USMLE metabolic alkalosis Nacl detection in macula densa?

[lundastar]

using diuretics(loop diuretics, thiazide..) , there is metabolic alkalosis because the ECFvolume is decreased (d/t urine increased) --> RAAS Activation -->metabolic alkalosis.

but.. I have a confusion that if we use loop diuretics or thiazide there is much NaCl excretion at TAL or DCT.
THEN, Macula dense which is located in Distal end of loop of henle will detect HIGH NaCl.. --> then it should be NOT RAAS activation...?

Help me Thank you in advance!

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[Ven0m]

using diuretics(loop diuretics, thiazide..) , there is metabolic alkalosis because the ECFvolume is decreased (d/t urine increased) --> RAAS Activation -->metabolic alkalosis.

but.. I have a confusion that if we use loop diuretics or thiazide there is much NaCl excretion at TAL or DCT.
THEN, Macula dense which is located in Distal end of loop of henle will detect HIGH NaCl.. --> then it should be NOT RAAS activation...?

Help me Thank you in advance!

Well if we know for a fact that the metabolic alkalosis with these diuretics are because of the RAAS, then it would be logical to assume that the decrease in ECF volume produces a net increase in RAAS activity even though there might be more NaCl being sensed at the macula densa

 

[aldol16]

RAAS is activated either by reduced Na reaching the macula densa or by reduced perfusion pressure of the macula densa. Since diuretics reduce ECF, this will reduce perfusion to the macula densa.

 

[TheIllusionist]

Macula detects flux (rate of flow through the surface) of NaCl.

Abstraction

1. Pre-Diuretics

Let's say your density of Na Cl is 100 Na Cls/ Liter in your blood. Let's say 1L at volumetric baseline flows past the macula densa per second.
Therefore, at baseline, the macula densa detects 100 Na Cls floating past it per second.

2. Post Diuretics

Over time you get volume contraction with diuretics. So flow rate will decline. So even if the solution flowing through the macula dense let's say has 120 Na Cls/L, if the flow rate say cuts in half to 500ml/sec instead of 1L/sec before the diuretic volume depletion, the overall number of NaCls passing through the macula densa per second is 60 vs. 100 (baseline).

This causes activation of RAAS. RAAS leads to aldosterone which leads to metabolic alkalosis and hypokalemia.