absorption via Na/H+
• NET RESULT= NaCl IN LUMEN

Steroids= increase transcription of proteins that generally facilitate Na+ & water uptake

• Na+/K+ ATPase
• Na+ Channels
• Na+/H+ exchanger

Thus,
Intracellular messengers= less water retention
Steroids= retention

Question 11

What are the neuronal pathway & agents that lead to fluid secretion?

Answer 11

ACh
VIP
5-HT

**All increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger

Question 12

What are the paracrine/immune pathway & agents that lead to fluid secretion?

Answer 12

Prostalgandin
Histamine

*****Both increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger

Question 13

What are the bacterial agents that lead to fluid secretion?

Answer 13

Cholera toxin
E. coli toxins

*****Both increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger

Question 14

What are the endocrine mechanisms that promote fluid absorption?

Answer 14

Mineralcorticoids
Glucocorticoids

**Both upregulate the transcription of Na+ channels & Na+/K+ ATPase

Question 15

What is the role of somatostatin in fluid absorption?

Answer 15

Increased Na+/H+ exchanger leading to fluid absorption

Question 16

What is the difference between osmotic diarrhea and secretory diarrhea?

Answer 16

Osmotic= diarrhea caused by a nonabsorbable nutrient

Secretory= diarrhea caused by intestinal secretion of fluid and electrolytes

Question 17

List the causes of osmotic diarrhea.

Answer 17

Disaccharide deficieicny 
Pancreatic enzyme def.
Nutrient binding sub. 
Loss of enterocytes 
Bacterial overgrwoth 
Antacids

Question 18

What are the causes of secretory diarrhea?

Answer 18

Enterotoxins
Inflammatory cytokines
Tumor

Question 19

What are the causes of both osmotic and secretory diarrhea?

Answer 19

Inflammatory conditions

Infectious disease

Question 20

What are the different causes of constipation?

Answer 20

1) Diet/ Lifestyle
2) GI
3) Pharmacological
4) Endocrine
5) Neurogenic

Question 21

How do the small & large intestine compare in net absorption & secretion?

Answer 21

Small intestine=

• Absorb: Na+, Cl-, K+ & H2O
• Secrete: Bicarbonate

Large intestine=

• Absorb: Na+, Cl-, & H2O
• Secrete: K+ & Bicarbonate

Thus, the MAIN difference is that K+ is ABSORBED in the small intestine & SECRETED in the large intestine