Lecture 8 - Nephron Sites of Renal Sodium Reabsorption Flashcards
What determines the size of the extracellular volume?
Amount of sodium excreted
Average daily sodium intake?
50-300 mEq/day (usually 150 mEq/day)
Average daily sodium excretion?
50-300 mEq/day (usually 150 mEq/day)
How is sodium excreted?
- MAINLY urine
- Small amount in sweat
- Small amount in feces
Normal daily filtration rate of Na+?
F = 100 mL/min x 0.140 mEq/mL = 14 mEq/min = over 20,000 mEq/day
Normal excretion fraction for Na+?
0.7%
Where is Na+ reabsorbed in the nephron? Provide %.
- Proximal tubule (60%)
- Thin ascending limb of the loop of Henle + thick ascending limb of the loop of Henle (25%)
- Distal convoluted tubule (5-7%)
- Collecting duct (3-5%)
How is Na+ reabsorbed in S1?
- SGLT2 with glucose
- AA cotransporter
- Lactate and H2PO4– cotransporters
- Na+/H+ lumenal exchanger
=> all secondary active transport mechanisms because all driven by the Na+/K+-ATPase on the basolateral membrane
How is Na+ reabsorption in S2 different than in S1?
Same as in S1, but upstream Na+ reabsorption has caused increased Cl- concentration in lumen => now there is also paracellular Cl- reabsorption which drives passive Na+ transport paracellularly
Concentration of Cl- in S2?
30% higher than what it is in plasma
Lumenal potential in S1? Explain why.
Slightly negative due to Na+ movement
Lumenal potential in S2? Explain why.
Slightly positive due to Cl- movement
How is Na+ reabsorbed in S3?
Same as in S2
Lumenal potential in S3? Explain why.
Slightly positive due to Cl- movement
How does the tubular concentration of Na+ over the plasma Na+ change throughout the proximal tubule?
It does NOT, because H2O moves with it at exactly the same rate
What solutes have a tubular concentration over plasma concentration that decreases throughout the proximal tubule? What does this mean?
- HCO3- (higher ratio than 2 others: reabsorbed at a lower rate)
- AAs
- Glucose
=> means they are being reabsorbed at a faster rate than Na+/H2O and so their lumenal concentration is plummeting
What solutes have a tubular concentration over plasma concentration that increases throughout the proximal tubule? What does this mean?
- Inulin => not reabsorbed at all
2. Cl- => not reabsorbed in S1, so lumenal concentration increases and then is reabsorbed at same rate than Na+/H2O
What is lumenal inulin concentration indicative of? Explain why.
Rate of water reabsorption since it is not reabsorbed
How is Na+ reabsorbed in the thin descending limb of the loop of Henle? Purpose?
NOT PERMABLE! Water moves out, Na+ lumenal concentration rises, which sets up a potential concentration gradient for the next step IF enough urea is in the interstitium
How is Na+ reabsorbed in the thin ascending limb of the loop of Henle?
Simple diffusion due to the fact that the total interstitial osmolarity is the same, BUT the urea osmolarity is much higher than the NaCl, so NaCl is reabsorbed
How is Na+ reabsorbed in the thick ascending limb of the loop of Henle? Effect on tubular fluid? Type of transport?
- Basolateral Na+/K+-ATPase pumps Na+ into the peritubular capillary
- Very unique carrier (does not utilize energy) on the lumenal surface: 1Na+/1K+/2Cl- all pumped in due to drive of sodium gradient => secondary active transport
- Na+ pumped out via basolateral ATPase and Cl-/K+ get pumped out together by transporter or simple diffusion
=> dilution of tubular fluid because water cannot follow (TIGHT segment)
PLUS:
- Na+/H+ exchanger
Can the unique lumenal transporter (1Na+/1K+/2Cl-) of the distal tubule be inhibited?
YUP - some powerful diuretics do this directly (e.g. lasix) and other block simple diffusion lumenal K+ channels, which indirectly inhibits the carrier as well
Lumenal potential in thick ascending limb of the loop of Henle? Explain why.
Slight positive potential due to the fact that the pump runs out of K+ (concentration in plasma and tubular fluid much lower) so high K+ inside the cell needs to diffuse out
How is Na+ reabsorbed in the distal convoluted tubule? What is important to note?
- Basolateral Na+/K+-ATPase pumps Na+ into the peritubular capillary
- Unique Na+/Cl- carrier (no energy required) that is sensitive to thiazide diuretics (different from those used in thick ascending limb of the loop of Henle)
