why is the urinary Na excretion high in SIADH? I thought it would be low as you are increasing blood volume.
What happened to the whole water follows salt and vice versa principle?
Let's back up and talk about what you probably already know, and progress to your question.
What is SIADH? It is the syndrome of inappropriate ADH secretion. Something goes wrong in the brain (tumors or mass effects) or in the lungs (paraneoplastic syndrome) and ADH is made.
What does ADH do? It holds onto water from the kidneys, thereby expanding the blood volume. What does it do? It holds onto water.
Water only. This is why the patient may become hyponatremic.
Why hyponatremic? Disorders of sodium can usually be thought of as a disorder of concentration. When free water (water only) is held on to, it effectively dilutes whatever is in the blood. Thus there is a dilutional effect to the blood. The concentration goes down of everything. The lab we measure (other than osmoles specifically), the routine lab we measure that gives us an idea of concentration of the blood, is sodium. Too much free water dilutes teh blood, dilutes the sodium, and you get a hyponatremia.
So how does ADH hold on to only free water? ADH increases the expression of, as well as insertion into the plasma membrane of, channels called
aquaporins. These insert into the collecting ducts and allow the passage of one thing. Water. Aquaporins allow
WATER to flow out of the collecting duct but
NOT SALT.
Why then does urine sodium increase? urine sodium concentration increases for the opposite reason of why the body gets hyponatremic. If water comes out of the urine to dilute the blood, then the water out of the urine must concentrate the urine. Hopefully, that was obvious.
An expanded blood volume (now all water) increase GFR at the kidney. More stuff is filtered. Think of it this way: the patient feels that there is too much water in their blood, and the kidneys are going to get rid of it. Everything in the filtered water goes with it (including sodium). Once in the kidney the kidney sees low concentration of sodium and says "wow, i need to pass out this dilute urine" but the SIADH says "HOLD ON TO WATER" louder than the kidney can say "get rid of water."
why is the urinary Na excretion high in SIADH? I thought it would be low as you are increasing blood volume.
Increased filtration + increased concentration of lost sodium = increased urinary sodium
What happened to the whole water follows salt and vice versa principle?
Usually thats the case. But Aquaporins allow flow of water only. This only happens in the collecting tubules. Urine is more dilute than blood, so water will flow away from the tubules if allowed to.
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