Exam #5: Fluid & Electrolytes Flashcards
Where is Na+ absorbed?
Generally, Na+ is absorbed in BOTH the small & large intestine
What are the mechanisms of Na+ absorption? Where are these transporters located? When are they primarily functioning?
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
Where is Cl- absorbed?
Cl- is absorbed in BOTH the small & large intestine
What are the mechanisms of Cl- absorption?
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
Where is Cl- secreted?
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What is the mechanism of Cl- secretion?
CFTR that is located in the apical membrane of the enterocyte.
*****Note that Na+ follows and causes water to stay in the lumen
What regulates the CFTR?
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
Where is K+ absorbed? Where is K+ secreted?
Small intestine= absorption
Large intestine= secretion
What are the mechanisms of K+ absorption? What is the mechanism for K+ secretion?
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
What are the signals that regulate electrolyte absorption & secretion?
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
What are the neuronal pathway & agents that lead to fluid secretion?
ACh
VIP
5-HT
**All increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger
What are the paracrine/immune pathway & agents that lead to fluid secretion?
Prostalgandin
Histamine
*****Both increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger
What are the bacterial agents that lead to fluid secretion?
Cholera toxin
E. coli toxins
*****Both increase 2nd messengers that stimulate the CFTR & Na+/H+ exchanger
What are the endocrine mechanisms that promote fluid absorption?
Mineralcorticoids
Glucocorticoids
**Both upregulate the transcription of Na+ channels & Na+/K+ ATPase
What is the role of somatostatin in fluid absorption?
Increased Na+/H+ exchanger leading to fluid absorption
What is the difference between osmotic diarrhea and secretory diarrhea?
Osmotic= diarrhea caused by a nonabsorbable nutrient
Secretory= diarrhea caused by intestinal secretion of fluid and electrolytes
List the causes of osmotic diarrhea.
Disaccharide deficieicny Pancreatic enzyme def. Nutrient binding sub. Loss of enterocytes Bacterial overgrwoth Antacids
What are the causes of secretory diarrhea?
Enterotoxins
Inflammatory cytokines
Tumor
What are the causes of both osmotic and secretory diarrhea?
Inflammatory conditions
Infectious disease
What are the different causes of constipation?
1) Diet/ Lifestyle
2) GI
3) Pharmacological
4) Endocrine
5) Neurogenic
How do the small & large intestine compare in net absorption & secretion?
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
