B-type natriuretic peptide (BNP) belongs to a family of protein hormones called natriuretic peptides. These natriuretic peptides have an important role in regulating the circulation. They act on blood vessels, causing them to dilate, or widen. They also work on the kidneys, causing them to excrete more salt and water. In addition, the natriuretic peptides reduce the production of various hormones that narrow blood vessels, boost the heart rate, or affect fluid retention; examples include adrenaline, angiotensin, and aldosterone.