Sodium Reabsorption Flashcards
What do we mean when we say the PCT solute exchange is Iso-osmolalitic?
Doesn’t change the osmolality of the tubular fluid
In the PCT, what membranes are Aquaporins found on?
Does water follow Na or does Na follow water
Both Apical and Basement membranes
Water follows Na+
Outline Bicarbonate reabsorption in PCT in 4 steps
- H2CO3 (either dissociated or associated) broken down into H2O and CO2 by Carbonic Anhydrase
- These diffuse into PCT, Carbonic Anhydrase in cell keeps them dissociated
- HCO3 leaves PCT with Na via Na-HCO3 on BM, then reabsorbed by peritubular capillaries
- H leaves PCT through NHE (Na moves in) where it can combine with more HCO3 ions, allowing cycle to repeat
What does Amiloride do in PCT?
- Blocks NHE antiport in PCT
- Prevetns 80% of action of Angiotensin II, that is secreting H2 ions from PCT into lumen
What does Amiloride do in DCT?
Acts as a diuretic inhibiting ENaC in DCT
What ion is predominantly absorbed alongside Na+?
Describe its reabsorption
Cl- reabsorption;
- Diffuse from PCT cell into peritubular capillaries through Cl channel on BM
- Cl-Base Antiport moves Cl into PCT cell, and Base into lumen
Other than through cell pumps/ channels, how can Na, Cl and Water leave lumen?
Paracellular diffusion through tight junctions
(Cl moves towards +ve charge building up in interstitium)
(Solvent drag, NaCl dragged along with water)
Explain how Carbonic Anhydrase inhibitors can be used as a diuretic
How big is this effect?
- H2CO3 not broken down into H and HCO3
- NHE antiport doesn’t function
- Na+ doesn’t move from Lumen into PCT then finally into Interstitium through Na-K ATPase (Therefore H2O doesn’t follow so more H2O is excreted)
- Significant but not massive effect
There is an increasing concentration gradient of solutes as we travel from Cortex to Papilla.
Compare H2O movement from/ in the Desending and Ascending parts of LoH
Descending: Water moves out into Interstitium
Ascending: No water movement as Ascending Limb is impermeable to H2O
Describe Na Reabsorption in LoH
- Na pumped out through BM via Na-K ATPase, whilst K is brought in from Interstitium
- Na, Cl and K passively diffuse into LoH cell from lumen via Na-2Cl-K symport on AM
- K diffuse out of LoH cells through ROMK channels on AM AND BM
- Cl diffuses out of LoH cell through Cl channel on BM
- K and Cl diffuse out of LoH cell through K-Cl symport on BM
Explain how Loop Diuretics work to reduce Na reabsorption in LoH
Name 1 of these
How potent are these?
- Block Na-2Cl-K symports
Furosemide
Most potent diuretics, cause massive loss of water
Outline the water permeability beyond the LoH
- Low H2O permeability of early DCT
- H20 permeability in Late DCT and CD dependent on Aquaporins (dependent on ADH)
Outline Na reabsorption in DCT
Include Ca channels/ pumps
- Na-K ATPase on BM drives Na-Cl symport on AM, bringing them into DCT Cell passively
- Na diffuse in through ENaC channels on AM
- K and Cl diffuse out through their own channels on BM
- Ca-ATPase and Na-Cl Antiport on BM move Ca out of DCT cell
- Ca channels diffuse into DCT cell through channels on AM
How do Thiazides affect Na reabsorption in DCT?
How does Amiloride work here?
Thiazides;
- Na-Cl symport on AM blocked
Amiloride;
- Blocks ENaC channels on AM
Early CD and Late DCT share many similarities, making it difficult to delineate where the change happens.
What are the 2 regions and cell types in the CD
Regions;
- Cortical Collecting Duct (CCD, descends through cortex)
- Medullary Collecting Duct (MCD)
Cell types;
- Principal cells
- Intercalated cells
