Na+ transport I and II
Terms
undefined, object
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- Interpret: [TF/P]x = 1.0
-
x is not reabsorbed or secreted
OR
x is reabsorbed in proportion to water
ex. all freely filtered items in Bowman's space and Na+ in proximal tubule - Interpret: [TF/P]x < 1.0
-
x is reabsorbed more than water
ex. glucose in proximal tubule (glucose reabsorbed more than water here) - Interpret: [TF/P]x > 1.0
-
x is reabsorbed less than water
OR
x is secreted
ex. urea in cortical collecting ducts in the presence of ADH (water reabsorbed and urea not reabsorbed) - What does the [TF/P]inulin measure?
-
water reabsorption
* amount of inulin is constant in the tube, but the concentration of inulin in the tube varies based on amount of water reabsorbed - If the tubular inulin conc is 1.0, and 50% of the water is reabsorbed, what is the new conc of inulin in the tubular fluid?
- 2.0
- What are inulin's 3 functions?
-
1) ECF marker
2) measures GFR (Cinulin)
3) measures water reabsorption - What does the "double ratio" tell us?
- fraction of the filtered load of a substance remaining in the nephron at any point
- Interpret a double ration for Na+ of 0.33 and a [TF/P]Na of 1.0. Where would this happen?
- At the end of the proximal tubule 67% of Na+ has been reabsorbed (33% still in nephron), and it has been reabsorbed in the same proportion as water.
- What is the MOST IMPORTANT function of the kidneys?
- Na+ regulation
- How much Na+ is ingested/excreted per day?
- about 150 mEq of Na+
- If the kidneys excrete more Na+ than is injecsted, what kind of 'balance' is the body in?
- negative Na+ balance
- Give the % of Na+ reabsorption by location in the nephron.
-
proximal tubule - 67%
thick ascending limb - 25%
early distal tubule - 5%
late distal tubule and collecting duct - 3%
(<1% is excreted) - What are the 9 main features of the EARLY proximal tubule?
-
1) isosmotic reabsorption
2) Na+ glucose cotransporter (Na+ phosphate and Na+ aa also)
3) Na+/H+ exchange on luminal membrane and Na+/K+ ATPase on basolateral membrane
4) lumen has (-) transepithelium difference due to Na+/glucose transport
5) [TF/P]Na and [TF/P]osm = 1.0
6) preferential reabsorption of HCO3- over Cl-
7) reabsorb most vital nutrients here!
8) reabsorb 33% of Na
9) Na/P cotransport inhibited by PTH - What are the 7 main features of the LATE proximal tubule?
-
1) isosmotic reabsorption
2) 33% of Na reabsorbed here
3) high [Cl-] in lumen
4) Cl- moves into cells and between cells down conc grad, and also via Cl-/formate exchange on luminal membrane
5) lumen (+) transepith pot diff created by Cl- diffusion
6) [TF/P]Na and [TF/P]osm = 1.0
7) still have Na+/H+ exchange and Na+/K+ ATPase on basolateral membrane - What happens to the isosmotic reabsorption process if more Na+ is filtered?
- nothing - isosmotic reabsorption is maintain by increasing Na+ reabsorption
- What is glomerulotubular balance?
-
a regulatory feature of the proximal tubule - filtration of Na+ balanced by reabsorption -> constant fractional reabsorption of 67% (normally) -> ECF Na+ content and volume are maintained
THE ONCOTIC PRESSURE OF THE PERITUBULAR CAPILLARY MAINTAINS THE GT BALANCE! - Explain what happens to the oncotic pressure of the peritubular capill blood and to the proximal tubule reabsorption of Na+ and water when ECF volume INCREASES.
- volume expansion = dilution = decrease in oncotic press in peritubular cap blood = decrease in Na/water reabsorption
- Can the % of Na reabsorbed in the proximal tubule change?
- yes - if ECF volume changes
- What happens to the amount of Na+ reabsorbed in the prox tubule in ECF volume expansion?
- fractional reabsorption is decreased (due to decreased oncotic press in PT capill), and excretion is increased
- What happens to the amount of Na+ reabsorbed in the prox tubule in ECF volume contraction?
-
fractional reabsorption is increased because of:
1)increase in oncotic press of PT capill
2) decrease in Pa, increase in sympathetics
3) decrease in Pa, increase in RAA, increase in AII, increase in Na/H cotransport - What is unique about the thick ascending loop of henle?
-
it is impermeable to water
(but DOES reabsorb solute)
*this is why it is called the Diluting Segment - How much of Na+ is reabsorbed in the Thick ascending limb?
- 25%
- What is [TF/P]Na and [TF/P]osm in the TAL?
-
<1.0
(because tubular fluid is more dilute than plasma) - What transporters are located in the TAL?
- Na/K/2Cl- in luminal membrance, and Na/K ATPase on basolateral membrane
- What hormone specifically stimulates Na+ reabsorption in the proximal tubule?
- Angiotensin II
- What is an example of a loop diuretic, and how does it work in the TAL?
-
furosemide - inhibits the Na/K/2Cl- cotransporter by binding to the Cl- binding site thus inhibiting Na+ reabsorption
(is a weak acid) - What is important about the luminal potential pressure here?
-
it is (+), and drives the reabsorption of Ca2+ and Mg2+ between cells
(IMPORTANT BECAUSE LOOP DIURETICS ABOLISH THE (+) LUMINAL POTENTIAL) - Is the 3-ion cotransporter in the TAL electrogenic?
- yes -- some K+ diffuses back into the tubular lumen giving the lumen a net (+) charge
- What is the [TF/P]Na and [TF/P]osm in the TAL?
- <1.0
- What characterizes the early distal tubule?
-
1) impermeable to water
2) "cortical diluting segment"
3) Na/Cl cotransporter in luminal membrane
4) 5% of Na+ reabsorbed here
5) thiazide diuretic works here (by inhibiting Na/Cl- cotransporter)
6) Ca2+ reabsorption stimulated by PTH
7) [TF/P]Na and [TF/P]osm <1.0 - Which is stronger between loop diuretics (furosemide working in TAL) or thiazide diuretics (working in the early distal tubule)?
- loop diuretics
- Which cells in the late distal tubule and collecting ducts reabsorbs Na+?
- principal cells
- What 2 hormones work in the late distal tubule and collecting duct areas?
-
1) ADH
2) Aldosterone - What are the 3 functions of principal cells?
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1) Na+ reabsorption (thru Na+ channels induced by aldosterone)
2) K+ secretion (thru K+ channels on luminal side induced by aldosterone)
3) water reabsorption (thru AQP2 channels induced by ADH) - What are the 2 functions of intercalated cells?
-
1) K+ reabsorption (thru H+/K+ ATPase - same transporter that secretes acid from parietal cells in the stomach)
2) H+ secretion (thru H+ ATPase upregulated by aldosterone) - What are aldosterone's 3 actions? IMPORTANT!!!
-
1) induces Na+ channels - increase Na+ reabsorption in the principal cells
2) adds K+ channels - increases K+ secretion in principal cells
3) stimulates H+ secretion in intercalated cells - What is responsible for "fine tuning " Na+ reabsorption in the nephron?
- the principal cells
- Which cells do spironolactone and amiloride (K-sparing diuretics) affect?
- the principal cells - by blocking Na+ channels