Renal Transport Mechanisms Flashcards
What gets reabsorbed in the Proximal Convoluted Tubule? Proximal straight tubule?
All glucose and amino acids should be reabsorbed here as well as most other filtered substances:
- sodium (70%)
- urea (50%)
- potassium (70%)
- phosphate (70%)
- Ca2+ (70%)
- Mg (30%)
- H2O (70%)
PST: ~15% of phosphate
In order for reabsorption to occur, what do things have to journey through?
- Stuff has to cross the apical border
- Navigate through the cell interior
- Cross the basolateral border
- Then enter the peritubular capillary
- Reabsorption is powered by Na-K-ATPase
- reabsorption usually driven by Na+ coupled with something else
How does stuff cross the apical border?
-by symporter or antiporter
How do things cross the basolateral border?
- Na+ gets out via Na-K-ATPase
- Special transporters handle the rest
What transporters exist in the PCT?
apical: SGLT, amino acid-Na+ symp, phosphate-lactate-citrate-Na symp, NHE (Na-H exchanger)
basolateral: Na-K-ATPase, GLUT, Amino Acid, bicarb, phosphate, lactate, citrate
Water moves transcellularly via AQP and bulk flow into blood
What gets reabsorbed form the TAL?
NO water !!
- Important for Na, K, Ca, and Mg (most happens here ~60%)
- NKCC2 (N, 2Cl, K) and ROMK are important here, back leak of K+ via ROMK is critical to passive paracellular diffusion of Mg and Ca from lumen
What is unique about the Distal Convoluted tubule?
-least amount of solutes reabsorbed here
-highly regulated, ADH and aldosterone, variable
secretion/reabsorption of solutes depending on
conditions
-Principal cells, alpha and beta intercalated cells
What do principal cells reabsorb and secrete?
reabsorb: Na+, and H2O
Secrete: K+
What do alpha intercalated cells reabsorb and secrete?
reabsorb: K+, HCO3-
Secrete: H+
What do beta intercalated cells reabsorb and secrete?
reabsorb: H+, Cl-
Secrete: K+, HCO3-
What gets reabsorbed in the DCT?
-relatively impermeable to water
-continued NaCl reabsorption via thiazide sensitive NCC
further dilutes tubular fluid
-Ca2++ crosses apically via TRPV5 and basolaterally via
NCE (Na-Ca exchanger)
-ENaC in late DT and collecting duct is acted upon by
aldosterone to reabsorb Na by upregulating its
expression
What channels allow principal cells and beta intercalated cells to secrete K+ into the tubular lumen?
BK and ROMK
How do alpha intercalated cells reabsorb K+?
- H-K ATPase moves K into the cells from the tubular
lumen. - K diffuses across the basolateral membrane through K
channels.
What are the most important factors that stimulate sodium reabsorption?
- Na+ deficiency
- low Na+ diet
- hyponatremia
- Na+ loss through severe diarrhea
What are the most important factors that stimulate sodium secretion?
- hypernatremia
- ANP
- Renal prostaglandins
What stimulates K+ secretion by principal cells and beta intercalated cells?
increased serum concentration and aldosterone
What stimulates K+ reabsorption by alpha intercalated cells?
(1) K+ deficiency;
(2) low K+diet;
(3) hypokalemia;
(4) K+ loss through severe diarrhea
What does ADH respond to? How?
- Hyperosmolality (very sensitive)
- Volume depletion
How:
1. Increases the water permeability of the principal cells of
the late distal tubule and collecting ducts.
•Aquaporins inserted into apical membrane of
principal cells (not present w/o ADH)
•Best known and most important mechanism!!!!
2. Increases urea permeability in the inner medullary
collecting ducts (but not in the cortical or outer
medullary collecting ducts)
3. Increases the activity of the Na+-K+-2Cl−cotransporter
(NKCC2) of the thick ascending limb
How is urine concentrated?
Countercurrent mechanisms (multiplier and exchanger)
Describe countercurrent multiplication.
- The single effect
•NaCl leaves ascending limb, interstitium becomes
hyperosmotic (salty medulla!)
•Water leaves descending
limb to equalize the interstitium osmolality
2. Fluid flow •Fluid always flowing through tubule •New fluid enters descending limb from above •Pushes tubular fluid downward •Gradient develops •Effect multiplies
What is countercurrent exchange?
• Passive movement of water from descending limb into
interstitium, and reabsorbed into vasa recta
• Increased osmotic gradient results in more water
reabsorption (ADH)
• Blood flow rate impacts equilibration
What creates medullary osmolality?
the presence of NaCl and urea
What is the equation fro osmolar clearance?
Cosm = (Uosm*V)/(Posm)
What is the equation for free water clearance?
Ch2o = V - Cosm or V - (Uosm*V)/(Posm)
when (-): excess solutes are removed, water conserved
when (+): water is being excreted, dilute urine
