Intestinal Transport of Fluid and Electrolytes Flashcards
How much fluid for
- Oral intake
- Total presented to the intestine
- Absorbed
- In stool
- 2 L
- 9 L
- 8.8 L
- 200 mL
What is the normal absorptive load for (in L)
- Duodenum/jejunum
- Ileum
- Colon-rectum
- 5.5 L
- 2.0 L
- 1.3 L
Maximum absorptive capacity for
- Small intestine
- Colon-Rectum
- 12 L
2. 4-6 L
Paracellular transport
Solutes/electrolytes move down electrochemical/concentration gradients through tight junctions between cells
The tight junctions are most permeable in the small intestine
Resistance/tightness increases as you move down the large intestine
Transcellular Transport
Solutes/electrolytes must cross both the apical and the basolateral membrane
Generally, the solute movements across at least one of these membranes must have an active component (requires energy to go against a gradient)
Where is Na absorbed
- Paracellularly
- Transcellularly
- Mostly in the initial/proximal portion of the small intestine
- Along the entirety of the small and large intestines
How is Na moved out of the cell basally?
Via the Na/K ATPase
2 co-transporters that carry Na and X into the enterocyte apically
SGLT1 (Na and glucose or galactose)
Na and an amino acid
Is Na absorption in the proximal small intestine cAMP sensitive or not? What is the clinical significance of this?
It is not cAMP sensitive
It means this process is not affected by most enterotoxins that contribute to diarrhea formation
What is the non-nutrient coupled mode of Na absorption in the proximal small intestine?
When Na absorption is coupled to proton exchange
In response to increased pH from bicarb, Na/H exchangers are activated apically (Na in, H out)
Difference between apical and basal Na/H exchangers (NHEs)
Apical: pH inducible
Basal: constitutive
They bring Na into the cell, pump H out
In the ileum and proximal colon, NHE activity is coupled to…
When is it important
Bicarb export and Cl input (apically)
Electrically neutral
Important between meals
Inhibited by activation of multiple signalling moieties (cAMP, cGMP, Ca), so is important in some diarrheas
How can Na be brought into the cell in the distal colon and rectum?
Through apical epithelial sodium channels (ENaC)
Capable of moving Na across the enterocyte epithelium against large concentration gradients due to the high activity of basal Na/K ATPase
ENaC channels are stimulated by ____ and it works on the channel in what 3 ways?
Aldosterone
- Opens ENaC (very rapid)
- Increases delivery of formed channels to apical membrane (rapid)
- Increased synthesis of ENaC and Na/K pump (slow)
Passive Cl absorption in the small and distal large intestine
Driven by a voltage gradient caused by movement of Na into the interstitium and by the presence of negatively-charge amino acids and carbs in the lumen
May occur transcellularly through Cl channels or paracellularly through tight junctions
Cl absorption in the distal small intestine/entire large intestine
Cl/HCO3 exchanger can act alone without parallel coupling
This is inhibited by the activation of multiple signalling moieties, and is important in the etiology of several types of diarrhea
Is water actively or passively absorbed? Does it move paracellularly or transcellularly?
Passively
Majorly paracellularly but can be absorbed transcellularly as well
3 ways water can move transcellularly
Diffusion through lipid bilayer (inefficient)
Aquaporins (efficient)
Co-transport (very effecient)
4 functions of intestinal fluid secretion
Propel substances out of the crypts
Maintain the fluidity of intraluminal contents
Maintain osmotic equilibrium
Dilute potentially injurious substances
What is secreted from duodenal
- Crypts
- Glands
- Enterocytes secrete bicarbonate rich fluid
2. Secretory portions in the submucosa (ducts empty into crypts) secrete an alkaline mucinous fluid that is protective
What is secreted from jejeunal and ileal secretion
Crypt enterocytes secrete an isotonic NaCl-containing solution
Drive by active secretion of Cl transcellular that relies partially on the CFTR channel
How do Cl ions get into the jejunal/ileal crypt cells basally?
Co-transported with Na and K via a triporter
How does Cl leave the cell in the jejunal/ileal crypts?
Through CFTR apically
These channels are regulated by Ca/cAMP/cGMP, which are regulated by secretagogues
5 factors influencing intestinal water and electrolyte transport
Hormones and paracrines produced by mucosal cells
Neurotransmitters produced by enteric neurons
Chemicals produced by subepithelial myofibroblasts and immune cells
Changes in the capillary blood and lymphatic flow
Smooth muscle contraction
2 major pathophysiological mechanisms for diarrhea
Decreased absorption of fluid and electrolytes
Increased secretion of fluid and electrolytes
3 reasons for diarrhea associated with decreased fluid and electrolyte absorption
Inhibited ion transport system
Presence of poorly absorbed luminal osmols
Increased propulsive activity causing decreased contact time
2 mechanisms for diarrhea associated with increased secretion of fluids and electrolytes
Stimulated anion secretion (bacterial toxins)
Secretion from hyperplastic crypts (chronic inflammation - celiac)
4 reasons for secretory diarrhea associated with celiac disease
Decreased brush border hydrolases resulting in unabsorbed osmols
Villus atrophy (malabsorption)
Crypt hyperplasia (increased secretion) MAIN
Inflammation induced hyper-secretion by crypt enterocytes
How does the cholera toxin work?
Causes Cl secretion and prevents NaCl absorption
Nutrient coupled Na absorption is NOT effected (so treat by giving Na/glucose)
