B-type natriuretic peptide
Picture the heart's pumping chambers as balloons. When they are overfilled and stretched, they send out a chemical distress signal called B-type natriuretic peptide. Measuring this hormone in the blood gives a quick read on whether the heart is under pressure from too much fluid, which is the hallmark of heart failure.
BNP is made and released mainly by the ventricles in response to wall stretch and increased pressure. Once in the blood, it acts as a hormone that promotes salt and water loss through the kidneys and relaxes blood vessels — the body's own attempt to relieve the overloaded heart. A high blood level reflects how hard the heart is straining.
BNP is most valuable for sorting out breathlessness: a low level makes heart failure unlikely, helping point to lung or other causes instead. But the number is not perfect — it rises with age and kidney disease and can be lower in obese patients — so it supports, rather than replaces, the overall clinical picture and imaging such as echocardiography.
The newer heart-failure drug sacubitril-valsartan raises measured BNP because it blocks the enzyme that breaks BNP down, so NT-proBNP is often preferred for monitoring patients on it.