Distal Nephron Flashcards
Na-K ATPase effect on intracellular space
Pumps 3Na out and 2K+ in rendering the intracellular space more negative and allowing for Na to pump in on the apical side
Character of fluid entering thin loop of henle
- Na in same concentration as plasma
- Cl in higher concentration than in plasma due to bicarb reabs in the PT
- Little if any glucose, AAs, or protein
- Minimal K+
- Lots of NH4+ which was synthesized in the PT
- 20% of filtered phosphate (rest was absorbed in the PT)
How much active transport is there in the thin limbs?
Little if any, not even many Na-K ATPases
Permeability properties of the thin descending limb
- Permeable to water with low perm to Na and Cl
- Water is extracted from the lumen and goes into the hypertonic interstitium of the medulla
Permeability properties of the thin ascending limb
Impermeable to water and largely permeable to Na and Cl so salt DIFFUSES out of the tubule
What is the net effect of the thin descending and thin ascending loops?
To reabsorb water and Na isotonically
Tubular fluid content by the end of the thin limbs
- 15% of NaCl, 15% water and 15% of Ca2+ remains (70% was reabs in the PT, 15% in the thin limbs)
- 10% filtered K+ and phosphate remains
- 80% mg
- no glucose, little bicarb
Permeability and charge of the thick ascending limb
- Water impermeable
- Powerful NaCl channels that dilute the urine
- Lumen is positive as compared to blood side
What channel is key to thick ascending loop fxn?
NKCC2 channel
- Na, K and Cl are pumped into the cell
- Na is actively pumped out via Na/K ATPase
- Cl leaves the cell because the inside of the cell is negative
- K+ is recycled through ROMK
The lumen positive potential pushes Mg and Ca out to the blood to get reabs here
What % of filtered solute is reabs in the thick ascending limb?
about 25%
What is responsible for the pos potential in the thick ascending limb? What are the consequences of a more negative lumen?
The ROMK channel (ie K+ is responsible)
–if you block ROMK or have a defect here, the lumen will not have a pos potential and Mg and Ca won’t get reabs
Characteristics of the luminal fluid at the end of the thick ascending limb?
- 10% of NaCl was reabs and now 5% remains
- 0% of water was reabs because it was water impermeable, so now a hypotonic fluid remains
- No glucose or phosphate
- 70-80% of Mg is reabs and now 5% remains
Consequences of a defect in the thick ascending limb?
- More NaCl delivered to the distal tubule
- More Ca delivered to distal tubule
- Leads to a decrease in fluid retention, inc in aldosterone–>inc in K+ and H+ excretion
What charge does the tubular lumen have in the DCT?
Weakly + due to active Cl uptake and some K+ recycling
What channel is key to the DCT?
NCCT (Na-Cl co transporter)
Cl then passively exits the cell and goes into the blood
Ca and Mg handling in the distal nephron
Only place where active reabs occurs (DCT, CNT to CD). Mg is reabs earlier than Ca
Once in the cell, what buffers intracellular Ca2+?
Calbindin binds to it after it enters through the TRPV5 channel from the lumen and transports it to the blood side to be actively pumped out (ie the BLM side)
-process is controlled by vit D and PTH
Character of fluid leaving the DCT?
- Hypotonic
- 1-5% of filtered NaCl and K+ remaining
- Minimal Ca and Mg
- Some acid
- 20% of filtered phosphage
What is the major ion process in the cortical collecting duct?
Na passively enters through ENaC, K passively leaves through ROMK on the apical side
–>last bit of Na is reabs here
What is the charge of the lumen in the cortical collecting duct?
negative
Via what channels is K+ reabs in the cortical collecting duct?
Through low or intermediate conductance K channels
Effects of aldosterone
Work on the CCD to increase ENaC channels, stimulate the Na-K ATPase, ROMK and increase H+ transport into the lumen
What type of cell secretes H+?
Intercalated cell in the CCD
What enzyme makes bicarb
Carbonic Anhydrase
Where does active urea transport occur?
CD. Urea concentration here helps with urinary concentration and inc tonicity of inner medulla. ADH increases its transport.
Functions of the collecting duct
- Regulate final urine composition of Na, K, bicarb and H+
- Regulate water retention/elim
- Contributes to the formation of high renal interstitial osmol so that the concentration of urine is ideal
Content of fluid leaving the CD
All depends on what the organism needs!
