A heart that doubles as a gland
If RAAS were the only volume system, the body could only ever tighten — there would be no easy way to *let go* of fluid. The counterweight is built into the heart itself. The heart's muscle cells, especially in the atria, store hormones and release them when the chambers are stretched by too much returning blood. Stretch means “too full,” and the heart answers by secreting atrial natriuretic peptide (ANP) and, mainly from the ventricles under strain, B-type natriuretic peptide (BNP). The word natriuretic literally means “making the kidney pee out sodium.”
RAAS and the peptides as opposites
The simplest way to hold the natriuretic peptides in mind is as the mirror image of RAAS — a clean case of hormone antagonism. Where RAAS *keeps* salt, the peptides *dump* it (natriuresis). Where angiotensin II *tightens* vessels, the peptides relax them. Where aldosterone is *encouraged* by RAAS, the peptides actually *suppress* renin and aldosterone. Two opposing forces, and your actual volume sits wherever their tug-of-war balances.
RAAS (volume LOW) Natriuretic peptides (volume HIGH)
Trigger low pressure / salt heart wall STRETCH
Sodium reabsorb (keep it) excrete it -> NATRIURESIS
Vessels constrict (tighten) dilate (relax)
Volume effect raise it lower it
Renin/aldost. turn UP turn DOWN
<----- pulling toward more volume | pulling toward less volume ----->
actual volume rests at the balance pointWhy BNP shows up in clinic
There is a very practical payoff. When the heart is failing, the chambers are chronically over-stretched, so they pour out BNP continuously. A blood test for BNP (or its companion fragment NT-proBNP) therefore acts as a stretch gauge for the heart. A breathless patient with a sky-high BNP probably has heart failure; a normal BNP makes it unlikely. The hormone the heart releases to *unload* itself doubles as the signal doctors read to know how *overloaded* it is.