B-type natriuretic peptide

B-type natriuretic peptide is a hormone the heart releases as a “pressure-relief valve.” When the heart’s walls are stretched by too much blood volume, the muscle itself secretes this peptide to tell the body to shed salt and water and to relax the vessels — easing the load on the heart.

Despite an old name (“brain” natriuretic peptide, where it was first found), its main source is the ventricular muscle of the heart, which makes more of it when wall tension rises. Once in the blood it promotes natriuresis (sodium excretion in urine), increases water loss, widens blood vessels, and dampens the renin-angiotensin-aldosterone system. In effect it is the natural counterweight to the salt-and-water-retaining hormones, pushing in the opposite direction.

Clinically, BNP is best known as a blood test. Because failing, overstretched hearts pour it out, a high BNP level helps diagnose and gauge the severity of heart failure and track its treatment. A laboratory-stable fragment called NT-proBNP, made when the precursor is cleaved, is often measured instead because it lasts longer in the blood.