Renal V: Tubular Functions Along Nephron Flashcards
Where is the only place in the nephron in which sodium is NOT reabsorbed?
thin descending limb
Where is the majority of Na+ reabsorbed in the nephron?
- PT (67%)
- TAL (25%)
Excretion of Na+ is ____% of filtered load
less than 1
How are the mechanisms for Na+ reabsorption in early and late proximal tubules different?
- early: Na+ reabsorbed primarily w/ HCO3- and organic solutes (ex: glucose, amino acids)
- late: Na+ reabsorbed primarily w/ Cl-
Moving along the proximal tubule, why do the values for [TF/P]Na+ and [TF/P]osmolarity both remain at 1?
Na+ and total solute are reabsorbed in proportion to water (isosmotic reabsorption)
How do [TF/P]glucose, [TF/P]aa’s, and [TF/P]HCO3- change along the early proximal tubule?
they fall below 1 because reabsorption of these substances exceeds water reabsorption
Describe glomerulotubular balance.
It is the major regulatory mechanism of the PT, and it describes the balance b/t filtration (at glomerulus) and reabsorption (at PT). It ensures that a constant fraction of the filtered load is reabsorbed by the PT despite variation in GFR. It occurs by mechano-transduction of the microvilli of PT cells.
What is the normal fraction of the filtered load that is reabsorbed by the PT?
67%
Do the thin limbs have active or passive permeability properties?
passive
Describe the permeability of the thin descending limb.
passive H2O reabsorption, therefore Na+ and Cl- impermeable
Describe the permeability of the thin ascending limb.
passive Na+ and Cl- reabsorption, therefore H2O impermeable
Does the thick ascending limb have active or passive permeability properties?
active
Describe the permeability of the thick ascending limb.
It reabsorbs ~25% of the filtered Na+ via active transport, and it is impermeable to water.
Why is the thick ascending limb called the diluting segment?
NaCl is reabsorbed but water is not reabsorbed with it, as the thick ascending limb is impermeable to water. Thus, there is more H2O and less Na+ in the thick ascending limb, so the tubular fluid is more dilute.
Which part of the nephron is referred to as the diluting segment?
thick ascending limb
What is the site of action of loop diuretics?
thick ascending limb
Do loop diuretics act on the PT? Why or why not?
No, because we don’t want to interfere with the 67% reabsorption that happens at the PT (and loop diuretics inhibit reabsorption)
What effect do loop diuretics (like furosemide) have?
They act at the thick ascending limb and inhibit Na+ reabsorption, resulting in excretion of 25% of the filtered Na+.
Describe the permeability of the early distal tubule.
It is impermeable to water (like the thick ascending limb), and it reabsorbs ~5% of the filtered Na+.
How much filtered sodium do the late distal tubule and collecting duct reabsorb?
only 3%
What are the major cell types that line the late distal tubule and collecting duct?
principal cells and intercalated cells (alpha and beta)
What are some main differences b/t the early DT and the thick ascending limb?
The early distal tubule is…
- a NaCl co-transporter (transports 1 Na+ and 1 Cl-, so 2 ions that make the process electroneutral)
- inhibited by thiazide diuretics (organic acids)
The luminal membrane of principal cells contain which type of channels?
ENaC (epithelial Na channels)
Where are principal cells found?
the late distal tubule and collecting duct
ENaC’s are only found in ________.
principal cells
Why does the late distal tubule and collecting duct have a high negative potential on the luminal side?
this is due to reabsorption of Na+ (Na+ is pretty much entirely depleted from the tubule by this point)
Principal cells are involved with K+ ______, while alpha-intercalated cells are involved with K+ _______.
secretion; reabsorption
What do K+ sparing diuretics do?
They inhibit K+ secretion by principal cells (due to inhibition of Na+ reabsorption).
